180 INDUSTRIAL A X D ENGINEERIiVG CHEMISTRY VOl. 21, XO. 2

of the chemical constitution and structure. A glass melted from moist raw materials differs in working quality from a glass melted from the same materials dry. Chemically they do not differ. Two glasses otherwise identical, both without visible strain, will have different viscosities, depending on heat treatment. What structural differences exist among these glasses identical in chemical composition yet unlike in physical properties, we do not know. Nor has a systematic determination been made of equilib- rium conditions among glass components. Investigation of these subjects and others as purely scientific problems undoubt- edly would yield results of immediate practical application.

A major task of the glass technologist today is the delivery to glass-working machines of material that is both homogeneous and uductuating as to chemical composition, as to temperature, and as to physical properties. The hand worker can adapt his ma- nipulation to a varying material. For the machine the material should be constant. Present glass-melting methods are poorly adapted for delivering such a product.

One reason why glass is not homogeneous is that it dissolves the clay walls of the tank. A long step toward meeting the crying need of the industry for a suitable refractory in which to melt glass has been taken by Doctor Fulcher, inventor of cast refractories, which are aluminum silicates and refractory oxides handled as in iron foundry practice, the material being melted in an electric furnace and cast in molds for tank blocks and other shapes. The cast block is more resistant to the corrosive attack

of glass than the ordinary clay block, which lasts in exposed positions a t best a year and sometimes only a few months.

Lack of homogeneity is caused also by temperature differences in the glass, and such differences, as well as an excessive waste of fuel, are to be laid to the conventional design of the glass-melting tank. I n the melting process, which is a large item of cost, only about 10 per cent of the coal used is actually required to raise the raw materials to temperature and to melt them. Ninety per cent of the heat value is lost. The excessive consumption of fuel is caused chiefly by the practice of maintaining a t or near melting temperature a mass of glass out of all proportion to the quantity actually being worked. For machines working, say, 50 pounds a minute we hold 6000 times that amount, or 160 tons a t white heat 24 hours a day, 7 days a week, and thus cause losses by radiation and otherwise amounting to many times the total heat theoretically required.

An abundance of interesting and fundamental work, then, re- mains to be done in glass, both in pure science and in its appli- cation. One of my fellow students in Ostwald’s laboratory, in Leipzig, thirty years ago, who was laboring as we all were to give birth to something that might charitably be regarded as a con- tribution to science, was in the habit of lamenting bitterly that the easy things had all been done. In glass not even the easy things have all been done, and help is needed from those who like to do the hard things. To workers in science we suggest the field of glass investigation as attractive and worthy.

CHANDLER LECTURE

The Chandler Lecture for 1928 was delivered at Columbia University on December 7, by John Arthur Wilson, of Mil- waukee, Wis., chief chemist of A. F. Gallun and Sons Company and consulting chemist and director of research for the Mil- waukee Sewerage Commission. In presenting the medal to Mr. Wilson, Dean George B. Pegram stated that the medalist was best known “for the way in which he has applied the most modern concepts of chemistry to one of the oldest industries, the making of leather,” and described his achievements as follows:

Mr. Wilson’s published researches in physical chemistry, colloid chemistry, and the chemistry of proteins ; his application with great daring and acumen of wide and exact knowledge of the most modern advances in chemistry to the complex problems of leather chemistry, resulting in valuable improvements in processes; and his distinguished public service in introducing improvements in the process of sewage treatment that has not only made operable a sewage disposal plant for his own city of half a million people which is a valuable object lesson for all our cities, but has made it operable in such a way that it may soon be returning revenues to the city, are achievements that have placed him in the front rank of chemists.

. . . . . . .

The Charles Frederick Chandler Foundation was established in 1910, when friends of Professor Chandler presented to the trustees of Columbia University a sum of money, and stipulated that the income was to be used to provide a lecture by an eminent chemist and also a medal to be presented to this lecturer in further recognition of his achievements in the chemical field.

The previous lecturers, with the titles of their lectures, are as follows: 1914

1916

1920

1921

1922

1923

1925

1926

1927

L. H. Baekeland

W. F. Hillebrand

W. R. Whitney

F. G. Hopkins

E. F. Smith

R. E. Swain

E. C. Kendall

S. W. Parr

Moses Gomberg

Chemistry and Leather John Arthur Wilson

A. F. GALLUN & SONS COMPANY, MILWAUKEE, WIS.

Some Aspects of Industrial Chemistry [Vol. 6 769 (1914)l

Ou; Analytical Chemistry and Its Future [Vol 9 170 (1917)I

The Lktl;st Things in Chemistry [Vol. 12, 599 (1920)l

Newer Aspects of the Nutrition Problem [Vol. 14, 64 (1922)l

Samuel Latham Mitchill-A Father in American Chemistry [Vd. 14, 556 (1922) ]

Atmospheric Pollution by Industrial Wastes [Vol. 15 296 (1923)l

Influence hf the Thyroid Gland on Oxidation in the Animal Organism [Vol. 17,525 (1925)

The Constitution of Coal-Having Speciall Reference to the Problems of Cirboniza- tion [Vol. 18, 640 (1926)l

Radicals in Chemistry, Past and Present [Vol. 20, 159 (1928)l

INCE the dawn of civilization, leather has been one of the world’s most important commodities. It has become so much a part of our everyday life that we should find our-

selves in a quandary if it were suddenly taken from us. And yet, after thousands of years of daily use, its properties remain but poorly defined. Leather is not a simple and homogeneous material of definite properties. On the contrary, it is of very

variable chemical composition; it has an exceedingly complex and variable physical structure; and every variation in composi- tion or structure causes some corresponding change in properties and in serviceability. Unfortunately, the relations involved are not yet well understood.

Occasionally we find glaring examples of the far-reaching effects of our ignorance in this respect. It will be sufficient to cite one.

S

February, 1929 INDUSTRIAL AND ENGINEERING CHEMISTRY 181

The results of one of our recent investigations have indicated that the great majority of people now suffer unnecessary foot discom- fort because of the methods employed in tanning the leather used in making their shoes. The discomfort arises from an excessive shrinkage and expansion of the leather with changing atmospheric conditions, which can be overcome to a very considerable extent by changing the method of tanning the leather. Very few people, if any, previously suspected that the discomfort was in any way related to the composition of the leather.

Because of the important part which leather plays in the daily life of nearly every civilized human being, it is apparent that a great service can be rendered to mankind by the development of a scientific control of all the important properties of leather. Chemists have appreciated this fact for more than a century, but the task has been too great for their limited facilities. It involves studies of the materials used in making leather and of their chemical reactions, as well as measurements of properties of leather which are not well defined. A study of the raw skin alone offers seemingly infinite diffi- culties. Its structure is very complex and varies according to the kind of animal, its age, food, habits, and climatic conditions under which it lived, and also according to the particular location in the skin. Animal skin contains a number of ‘different proteins, fats, and other materials in variable propor- tions. Few materials known to the chemist are so complex as the proteins and the tannins which a r e employed to convert protein matter into leather. In the manufacture of leather, one also encounters bacteria, molds, enzymes, complex inorganic salts, emulsions of various kinds of oils, dyestuffs, and finishing materials, including waxes,

heat from the body, using the evaporation of water to accelerate the heat escape, when necessary, and supplying an oily material to the surface of the skin to retard the loss of heat when the ex- ternal temperature falls. The skin is also an organ of sense, equipped with nerves sensitive to touch, pain, heat, and cold. It is an organ of secretion and excretion and is supplied with glands, ducts, muscles, and blood vessels. It is a covering protecting the body against bacterial infection and acting as a buffer against shocks and blows. In strong sunlight it is capable of developing color filters to protect the underlying tissues from the destructive action of the ultra-violet rays of the sun. The many intricate functions of the skin are associated with a structure and chemical composition that are exceedingly complex and variable.

John Arthur Wilson

gums, resins, soluble proteins, and lacquer and varnish materials. To solve the basic problems of leather chemistry actually requires the elucidation of the basic problems of most other branches of chemistry.

During the past decade leather chemistry has kept pace with the unprece- dented speed of development of other branches of the science. In the task of producing a more serviceable leather under scientifi- cally controlled conditions, a few definite results have already been obtained and we may confidently look forward to further important developments in the near future.

In this lecture an attempt will be made to portray the present status of leather chemistry in such manner as to be of interest to chemists in general. This involves descriptions of the structure, composition and chemical reactions of animal skin, the basic principles underlying the major processes of leather manufacture, and the effects of variations in operations upon the finished leather. For accounts of the development of each theory and the objections raised against it, reference must be had to the litera- ture, for time will not permit us to do more than to state the theory that appears most plausible a t present. Any individual theory presented must be accepted only with reservations. The object here is merely to present a picture of leather Chemistry as a whole.

Animal Skin

However, the outlook is far from being hopeless.

Many of the important properties of leather depend upon the structure of the skin from which it was made. A knowledge of the functions of skin will enable one to understand this structure more clearly. One of the most important functions of the skin is to keep the body temperature constant. It is supplied with a wonderfully delicate mechanism which controls the escape of

The skin is divided sharply into two layers, distinct both in structure and origin: a rela- tively very thin outer layer of epithelial tissue, the epidermis; and a much thicker layer of connective and other tissues, the derma or corium. Raw skin, as an article of commerce, has also a third layer, the superficial fascia, known to the tanner simply as “flesh” and containing both adipose and areolar tissues. I n life the areolar connec- tive tissues connect the skin proper very loosely to the underlying tissues of the body. The derma lies between the superficial fascia and the epidermis. In the preparation of skin for tanning, except in special cases, such as the tanning of fur skins, the flesh and the entire epidermal system must be re- moved, leaving the purified and unharmed derma to be converted into leather.

Figure 1 is a photomicrograph of a cross section of cowhide cut from the butt. It will serve to show the general structure of skin. The epidermis appears as a thin, dark

line forming the upper boundary of the section and occupying barely 0.5 per cent of the total thickness, the rest being the derma, the areolar tissue having been removed from this portion of the hide in flaying. The epidermis is made up of cellular strata originating from the ectoderm, the outer layer of the young em- bryo; and the derma is derived from the mesoderm, or middle layer. These two layers grow independently throughout life and differ materially in both chemical and physical properties. The tanner makes good use of this difference in the preparation of skin for tanning.

The top fifth of the section shown in Figure 1 has a structure very different from that of the rest of the section and has been called the “thermostat layer,” which indicates its primary func- tion. The lower portion is known as the reticular layer because of the network appearance of the fibers of connective tissue. It is very advantageous to deal with these two layers separately, because the structure of the reticular layer determines many of the physical properties of the leather, such as tensile strength, solidity, resilience, temper, etc., while the structure of the thermo- stat layer determines more particularly the appearance of the leather. In making the finer grades of leather, a great deal of attention must be paid to the thermostat layer. The reticular layer is made up chiefly of interlacing bundles of fibers of white connective tissue, composed of the protein collagen. Collagen is the leather-forming substance of the skin. It is insoluble in cold water, but hot water converts it into gelatin and dissolves it. The reticular layer also contains blood vessels and nerves and a very small amount of yellow connective tissue made up of the protein elastin. It also contains blood and lymph. In some animals part, or even all, of the fibrous portion may be replaced by adipose tissue containing an abundance of fat cells. The fatty tissue may even extend up into the thermostat layer. Since

182 INDUSTRIAL AND ENC

this fatty tissue has no value for leather-making, the value of the skin decreascs in proportion to the amount of fatty tissue present.

Figure 2 shows a portion of the thermostat layer of Figure 1 a t higher magnification. The individual cella of the epidermis can be differentiated. The epidermis may be likened to layers of bacteria clinging to the surface of the true skin. The portion of epidermis in contact with the true skin is a layer of living epi- thelial cells, rather elongated in shape. In reproducing, each cell increases in height and then subdivides, forming two cells, one

a b o v e t h e o t h e r .

on continuously. the food being supplied by derma, the epi- d e r m i s h a v i n g no blood vessels of its own. As the older cells are pushed out- w a r d , f o o d i s no longer available and the cells die, dry up a n d become scaly, and are g r a d u a l l y worn a w a y . This scaling is often very n o t i c e a b l e on the scalp in the form of dandruff.

T h e independent growth of the epi- dermis and derma in- volves a number of important a p p e n d - ages of the skin. In the epidermal system tlie reproduction of epithelial cells pro- duces, not only the ep idcrmis , but also the hair and the oil and sweat glands. 4 glance a t Figure 1 will show t h a t t h e skin contains many

I I This p~ocess is going

pockets. A t the bot- tom of each pocket, or hair follick. thcre

Figure I-Verrlcal Secflon of Cowhide Taken from Butt. 16 x

is available a supply of nourishment from the true skin, which is used by the epithelial cells in the pocket and causes them to re- produce very rapidly. The cells become so numerous that many are pushed up out oi the pocket. As they are forced out of the region containing bod, they die and become glued together farming a hair. The hair is molded into the shape of the poc- ket and is either straight or curly, according to the curvature of the pocket. TT7hen the tiny blood vessels supplying a hair follicle become hardened with age, they do not permit the pas- sage of the larger particles containing pigmenting substances and so cells are reproduced containing no pigments and the hair becomes gray. When all flow into the follicle is Cut off, all the cells die and baldness results. When a hair is pulled out, another soon forms to replace it.

Attachcd to the bottom of each hair follicle and extending obliquely upward through the thermostat layer, almost to the surface of the skin, is a bundle of "on-striated muscle tissue, known as the erector pili muscle. This is clearly shown in Figure 2. Just above this mosfle and emptying into the hair follicle a t about its midpoint is a group of sebaceous 01 oil glands. Just below the bottom end of the muscle there is an open space

ZNEERING CHEAUSTRY Vol. 21, No. 2

containing a collapsed sac, which consists of sudoriferous or sweat glands. These glands have ducts leading to the surface of the skin and they supply water, the evaporation of which carries away the excess heat developed by the body reactions and pre- vents overheating of the body.

When the outside temperature falls, the pilo-motor nerves communicate with the erector pili muscles, causing thcm to con- tract. The appearance to the naked eye is the phenomenon known as gooseflesh. The muscle is actually putting pressure upon the sebaceous glands. The oil cells are broken down and the oily material is discharged into the hair follicle and from thence to the surface of the skin, where it retards the evaporation of water and conserves some of the heat of the body. The proper balance between the operations of the two kinds of glands keeps the body temperature constant. The sweat glands assist also in the discharge of waste materials from the blood. When the sebaceous ducts become clogged with dirt, the oil is trapped and the pressure behind it causes the appearance of blackheads. Various kinds of pimples result from improper functioning of the oil glands due to improper dirt. The high development of these glands in thc sheep and some other animals crowds and distorts the hair follicles so that they are curved and cause the production of curly hair.

The thermostat layer contains also many fibers of yellow con- nective tissue, made up of the protein elastin. The names elas- tin, collagen, keratin, etc., arc used as though they indicated in- dividual chemical substances, but they probably cover whole classes of closely related protein substances. Very few elastin fibers are found in the reticular layer. They seem to play some part in the operation of the thermostat mechanism.

Collagen is the most abundant constituent of the skin and the &ost important to the tanner, because i t is the material finally converted into leather. The fibers farming the upper boundary of the derma arc composed of protein matter resembling collagen, but diffrring from collagen in resistance to hydrolysis under C~I- tain conditions. Keratin is the chief protein constituent of the epidermis and hair; it is removed as far as possible from the skin before tanning, except in the case of furs. The elastin ap- pears to be of no valuc in leather-making, but the fibers of elas- tin arc too tinyaiid too fewin numbertohave mucheffectupouthe properties of the leather if not rcmovcd. The albumins, globu- lins,and mucins are usuallyleached out of the skin in the opera- tions of soaking, liming, and bating, prior ta taming. Thc sugars and inorganic salts arc not present in sufficient quantities to have much effect. In a fewtypes of skins the fats, lecithins, and choles- trrols are sufficiently abundant to cause trouble, if not removed prior to tanning.

S t ruc ture of Proteins

Lcathcr chemistry is very greatly concerned with the molecular structure of the proteins, particularly collagen. Unfortunately the structure of the proteins is not definitely known. Pischer demonstrated that condensation products of amino acids have properties that would class thcm as proteins, provided there is a sufficient number of amino acid residues in the molecule. He prepared one containing 15 glycine and 3 leucine residues, which had a molecular weight of 1213. It gave the biuret test for pro- tein, piecipitated tannin from solution, and would have been classed as a protein had i t been found in nature. Abderhalden and Fodor later succeeded in preparing a polypeptide containing 15 glycine and 4 leucine residues and having a molecular weight of 1326. Fischer pointed out that this polypeptide was redly only one of 3876 possible isomers, without considering the tautom- erism of the peptide linking. When more than two kinds of amino acids arc involved, the possible number of isomers increases very rapidly. If a protein be imagined made up of 30 molecules of 18 different amino acids, one taken twice, one three times, another three, one four, one five times, and thirteen taken once

February, 1929 INDUSTRIAL AND ENGINEERJNG CHEMISTRY 183

each, there would be loz7 isomers, even if there were no tautom- erism of the peptide group and if the linking took place only in the simple way as with monoamino-monocarboxylic acids. Hol- leman has pointed out that it is possible for each of the different kinds of living material to have its own individual protein and that the infinite variety of forms found in organic nature is partly the result of isomerism in the protein molecule.

As the work on protein structure developed, Abderhalden and others were led to the view that the proteins contain dioxopipera- zine nuclei capable of keto-enol tautomerism, thus

NH NH /\

€10.: 5 . R /\ 0:F HC.R

IH v NH

where R represents an amino acid or polypeptide group. Herzog and k n e l l examined collagen with the x-ray spectroscope and concluded that it is much more simply constructed than had previously been supposed, being composed of a material having a molecular weight of about 700. This value is interesting in view of the work done in our own laboratorics which indicates a combining weight of 750 for collagen.

In proteins like collagen, which have a jelly structure, it seems highly probable that the polypeptide or dioxopiperazine groups form continuous networks throughout the entire mass, there being no individual molecules in the orthodox sense of the term, just as we now know that a crystal of sodium chloride contains sodium and chlorine atoms, but no discrete molecules of sodium chloride‘. The protein structure is pictured as a three-dimen- sional network of atoms with interstices of such magnitude as to permit the free passage of water molecules and of simple ionogens.

Protein Equilibria

If a skin is immersed in water and then acid is added very gradu- slly, the collagen fibers begin to swell by absorbing the aqueous solution. As more acid is added, the degree of swelling increases to a maximum and then decreases as still more acid is added. The addition of salt causes a decrease in swelling. This effect was frequently noticcd by tanners of the nineteenth century. hut chemists were unable to explain it. Toward the end of the last century the late Professor Proctcr, known as the “iather of leather chemistry,” concluded that any hope of developing a science of leather chemistry depended upon finding a solution of the phenomenon of protein swelling; unless one understood the comparatively simple phenomena observed in pickling, one could hardly hope to understand the much more complex phenomena observed in tanning. He devoted the remainder of his life primarily to this problem. He chose what he considered the simplest possible case, the swelling of a strip of purified gelatin in contact with a solution of varying concentrations of hydro- chloric acid and sodium chloride. The work finally culminated in the Procter-Wilson theory of swelling.

It would be going too far afield to give the development of the theory here. We shall, rather. attempt to draw a picture of the molecular mechanism of swelling without considering the mathe- matical relations involved. Consider a strip of gelatin immersed in a solution of hydrochloric acid. Water and acid molecules penetrate into the interstices of the protein network. It has been demonstrated that polypeptide and dioxopiperazine groups do not lose their reactivities when built up into protein-like structures. The protein contains trivalent nitrogen, which reacts with hydrochloric acid, farming a substituted ammonium chloride. The remarkable thing about this salt is that the cation is insoluble and the anion very soluble. The cation is part of the protein network and is not in true solution. The effect is to con- fer upon the network a positive electrical charge. The anion is in

true solution in the water present in the interstices of the protein network. This solution will contain hydrogen and chlorine ions in equal number derived from the ionization of the acid and, in addition, i t will have the chlorine ions from the substituted am- monium chloride, which we may call gelatin chloride. In other words, the solution in the interstices of the protein network wiU have 8 greater concentration of chlorine ions than of hydrogen ions. In the outer solution, free from protein, hydrogen and chlorine ions ate present in equal concentrations.

This is essentially the condition described in Donnan’s now famous theory of membrane equilibria. It is obvious, from Don- nsn’s reasoning, that the total concentration of chlorine plus hydrogen ions is p ra te r in the jelly solution than in the external solution at equilibrium. This difference is the mcasuxe of a force tending to cause ions to pass from the jelly to the external solution. The conditions here are differat from those pictured in Donnan’s theory in that there is no membrane. The ions in actual solution in the jelly find no mechanical obstruction to their passage into the external solution, but are held back by the attraction of the anions for the positive charges on the protein network. The effect is that of a force pulling on the network and t ad ing to drag i t out through the external solution. Gelatin jellies follow Hooke’s law up to an elastic limit and the increase in volume is directly proportional to the excess in concentration of all ions of the jelly solution over that of the external solution. For any special case the quantitative relation involves two coil- stants, the hydrolysis constant of the substituted ammonium chloride and the constant corresponding t o the bulk modulus of elasticity of the protein. Given these constants, all the relations can be calculated by simple ihermodynamic reasoning. Thi3 was done by Wilson and Wilson for the gelatin-IIC1 cquilibrium ovcr

Figure &-Vertical Section of Thermostat Layer of Cowhide. Taken from Same Section as Thar of Figure 1. 75 x

184 INDUSTRIAL AhW ENGINEERING CHEMISTRY Vol. 21, No. 2

the entire swelling range studied by Procter, and the agreement between calculated and observed results for degree of swelling and distribution of ions between the two phases was absolute, nitkin the limits of experimental error.

This work not only furnished a new basis for leather chemistry, but i t started a new trend of thought in other branches of chem- istry, an outstanding example being the work of the late Jacques Loeb on colloidal behavior. It also opened up a new field of in- vestigation on the nature of aqueous solutions in the region of electrically charged surfaces. When a charged particle is sus- pended in an aqueous solution of some ionogen, the ions tend to distribute themselves between the bulk of solution and the thin film of solution, wetting the particle in a manner analogous to the distribution of ions between an aqueous solution and a gelatin jelly. It has not yet been found possible to measure concentra- tions directly in such thin films of solution, but by studying the relations involved in the case of the jellies mu& has been learned of the relations involved in the case of colloidal dispersions.

Figure 3-Verticsl Section of Caifskin from Buff.

The apparent attraction of the gelatin for the aqueous solution is the result of the acquisition by the solution of electrons belonging to the atoms in the protein network. A similar condition ob- tains in the case of an aqueous dispersion of electrically charged particles: the charging endows the barticles with an attraction for the water that increases the stability of the dispersion. The principles learned in this work on leather chemistry were applied also to the filtering and drying of sewage sludge and made p~ssihle the successful operation of the great sewage disposal plant a t Milwaukee, conceded by many to be the most efficient sewage dig. posal plant in the world.

These few examples emphasize the direct bearing of the funda- mental problems of leather chemistrr upon the fundamentql problems of many other branches of chemistry.

Taken after 40 Hours In Lime ~ i ~ ~ o r . 25 x

Curing

After flaying, it is customary to cure the skin by packing it in sodium chloride. This protects i t against bacterial damage until it is ready to be worked in the tannery. The salt diffuses into the skin, gradually saturating the water remaining in it. During this process the skin gives up some of its water, which flows away as brine. The fresh skin contains about 62 per cent of water and the cured skin, about 40 per cent. When the skinsare washed to remove blood and soluble proteins before salting, the curing is much more effective against bacterial damage. In general, the greater the amount of soluble protein matter present, the greater is the concentration of sodium chloride required to prevent ap- preciable bacterial action.

Sodium chloride and a number of other halide salts catalyze the hydrolysis of collagen. Pfeiffer found that halide salts form definite and fairly stable compounds with both amino acids and dioxopiperazines. Thomas found that those salts which most readily form such compounds are most active in increasing the hydrolysis of collagen. While halides catalyze the hydrolysis, sulfates inhibit it. This has raised the question as to the relative merits of sodium chloride and sodium sulfate as CUI- ing agents. In practice the hydrolysis produced by sodium chloride must be very small. On the other hand, the bacterial damage resulting from the use of insufficient salt may be v e ~ y great. At low temperatures the solubility of sodium sulfate is very much less than that of sodium chloride. In comparing the two salts in a practical way, it is not easy to ditrerentiate between the effects of the salts directly upon the collagen and the effects of the salts upon bacterial activity. On the other hand, the hydrolytic activity of the chlorides of calcium and magnesium is so great that skins would be destroycd if saturated with either. Even fully tanned leather will shrivel up when soaked in saturated solutions of calcium or magncsium chlorides.

Soaking a n d Fleshing

When the awed skin reaches the tannery, it is washed free from blood and dirt end then soaked in cold water to leach out the soluble protein matter and to allow the collagen fibers to absorb water. A uniform distribution of water throughout the skin is essential to permit satisfactory fleshing. After a t least a prelimi- nary soaking, the areolar tissue, or flesh, is removed by placing the skin in a machine which forces the flesh side of the skin against a revolving roller set with sharp blades. After fleshing, the skins are sometimes soaked again in order the more completely to re- move soluble proteins.

If soaking is unduly prolonged in warm water, the skins are certain to suffer from bacterial damage, unless protected by some antiseptic. Bacterial activity can be greatly retarded by the ap- plication of about 100 parts of chlorine per million of water. It is the proteolytic enzyme secreted by the bacteria that does the real damage. When water colder than about 12" C. is used for soaking, in the ordinary way, antiseptics are unnecessary. because enzyme activity a t this low temperature is so slow that the soak- ing operation is completed long before any appreciable damage is done. It is probable, also, that much less enzyme is produced a t this temperature than at temperatures 10 to 20 degrees higher.

Unhairing

Only the conncctive tissues of the skin are convertedintolcather. In separating the hair and epidermis from the derma, use is made of the diflrrence in reactivity towards acids and alkalies of kern- tin, the chief protein of the epidermal system, and collagen, the protein of the white connective tissues. In contact with alkaline mlutions, keratin is much more rapidly hydrolyzed than collagen; in contact with acid solutions, collagen is much more rapidly at- tacked than keratin. This was known to the ancient tanners. who used limewater to looscn the hair. If a calfskin is soaked in

February, 1929 INDUSTRlAL AND EhrQIhrEERINQ CEIEMISTRY 185

saturated limewater at 20" C. for abo& 5 days, the hair and epi- Vegetable Tanning dermis will become loosened from the derma so that they can be rubbed off with a blunt blade.

For many years tanners have known also that this action can he greatly accelerated by the addition of a small quantity of soluble sulfide. Figure 3 shows a cross section of a calfskin after soaking in saturatcd limewater containing 0.7 gram per liter of sodium sulfide for 2 days a t 25" C. The alkali has destroyed the epidermal cells which rest upon the true skin'. The older cells, forming the corneous layer of the epidermis, still cling together and appear as a continuous mass somewhat separated from the der-. I t is thrown over a rubber slab and forced against a roller Set with blunt knife blades which rub off the loose hair and epidermis. A somewhat similar machine then scuds out the glands and epithe- lial cells still lying in the hair follicles.

The role played by sulfide in accelerating hydrolysis of keratin has long puzzled chemists. Merrill showed that keratinremovec, sodium sulfide from solution, but that collagen does not. Nor does sulfide accelerate the hydrolysis of collagen by alkalies. The sulfide apparently reacts with keratin, producing a product which is much more rapidly hydrulyzrd by alkali than the original keratin. Many lines of investigation have brought forth the suggestion that the sulfide reduces the cystine residues of the+kera- tin molecule, effecting a break between the adjoining sulfur atoms. This view led Merrill to predict that any strong reducing agent, such as stannous chloride, should accelerate the hydrolysis of keratin hy alkaline solutions, and his predictions were confirmed by experiment. Stannous chloride acts like sodium sulfide in the unhairing process.

Thc skin, after liming, is unhaired by machine.

Bat ing

Perhaps the most curious of all the processes involved in making leather is that of bating. For centuries this was one of the mys- terious processes of the taiinery and, until comparatively recently, i t was casiiy the most disgusting. It has been so completely en- shrouded in secrecy, jealously guarded, that it has not been possible to trace its origin. However, the process as handed down by the generations past is still an unpleasant memory to many tanners living today. It consisted in digesting the limed and unhaired skins in a warm infusion of the dung of dogs or fowls until all plumpness had disappeared and the skins had he- come so soft as to retain the kprcssion of thumb and finger when pinched and suffxciently porous to permit the passage of air under pressure. When this process was omitted, the leather always lacked its otherwise fineness of appearance.

Wood demonstrated that the active principle of the dung was a tryptic enzyme. The tanning world was very quick to abandon the use of oiIensive dungs and to replace them by pancreatic enzymes. However, it was not until very recently that chemists discovered the part played by the enzyme. Leather made from unbated skins has a certain characteristic, but undesirable, grainy appearance. Wilson and Daub studied the action of trypsin on limed calfskin under the microscope and found that it hydro- lyzes the tiny elastin fibers in the thermostat layer. In practice bating is not carried this far. Wilson and Merrill demonstrated that the value of bating comes from the hydrolysis by the trypsin of keratose, a degradation product of keratin. Keratose is sol- uble in neutral or alkaline solutions, but is precipitated at its isoelectric point, pH = 4.1. It is formed in the thermostat layer during liming. If not removedpriorto taming, itbecomes precipitated in the thermostat layer by the acid tan liquors and thus causes the roughness of appearance characteristic of unbated skins. This discovery made possible the scientific control of a Process that had given the tanners more trouble than any other involved in the making of leather. The bating value of any enzyme preparation can now he determined easily by measuring its activity upon purified keratose.

After the skin has been bated and rinsed, it is ready to be tanned. Thousands of years ago the discovery was made that the properties of skin substance change completely when the wet skin is brought into contact with the aqueous extract of those f o r m s of p l a n t l ife which have since come to be classed as vege- t a b l e t a n n i n g mate- rials. A t a n l i quor may be made by grind- ing the bark of a tree and leaching it in a coffee percolator, just as one would prepare coffee. When a piece of bated skin is sus- pended in suchaliquor, it becomes colored a tan shade. The color substances diffuse into the protein fibers and render them imputrcs- cible. The active prin- ciple of the extxact is called tannin. Colla- gen and tannin com- bine to form a new subs t ance , l ea the r , which is very much less readi ly hydro lyzed t h a n collagen. I n practice the batedskins are kept suspended in tan liquors until all of the collagen has been converted into leather. Because of the very low rate of d i f fus ion of tannin into the intcrior of the skin, this usually

for very heavy hides. A voluminous literature now describes the attcmpts to isolate

and identify the active tanning principle af various lorms of plant life. Fischer isolated the tannin from Chincse nutgalls and then succeeded in synthesizing it, He prepared penta-m- digalloyl-6-glucose, which proved to he an isomer of the tannin from nutgalls. I ts formula is

F - - 1

HC-C-C-C-C-CH

where R is the radical

OH HO OH

-0.- O O H

O--OC

Fischer's success spurred on studies of tannins from-different Sources, and it was found that they differ considerably in:composi- tion and in properties. In spite of the voluminous literature on tannins which has been built up, the compositions:of the tannins

186 INDUSTRIAL A N D ENGINEERING CHEMISTRY Vol. 21, No. 2

most widely used in making leather have escaped detection, The chemistry of the tannins is evidently still in its infancy,

Even though the compositions of both collagen and the tannins are not definitely known, one may speculatQ from generalities, as to the nature of their combination. Tanning appears to be analogous to the combination of a weak base with a weak acid. Collagen is an ampholyte with an isoelectric point a t pH = 5. In practice, tan liquors have a pH value less than 5 . In contact with solutions more acid than pH = 5, collagen acts as a base. Tannin, on the other hand, acts like a weak acid; in an electrical field it migrates to the anode. Collagen may be classed as a weak nitrogen base and the tannins as phenolic substances.

Baeyer and Villiger studied the combination of phenolic sub- stances with weak bases. One molecule of diethylenediamine will combine with one molecule of phenol. Two molecules of quinoline will combine with one ‘molecule of resorcin. This type of combination persists through all degrees of complexity of the nitrogen bases and phenols from aniline and phenol to protein and tannin, lending great support to the simple chemical theory of vegetable tanning. Phenols combine similarly with oxygen bases. This has led Freudenberg to suggest that the tannins may become attached to both oxygen and nitrogen atoms in the collagen molecule.

If the combination of collagen and tannin is really that of a weak base with a weak acid, it follows that the tanning of collagen will reduce its capacity for combination with acid. Wilson and Bear demonstrated that the capacity of collagen to combine with sulfuric acid decreases as it becomes more heavily tanned.

It also follows from the theory that an increasing degree of tannage must lower the isoelectric point of collagen. Gustavson tested this, using Loeb’s dye technic to measure the isoelectric point of the collagen-tannin compound. Tanning collagen with an extract obtained from quebracho wood caused a reduction of its isoelectric point from a pH value of 5.0 to 4.0; tanning with hemlock bark extract caused a reduction to 3.9. The theory was confirmed in another way by Thomas, who showed that de- aminization of collagen greatly reduces its capacity to combine with tannin.

Because of the probability that the various tannins present in a tan liquor have very different molecular weights, attempts a t calculating combining ratios for collagen and tannin involve much speculation. In its combination with hydrochloric acid, collagen exhibits a combining weight of about 750. If each digalloyl radical in pentadigalloyl glucose is capable of combining with collagen, we arrive a t a combining ratio of 340 parts of this tannin to 750 parts of collagen, or 45 per 100 parts of collagen. The great majority of analyses of vegetable-tanned leathers at our disposal show a ratio of combined tannin to collagen lying be- tween 45 and 90, suggestive of monotannates and ditannates of collagen.

Chrome Tanning

Although vegetable tanning is of ancient origin, chrome tanning is a development of only the last forty years. Most of the world’s supply of light leather is now chrome-tanned. A chrome-tan liquor usually consists of a solution of basic chromium sulfate. It can easily be prepared for study in the laboratory by bubbling sulfur dioxide through a solution of sodium dichromate until the reduction is complete. Usually the skins are pickled before tanning in order to make them uniform in composition. Bated skin may contain variable amounts of calcium carbonate, which would disturb the tanning process. A common method of pickling consists in soaking the skins in a 12 per cent solution of sodium chloride to which sulfuric acid is added to bring the solu- tion to a definite final equilibrium concentration. The skins are then transferred to a revolving drum and tumbled in a chrome liquor until tanned to the desired degree, which may require only a few hours or 2 or 3 days. The chromium salt penetrates .~~~ -~

the skin very rapidly as compared with vegetable tannin, colors it green, and renders it capable of withstanding the action of boil- ing water. The compound of collagen and chromium is extremely resistant to hydrolysis.

It was but natural to suppose that chromium forms salts with collagen analogous to the chromium salts of amino acids or of dioxopiperazines. The equivalent weight of chromic oxide is 25.3. Taking the combining weight of collagen as 750, a com- bining ratio was cilculated of 3.4 parts of chromic oxide per 100 parts of collagen. Leathers have been made with 3.4, 6.8, 13.5, and 27.2 grams of chromic oxide combined with each 100 grams of collagen under conditions which favored the suggestion that they represented what might be termed monochrome, dichrome, tetrachrome, and octachrome collagen, respectively.

During tanning the pH value of the chrome liquor usually lies between 3 and 4. At first sight it might be wondered how col- lagen could act as an acid in contact with a chrome liquor having an acid reaction much greater than the isoelectric point of col- lagen. The thought involved in the theory is that the ioniza- tion of collagen as an acid never becomes reduced to zero, even though it may become extremely small with increasing acidity. This means that, even if the electrical charge on the protein struc- ture is predominantly positive, there still remains a very small, but finite, number of negatively charged groups scattered throughout this structure. Chromic ions diffuse into the collagen jelly and combine with these negatively charged groups wherever encountered. The ion which first combines with the protein may have only a single positive charge and might be indicated by the simplified formula, Cr(0H):. Having neutralized the electrical charges on each other, both the collagen and chromium compounds become capable of ionizing further, the chromium group giving off another hydroxide ion and the collagen another hydrogen ion. With a repetition of this process, all three bonds of the chromium become united directly with the collagen struc- ture. The fundamental assumption underlying this view is that, however small may be the concentration of negatively charged groups in the collagen structure under the conditions of tanning, it is very much larger than would result from the dissociation of the chromium compound of collagen. The remarkable resistance of chrome leather to hydrolysis is in line with this view.

Recent work has made it quite clear that this simple theory of the combination of chromium and collagen does not represent the whole fact. Many phenomena have been observed which could be explained only by introducing Werner’s coordination theory, which views a chromium ion, not as a chromium atom with three positive charges, but as an electrically charged micell or nucleus in which a central chromium atom is surrounded by six coordinatively bound groups. Gustavson has shown that it is such a micell as a whole that combines with the protein.

Some atoms are able to combine with others, not only by means of their recognized primaryvalency forces, but also by means of ad- ditional forces called auxiliary valencies. According to Werner’s theory certain atoms tend to draw to themselves, in the form of surrounding shells and by forces other than primary valency, a number of other atoms or coordinated groups. The central atom with its coordinated groups constitutes a nucleus outside of which are located the atoms or radicals which are held to the rest of the molecule by primary valency forces. The coordination number of an element indicates the number of groups which an atom can hold in this surrounding shell. Most metals have a coordination number of six, while some of the non-metallic ele- ments have a coordination number of four.

The first, called the a form, is a violet salt of the formula CrC13.6Ht0. All of its chlorine atoms are precipitated from solution by the ad- dition of silver nitrate. The second salt, called the B form, is a green salt of the formula CrCla.5H20. Only two-thirds of its chlorine is DreciDitated from solution by silver nitrate. The

Three different forms of chromic chloride are known.

February, 1929 INDUSTRIAL AND ENGINEERING CHE.MIISTRY 187

third salt, callrd the y farm, is a green salt of the formula CrCIH.- 4H,O. Only one-third of its chlorine can be precipitated from solution by silver nitrate. The structural formulas for these three salts, according to Werner's theory, are as follows:

The nudeus of the a form has three loose electrons which go to complete the octet in each of the outer shells of electrons of the three chlorine atoms, converting them into chloride ions capable of precipitation by silver nitrate. Under certain conditions one chloride ion will penetratc into the nucleus, carrying its extra electron with it and displacing one of the six cwrdinatively bound water molecules. The number of coordinative groups is kept at six, but now there arc only two positive charges on the nucleus, one of the three loose electrons having been returned to the nucleus by the incoming chloride ion. When the chloride ion enters the nucleus it ckases to be a separate ion, but forms part of the complex which constitutes the nucleus. Since only two chloride ions are left, only two-thirds of thc total chlorine can now be precipitated by silver nitrate. When the second chloride ion entcrs the nucleus, replacing a second water molecule, only one chloride ion is left capable of precipitation by silver nitrate. The nucleus then has only a single positive chargc.

Reasoning by analogy to other compounds, we can picture four more chromium chlorides. I n the presence of hydrochloric acid and sodium chloride it is possible for all coordinativcly bound water molecules to be displaced by chloride ions in the following order:

Sodium pentachlore-aquo chromiatc Sodium hexiiehioro chromiite

The trichloro-triaquo-chromium is not known, but a corre- sponding alcoholo compound has been prepared. The chromiates exist in the prescnce of a large excess of hydrochloric acid, al- though they do not form nearly so readily as the corresponding oxalato or similar compounds. An interesting and important fact is that a solution of chromium salts may contain chromium nuclci of variable electrical charge from three positive to thrce negative chargcs per atom of chromium. In the electrophoresis of ordinary chrome tanning liquors it is usual to find bath anodic and cathodic migration of the chromium a t the same time.

When alkali is added slowly to a solution of a chromic salt, very complex nuclei are formed. owing to the tendency for chromium atoms to share hydroxo-groups, thus:

The cohrdinativcly bound groups about a central chromium atom may be likened to the electrons in the outer shell of an atom and the central stom to the nucleus of an atom. The two hydroxo- groups may be likened to the two valence electrons that hold two chlorine atoms together as a molecule. This process of

building up complex chromium nuclei has been called olhication. It may be extended indefinitely by displacing aquo-groups by hydroxo-groups. By adding 2.5 equivalents of sodium hydroxide per mol of chromic chloride very slowly, allowing sufficient in- tervals between additions for olification to take place, Bjerrum succeeded in preparing an ol-compourid with a nucleus containing 12 chromium atoms.

Gustavson has devised a very ingenious method of studying the composition of positively charged chromium nuclei in which the solution is allowed to react with sodium permutite and the in- soluble chrornium compound of permutite which is formed is washed free from the adhering solution and analyzed. His in- vestigations with this method have thrown much light on the mechanism of chrome tanning. It is evident from his work that it is the entire chromium iiucleus which combines with collagen in tanning.

Werner's theory presents threc diffcrent types of possible com- binations of chromium and collagen: ( I ) the acidic groups of the protein may combine with a positively charged chromium nu- cleus; (2) the basic groups of the protein may combine with a negatively charged chromium nucleus; (3) certain groups of the protein may penetratc into the chromium nucleus, becoming co- ordinativcly bound and replacing other groups from the nucleus. By combining types (2) and (3 ) one can con- ceive a combinatimi. in which the chromium is bouiidto the protein by nine bonds per chro- mium atom. When six negative protein groups occupy all six co5rdina- tive positions about the central chromium atom, the nucleus ac- quires three negativc charges, p e r m i t t i n g further c o m b i n a t i o n with three basic protein groups.

Gustavson t a nn P d col lagen w i t h both cationic and an ionic c h r o m i u m nuclei . When a n i o n i c chro- mium was used, the iso- e l e c t r i c point of the collagen droppcd from pH = 5 to pH = 4, just as in the case of vcgelablc tanning, in- d i c a t i n~ combination - with the basic protein groups . Whencolla- Skin a8Thsf S h o w n i n F i ~ u r e 4 . 75 x gen was tanned with

Fisure §-Vertical Seftlon of Chrome- Ssme Tanned Calf Leather from Buff .

cationic chromium, the isoelectric point rose from 5 to 6, indicat- ing combination with the acidic protein groups. I Tlie isoelectric points were determined by the dye technic.

A mass of experimental data accumulated during the past few years indicates that the chief reaction occurring in ordinary chrome tanning is the combination ol cationic chromium nuclei with collagen acting as an acid, but that all threc types of cam- bination occur to some extent.

Miscellaneous Tsnnages

Many salts of heavy metals are capable of tanning collagen, but For this none are quite so satisfactory as those of chromium.

188 INDUSTRIAL A N D ENGINEERING CHEMISTRY Vol. 21, No. 2

reason chromium salts are the only ones used very extensively. Studies of the tanning action of ferric and aluminum salts by Thomas indicate a mechanism similar to that of tanning with cationic chromium.

The tanning action of fish oils has been known for many cen- turies. In the modern manufacture of chamois leather the tan- ning agent is cod-liver oil. The leather is made from the reticular layer of sheep skin, which is split from the grain layer so that the two layers may be tanned separately by different methods. The flesh splits are pickled and then swabbed with cod oil, after which they are pommeled in a specially built machine. They are spread out to cool, re-oiled and pommeled, alternately. Dur- ing the process acrylic aldehyde and other pungent products are evolved. By the time the tanning action is completed, practi- cally all of the water has been replaced by oil. The leather is washed in a warm solution of sodium carbonate to remove the free oil and any free acid left from the pickle. The leather is then washed in running water, dried, bleached in strong sunlight, staked, and buffed to make it smooth. Chambard and Michallet showed that chamoising consists of a chemical reaction involving collagen, the free fatty acids of an easily oxidizable oil, oxygen, and water, in which collagen is converted into a different protein more resistant to hydrolysis than the original collagen.

Payne and Pullman patented the use of formaldehyde as a tanning agent in 1898. Since then, many studies have been made of its combination with collagen and with gelatin. Formaldehyde combines with amino acids and dioxopiperazines, the points of at- tachment being the nitrogen atoms. In proteins it is evident that the formaldehyde becomes attached to the basic groups, as indicated by the shift in the isoelectric point of collagen to a more acid region and the fact that a formaldehyde tannage re- duces the capacity of collagen to combine with acid, tannin, or chromium. It will be remembered that deaminization of collagen has a similar effect.

Quinone has been found to be a very powerful tanning agent, rendering collagen quite inert in boiling water. Meunier has suggested that the mechanism of quinone tanning is indicated by the following equation :

0 RNHz + 2CsH402 = R.N(0)C~H~ + Ce.Hk(OH)%

Collagen Quinone Quinone leather Hydroquinone

This was confirmed by Thomas and Kelly, who showed that quinone actually does combine with the basic groups of the col- lagen.

In 1913 Stiasny presented to the world a new kind of tanning material consisting of sulfonated aromatic compounds condensed with aldehyde in such manner as to form soluble products. Wolesensky has suggested that these compounds, called syntans, enter into chemical combination with collagen through the inter- action of the sulfonic groups with the amino groups of the protein. These compounds are now used widely in admixture with vege- table tanning materials, whose action they modify in a desirable way. The waste sulfite liquors from paper mills furnishes a valuable tanning material called sulfite cellulose, which resembles syntans in certain properties.

It is apparent, from the wide variety and chemical nature of the materials used to tan animal skin and the differences in prop- erties of the leathers produced, that no one chemical equation can be given which will cover all tanning reactions. It is possible, however, to generalize. Collagen and gelatin exhibit marked attraction for water and are readily hydrolyzed. When they undergo chemical changes which markedly decrease their at- traction for water and tendency to hydrolyze under a variety of conditions, they are considered to have been tanned. There are probably a number of definite points in the protein molecule where hydrolytic splitting takes place or where water or highly ionized molecules may become attached. When a substance

combines with the protein a t these points and becomes so firmly attached as to prevent any further combination with water or highly ionized molecules, the effect is one of tanning and the sub- stance is listed as a tanning material. It is possible that the same effect may also be obtained through some rearrangement in the protein molecule not involving actual combination with some other material.

Fat-Liquoring

Although the tanning of animal skin lessens the tendency for the fibers to stick together upon drying, it does not lubricate them so that they slip easily over one another. In fact, when leather is dried immediately after tanning, without further treatment, it is usually very stiff and will crack upon bending sharply. In order to give it the desirable softness and pliability and to increase its tensile strength and resistance to tearing, oils and greases are incorporated into it to lubricate the fibers.

For the finer leathers, such as calf and kid, this is done by tumbling the wet leather in an emulsion of oil in water, a process called fat-liquoring. A fat-liquor emulsion usually consists of an oil, such as neat’s-foot or cod-liver oil; an emulsifying agent, such as soap, sulfonated oil, moellon degras, or egg yolk, or com- binations thereof; a material to adjust the p H value, such as borax or sodium carbonate; and water. When any of the more common types of leather are brought into contact with water, they acquire a positive electrical charge with respect to the water. On the other hand, the oil globules suspended in the fat-liquor possess a negative charge. When leather and fat-liquor are brought together, the positively charged leather structure and negatively charged oil globules tend to neutralize each other and the emulsion is broken, not by the globules coalescing with each other, but by their condensing on the surface of the leather fibers.

In fat-liquoring, the oil globules do not completely penetrate the leather, but become lodged in the outer layers. When neutral oils are used, they tend to penetrate more deeply into the leather during the subsequent drying, replacing the water lost through evaporation. This type of penetration also occurs where wet leather has been swabbed with oil or grease instead of being fat- liquored. The extent of penetration of the oil into the leather has a marked influence upon its physical properties. In chrome calf leather, much used for shoe uppers, we have found the most de- sirable physical properties to result from a rather high concentra- tion of fat in the outer layers, the amount decreasing to zero as the middle of the leather is approached.

One of the most common fat-liquors for chrome calf leather is a mixture of sulfonated oil and egg yolk. Unlike neutral oils, sul- fonated oils do not diffuse towards the middle of the leather dur- ing drying. The fixation of sulfonated oil by chrome leather re- sembles the fixation of tannin by raw skin in vegetable tanning. Apparently chemical combination takes place between the leather and the oil, which explains the failure of the oil to shift its posi- tion during drying. The nature of this fixation has received much attention, It now appears probable that the sulfo-fatty acid radical penetrates into the chromium nucleus, becoming coordi- natively bound and displacing other groups. It has frequently been observed that chrome leather loses some of its resistance to boiling water upon being fat-liquored with sulfonated oil. This can be explained by the theory of chrome tanning just presented, which assumes that the force holding chromium and collagen together is the attraction of the chromium nucleus for its elec- trons which are held by the protein structure. Each sulfo-fatty acid radical which penetrates the chromium nucleus carries an extra electron with it and the attraction of chromium nucleus for the collagen structure is lowered correspondingly.

Various anions can be placed in a sort of electromotive series according to their ability to displace or be displaced by others from the

This view has been tested in a number of ways.

February, 1929 INDUSTRIAL AND ENGINEERING CHEMISTRY 189

chromium nucleus. For example, a tartrate ion will displace a phosphate ion and a phosphate ion will displace a sulfate ion, etc. When chrome leather is treated with sodium phosphate, the phos- phate ions displace sulfate ion from the nucleus, whereupon the leather appears to lose its power t o combine with sulfonated oil. The inference to be drawn is that the sulfo-fatty acid radical penetrates into the nucleus more rapidly than sulfate ion, but less readily than phosphate ion. The tartrate ion penetrates the nucleus so vigorosuly as to effect complete detannization of the leather.

While the foregoing reasoning is admittedly extremely specu- lative, it has proved very useful in explaining observed variations in fat-liquoring in actual practice. Since the value of a theory lies in its usefulness, the reasoning, however speculative, seems to be justified.

Coloring

The shade of color desired in a given piece of leather is produced by a combination of dyeing and finishing operations. Some- times leather is given the desired color in the dye bath and then finished in such manner as not to change this color, but often the finishing processes alter the color of leather, making it necessary to correlate the processes of dyeing and finishing in order to get the desired effect in the finished leather. Some leathers are colored before and others after fat-liquoring, while still others are given a bottom color before and a top color after fat-liquoring. It is possible to color some leathers satisfactorily by the applica- tion of only a simple solution of dyestuffs, but others require a series of baths of mordants, strikers, bottom colors, and top colors.

Tanners find it convenient to divide the dyestuffs used in color- ing leather into three classes-acid, basic, and direct. It has been shown that collagen combines with acid dyes only at pH values less than 5 and with basic dyes only at p H values greater than 5. This is in line with the amphoteric nature of collagen, which acts as a base at p H values less than 5 and as an acid at p H values greater than 5. The purely chemical theory of dyeing has been given very strong support by the work of Chapman, Greenberg, and Schmidt on the combination of acid dyes with gelatin. At pH = 2 they found that 1 gram of gelatin combined with 0.00104 gram equivalent of any one of six different acid dyes. This enabled them to calculate a combining weight of 962 for gelatin. Deaminization of the gelatin caused it to lose its capacity to combine with acid dyes by an amount equivalent to the nitrogen removed by deaminization.

Both chrome- and vegetable-tanned leathers have an acid reaction a t the time of dyeing. Since the protein of leather is never completely saturated with tanning material in practice, we should expect either kind of leather to combine vigorously with acid dyes; and this is the case. A t the low p H values ob- taining in leather-dyeing we should not expect combination to any appreciable extent between collagen and basic dyestuffs. However, combination may conceivably be brought about through the agency of some other material capable of forming stable compounds with both collagen and the basic dyestuff. Tannin is such a material. It precipitates basic dyestuffs so completely as to be used as a test for the presence of basic dyes in solution, and it forms extremely stable compounds with col- lagen. If the active valencies of the tannin are not completely saturated in the combination with collagen, then a further com- bination with basic dyes is possible. The fact is that basic dyes combine vigorously with the collagen-tannin compound a t a pH value of about 4, whereas they do not combine with collagen alone to any appreciable extent a t this pH value. Nor do they com- bine with chrome leather to any appreciable extent a t this p H value unless the leather has first been treated with tannin or some other mordant. The assumption is thus warranted that the com- bination of basic dyes with leather is between tannin or similar groups acting as acids and the dyes acting as bases.

The direct dyes are extremely weak color acids or their salts. Whether because of the extremely low power of dissociation of the color acid or some other cause, direct dyes do not combine readily with the protein of vegetable-tanned leather as is the case with acid dyes. But when skin is chrome-tanned, it takes up direct dyes from solution with such vigor that nearly all of the fixation takes place a t the surface and very little dye penetrates into the leather. Here the chromium is evidently acting as a mordant. It seems highly probable that direct dyes have a powerful tendency to penetrate into the chromium nucleus, be- coming coordinatively bound. Varo found that pretreatment of chrome leather with tartrate ion, which enters the chromium nucleus with very great avidity, causes the leather to take up direct dyes less readily, permitting them to diffuse into the leather. Similarly, Gustavson showed that tannin readily enters the chromium nucleus, which explains why chrome leather loses some of its affinity for direct dyes when retanned with tannin. It seems logical to assume that acid dyes combine with chrome leather in two ways-by direct union with the protein and by entering the chromium nucleus as pictured for direct dyes.

'

Finishing

Leathers which have been colored, fat-liquored, and dried rarely meet the demands of the ultimate consumer without further treatment. They may not have the desired temper; the grain surface may not be sufficiently lustrous and the color may lack the desired uniformity, tone and depth; defects in the grain may be too pronounced; and the leather may absorb water too readily. In finishing, the tanner tries to develop those finer qualities of the leather which are most appreciated by the consumer. The finishing operations may be divided into two classes. One in- volves the application to the leather surface of such materials as casein, albumin and other protein materials, gums, mucilages, resins, waxes, pigments, dyes, and lacquer materials. The other includes mechanical operations, such as staking, rolling, brushing, glazing, buffing, graining, embossing, trimming, ironing, and plating. In the preceding operations the skins are treated in lots of several hundred, but in finishing each skin receives individual attention and its own special series of operations. Finishing is really a fine art requiring great skill. Much of the value of the leather depends upon the ability of the finisher.

Properties of Leather

Users of leather know surprisingly little about its properties. Their choice is usually determined by appearance rather than by the possibilities of service. Makers of leather, on the other hand, merely strive to produce what users will be likely to choose. As a result relatively little effort has been made to discover and control those properties which really mean most to the ultimate consumer. This unsatisfactory condition is aggravated by the fact that studies of the properties of leather are exceedingly diffi- cult and time-consuming. In many cases it is not apparent just what properties are important for certain purposes. In other cases the properties may be recognized by experts, who have difficulty in describing them to laymen, the properties in question never having been named. Methods for measuring many proper- ties have yet to be developed and each property will have to be measured as a function of many different variable factors before any clear picture can be drawn. When the important properties of leather have been defined, there will still remain the task of educating the people who use it.

There is a ray of hope in the recent organization of a national committee for the purpose of recognizing, naming, measuring, and cataloging the important properties of leather. The chief aim of this committee is to render leather of the greatest possible service to mankind. There have already been discovered several quantitative relations between important properties of leather

Vol. 21, No. 2 190 INDUSTRIAL AND ENGINEERING CHEMISTRY

and controllable variable factors and the prospects are bright that the ultimate consumer will find a steady improvement in quality of leather.

When used for footwear, leather must have great strength and yet it must stretch sufficiently to permit the shoe to conform perfectly to the shape of the foot. It has been shown that the greatest variable factor iduencing strength and stretch is the extent to which the thickness of the leather is reduced by split- ting. When a sample of chrome calf leather was split into two layers of equal thickness, the grain layer was found to be only 26 and the flesh layer only 16 per cent as strong as the unsplit leather, per unit width. The resistance of the leather to stretch was found to vary directly with the strength. Increasing the oil content of leather caused an increase in strength with a decrease in resistance to stretch. Increasing the water content, which oc- curs naturally with increasing relative humidity of the atmos- phere, has a similar effect. There is a very great difference in both strength and stretch over the area of a skin. Quantitative relations have been plotted for both chrome- and vegetable- tanned calf leathers, but the task of utilizing the information for the benefit of the consumer is difficult. In the first place changes made to alter strength and stretch values also change other important properties of the leather. Secondly, while in- creasing ease of stretch enables the shoe more readily to conform to the shape of the foot, it also causes the shoe to lose its shape and fineness of appearance more quickly. This is very marked in comparing shoes of calf leather with those of kid leather, which stretches much more easily.

Another important property of leather is its power to ventilate the foot. It can be made water-repellent from the outside while still retaining the power to pass water from the foot to the outside air. Tests have shown that a good shoe-upper leather will transmit water from a moist to a dry atmosphere about 70 per cent as fast as though the two atmospheres had direct contact over the same area. This power is decreased in proportion to the kind and amount of finishing material, such as casein, wax, lacquer, etc., applied to the surface. A small amount of finishing material very greatly increases the resistance of the leather to wetting. With the first application of finishing material, the water-repellence of the leather is increased out of all proportion to the decrease in ventilating power. Quantitative studies of these relations have

In this respect it is a very remarkable material.

made it possible to increase the serviceability of a shoe very greatly.

The comfort of a shoe is also largely determined by the temper, elasticity, flexibility, and resilience of the leather. These proper- ties are greatly influenced not only by the amount of oils in- corporated in the leather, but also by their distribution through- out the thickness of the leather. Investigations in this field are complicated by the necessity of analyzing the leather a t dif- ferent depths and a t different locations over the area of the skins.

In the early part of this lecture a property was mentioned which greatly affects the wearer of shoes-namely, the tendency for the leather to suffer dimensional changes with the relative humid- ity of the atmosphere. Many people have attributed to their toes the power to foretell changes in the weather, little realizing that the pain was merely an indication that the leather in their shoes was shrinking. In going from a dry to a moist atmosphere, chrome leathers increase in area by an average of about 18 per cent. They shrink in area correspondingly when the relative humidity falls. Vegetable-tanned leather, on the other hand, undergoes changes in area only one-third as great as this. For reasons that had nothing to do with the comfort and happiness of the ultimate consumer, it became desirable for the tanner and shoe manufacturer to have about 95 per cent of all the sole leather vegetable-tanned and about 95 per cent of all the shoe-upper leather chrome-tanned. When this was brought about, no one suspected the differences of shrinkage and expansion of the two kinds of leather. The shoe upper is thinner and tends to reach equilibrium with the air much more quickly than the very thick sole, and so the changing size is much more effective when the upper is chrome-tamed. Nearly everybody wears shoes with chrome-tanned uppers, which are subject to these great size changes with changing atmospheric conditions. For this reason most people suffer unnecessary discomfort. The tanner will use any method of tanning which the consumer demands, but, like most people, is slow to make a change until the demand is urgent. The interesting scientific fact is that the kind of tannage so greatly influences the power of leather to take up water and to change in size.

It has not been possible, in this lecture, to do more than give just a glimpse of leather chemistry as a whole and to show what the leather chemist is doing to make the footsteps of his fellow man a little less weary.

A N f l - A Quantitative Relations of the = [AA'+l - 1 1 where L = weight of solute recovered

W = weight of solute fed N = number of tanks A =

Countercurrent Washing Process

werght liquor transferred as liquid weight liquor transferred adhering to solid Editor of Industrial and Engineering Chemistry:

In the article by Ludwik Silberstein under this title, IND. ENG. CHEM., 20, 899 (1928), the author's mathematical analysis leads to equations which are correct, but the work is needlessly involved. The consideration of limits gone through to find conditions in the final steady state is unnecessary, as these relations can be derived very easily by simple material balances.

Moreover, the author has apparently entirely overlooked certain similarities in the equations he develops for systems of different numbers of tanks, and has therefore failed to discover that a single equation can be written for a system of any number of tanks. This has led him to an enormous amount of work in developing a new equation for each system, as the process is quite laborious for systems of three or more tanks.

Some time ago the writer had occasion to attack this problem, and developed and proved an equation applicable to a system of N tanks.

At the end of his paper the author gives an illustrative example, For the case which will now be solved by the above equation.

where N = 4 tanks, we have

700 375 A = - = 1.868

W = (0.40) (375) = 150 pounds

L = 150 [1.8686 - = 144 pounds recovered 1.86S5 - 1

~

150 - 144 = 6 pounds lost - = (6) ('O0) 1.6 per cent concentration, agreeing with the

375 author's figure.

SMITH D. TURNER HUMBLE OIL & REFINING COMPANY BAYTOWN, TEXAS October 13, 1928