vu1 18, so 9 _ _ _ ~ ~ _ _ _ _ INDUS’IXIAL AND ENGINEEM.VG CIfEWIS’I’KY .__ 934

Tanning 1876-1926’ BY John Arthur Wilson

A. F. GALLUX Br SONS

LTHOUGJI the taniiing inilustry l ias beeii flourishing for many thousands of years, the last fifty years mark A what is practically the beginning of the applic a t’ ion

of scieiice to tannery ~i~~etliwls. The great “father of leather chcmistry,” I’rof. Ji. It. frocter OS England, is still coil- tributing to the advaiicement of the science that owcs so mueii tu his genius and iiiitiativc. Soieiice has not yet changed the general principles of tnmiing wliich liare been evolved through all tlie ages of c~i~ilizatiom, but i t 11 a ~ a y one mystery after another, rcvcaliiig mucli of tiie meelsariism of tlie complex processes and Innking Iwssil)ln fairly rigid control of i~pcrtitioiis that often failed in the liaiiils of artisans of lifelong training. The result for the tanner Bas ireeii less troulile witli much fewer losses, less dependcnoe upon higldg skilled workmen and upon costly materials, iiiorc uniform operation, aiid better leather a t less cost.

Control of Bacterial Action

Tlic avcragc tanner of 1876 knew nothing at all ahout bacteria and coiisequently was bafiied hj- frequent recurrences of damage t u tlic stock in tlie processes preceding tanning. The finished leather might appear t o be pitt,cd, or to have wliat is called a loose or pipy grain, or aliow ttio pattern OS tiie blood vessel system upon the lcatlier surface, or miglit have its important properties affccted adversely in many other ways. Often darnages of this End had their origin in tlie soaking operation, in which the skins are cleansed and softened before liming. The soak vats occasionally became infected with certain kinds of proteolytic bacteria that caused destruction of the skin fibers in various ways and the leather finally obtained was of corresponi~ingly lowered quality. Sometimes severe losses were sustained before it was even suapected that the damage had its origin in the soak vats. In the modern tannery, bacterial nctiun in soaking is kept under control by regulationof temperature, frequent cliirnging of tlie water, and tlie uhe of antiseptics, such as chlorine, guided by bacteriological exaininatioiss of tlie water from the vats. Where the danger of contam- iiation is very great, it is a11 easy matter to reduce the bacterial count of water contaiuing skins from o \ w a million per cubic centimeter to a ricgligiblc quantity, without harming tlic skins, by the application US aboilt :dl to 100 parts per s nil lion of clilurine.

Liming

In liming, tu loosen the hair ami epidcrinia of the nkiiis, the tanner of fifty years ago would simply dump a pack of skins into a vat of water containing more calcium hydroxide tlian would dissolve. Usually tlie lime liquor was one that liad been used several times before arid contaiued accumu- lated calcium salts and protein decomposition products. At intervals of one to two days the pack was transferred to vats of freslm Jiquor, finally ending in liinewater freslily prepared. This procedure required from one to three weeks tu loosen the hair, depending upon the temperature of the water, which was usually not controlled. Since no means was provided for agitating the liquors, they were not kept uniformly saturated with lime and the results were corre- spondingly irregular. The skins also suffered more or less

1 Received May 10, 1D86,

co.. MILWAUEBII, Wlh

daruagc froin the long cuutikct wit11 the liiiie liquors necesiii- k t ed by t.lic crudeness of tlie operatiiin. In she-upper Icatlier this dsmage shows itself in the form of a marred appearance of the grain surftace and it wns tolerated only

ausc the public of 1878 w-as less esrrctiiig in its demands regnriling filiencss of appearance tlian tlre putilic~ of today.

In the modern tannery a cliemist controls the liiniiig pruccss. h i f o r m saturation of the liquurs with calcimii h y h x i d e is guaranteed by tlic use of aii ex system of nieclianical st,irriug sucli as a rotniting paddle wheel s1.t in the rat. Sitice it is desired to hydrolyze tlie keratinow proteins, but not tlie collagen, sodium sulfide is added and it. concentration controlled by chemical auaiysis. Sorlium suffide ciiinbines with lieratins, hut not with collagen, and tlic (xrmlmml formed is mucli nme readily hydrolyzed

alluili than t,he original keratiiis. The action is thus eilcd up and the hair and epidermis loosened before the

it~ts,cked tlic collagen appreciably. Pur each type of skin tile cliemist has found tliat tliere is an optimum temperature of liming at d i ieh the damnge to the collagen is least when the hair loosening is completed. N’itli proper control the liming can be completed in a single day am1 in only one liquor. Not only is tliere a consideral~le saving in time and labor, but the skins stiffer practically no damage and the finished leather is of higher and more uniform quality tlriin when linied by the best methods in vogue in 1876.

Bating

J’erliaps the most wllolesome advame made by scieiice in tanning was the substitution of purified trypsin for the otrnoxiuus dungs used in the ope ra t i~ i called liating. In 1876, after tlie skins had been limed, unliaired, arid washed, it was practically a uiiirersal rule to suak tliem in vats coritaii~ing warm infusious of the dung of dugs or fowls.

Modern Method ot Movine. Sk im from Vat to Vat By ineans 01 a crane end hoirf a P?ck of 400 skins can b e moved trot=

o m vat to Rnotber in lesa tlian a m i n u b Hormerly creh skin was moved sepnrrfely by hand.

This treatment prior to tanning gave a finished product with a rnucb smoother and finer grain surface than could he obtained by any other treatinent then known. The tanner liad no idea as to what constituent of the dung was responsible for tlre observed effects nor as tu tlie mechanism of the reaction taking place in tlie skin. Tlris made it difiicult

to control bhe operation of hating and consequently m!ir!li lenther was spoiled in this process. The difficulties en- countered in hating, or puoring, as it was sometimes called, are vividly pictured in a statement by the pioneer investi- gator of bating, the late Joseph T. Wood, of Nottingharn, in his book:

When learning thc trade as an apprentice cvery fault iu the leathers was attributed to this part of the work, and the troubles and miseries of the “pucr shop” first cviiscd me tu take up the studv 01 nueriur. I was dctcrmined to know the causes under- . . . lying the process. Puering is not only a filthy and disxusting

Experimenlal Tannery-A Part of Modern Chemical I.uborafocy

Wood analyzed the dung lilpi<Jrs, tested tliem for enzyme actiyity, and measured their pH values back in tho days, thirty to fort,?. years ago, when work of this kind was beyond tlie scope of many unix-ersitics. By a process of elimination lie concluded that the essential benefits of hating could be produced by soaking tlie skins in 3 solution of trypsin and huffer salts to give a pH value of about 8. This marked the beginning uf scientific and sanitary hating and the ma- jority of tlie tanneries of t.lie civilized world were very quick t,o adopt Wood’s new method.

In the last few years chemists have learned that the trypsin acts upon the elastin fibers of the skin and more particularly upon the keratose present in the skill and formed from the keratins during liming. The removal of keratose in bating is essential to make leather wit,h 3 very fine and smooth grain surface. Since trypsin act,s also upon collagen, undue prolongation of the bating process will result in damaged goods. ?he modern leather chemist has studied the effect of variation of enzyme activity, pI3 value, ternpera- ture, and time of bating upon the quality of the leather, and has estaldished systems of control that have changed what fifty years ago was a mysterious, treacherous, and disgusting operation int.o 3 ~rocess that is relatively simple, reliable, and clean.

Tanning

Fifty years ago this wits done almost entirely by soaking tlieni in watery cxtracb of barks, roots, nuts, or leaves, t l i e so-called vegetable tanniug process. ?lie active principle, tannin, diffuses slowly into t.lie jelly-like fihers of tlie skinsand com- bines with the collagen molecules, forming the impntrescible suhstanee called leather. Each tanner used to have his own secret combination of different tanning extracts, which often included expensive materials from distant lands. Usually the tanner Eoiind that he could not depart far from his own particular formulas without encountering trouble.

After hating, the skiris are ready to be tanrred.

Even wit11 rigid adlierence to his rules, Ire frequently found dark stains 011 his stock, or the grain surface became so rough as to lower the value of tlie leather very much: or the leather would crack upon beuding, or it was too stiff or too loose. The modern cliernist. has done away with innumerable difficulties that were common years ago and has made it possible to get as good results from cheap and plentiful tanning extracts as were ever obtained from the very ex- pensive ones. I& has sliown that the rate of diffusion of tannin into tlie skin and the rate oi comhination of tannin with ctillrtgoi~ are functionv of pII value, tctnperat.ure, and of conecnt.rations of tannin and nontannid, and he has estab-

1)s of control timt permit effi&irnt and uniform operritiw! even wlieii it is fouid necessary to replace one tanning extract, by anotlter.

111 tlie last fifty years a totally new inetiiod of tanning has cmiie to the fore in wliicli the tanning agent is basic ehro- ioiuin sulfate. ..\ tan liquor ready for use can be made by bubbling sulfur dioxide tlirongii a solution of sodium di- chromat,e until the rednction to chromic salt is complete. Usuiilly the bated skins an? first l~rought into equilibrium with 3 solutioii of sulfuric acid and sodium chloride of definite st.rengt11 and are tlicn drummed in chrome liquor until they beeomo resistant t o tlie action of boiling water. The chro- mium salts penetrate tlie skias rrmch more quickly than do tlic vegetable tannins and tlie combination of chromium and collagen is est.remely st,ahle.

The development of cliroine tanning was almost entirely dependent upon 3 knowledge of clreinist.ry, although i t is only very recenily tliat our ideas as to the mechanism of the rcactions involved have begun to s h a p themselves satisiactorily. In practice the yH value, temperature, and concentrations of cliromium and salts are carefully controlled. The process has proved of enormous value because i t has greatiy shortened the time of tanning and tlie risks involved. 3ow by far the greater portion of all shoe upper leather is made by this proccss. It has recently been shown, liowever, that its arm change with relative humidity is three times DS great as for vegetable-tanned leather and that it also loses its shape more readily. But these are disadvaiitages that concern the ultimate consumer, who must woner or later learn to judge quality from the properties of the leather. Meanwhile, competition forces operations into the least costly channels.

Fat-Liquoring and Coloring

After tanning, light leathers are usually fat-liquored and colored. In fat-liquoring, tho skim are tumbled in a re- volving drum containing a warm emulsion of oil in water. In practice, the oil globules carry a negative electrical charge and the surface of the leather fibers is positively charged with the result that the oil globules rapidly condense upon the leather surface. When the skins are removed from the drum t.iicy contain practically all of tlie oil originally emulsi- fied and tlie water is nearly oil-free. The skins are then hung up to dry and as the water diffuses out from the in- terior of the skiit and evaporates, the oil condensed upon the surface works its way in, lulxicating the fibers throughout and increasing the tensile strength and suppleness of tlie leather.

It happila that the uniformity of distribution of oil in the leather after drying is dependent upon the stability of the oil emulsion used in fat-liquoring. In the days gone by little was known of the factors determining stability, and the fat-liquoring operation lacked uniformity and was the source of much trouble in tlie form of cracky grain or ex- cessive firmnas or looseness. The development of colloid chemistry eleared away many of the mysteries of fat-liquor-

936 IXDUXTRIAL ALVD ENGINEERIiVG CHE.k!ISTRY 5-01. 18, KO. 9

ing of fifty years ago. This process has been made as sure and as simple as any in the tannery by the use of uniformly stable emulsions of scientifically selected oils and by the control of the nature and amount of soluble matter carried into the fat-liquor by the leather through proper regulation of the tan liquors.

I n coloring, the changes have been largely in the substi- tution of aniline dyes for the natural dyestuffs formerly used and in the selection of mordants and control of pH value and temperature. Modern leather shows an advantage over leather of fifty years ago, not only in the variety of shades available, bht also in the clearness, uniformity, and permanence of color. Great strides have also been made in the preparation of finishing or sizing materials and in methods of applying them to the leather.

Scientific Control Fifty years ago tanners had no chemical laboratories,

but today in a large and progressive tannery one may find five or six separate laboratories devoted, not only to routine analyses, but to research dealing with pure physical chemistry, colloidal phenomena, bacteria, enzymes, proteins, tannins, oil emulsions, histology of skin, photomicrography, and to many phases of chemistry not ordinarily taught at the university. These laboratories are continually throwing new light upon the processes, making more rigid the systems of control, and actually developing new methods.

Although fundamentally the processes are similar today to those in use in 1876, the development of scientific control of those processes has resulted in practical advances in leather-making that are nothing short of phenomenal.

Fifty Years in the Petroleum Industry' By Frank A. Howard

STANDARD DEVELOPMENT CO., NEW YORK, N. Y.

T HE first important product made by distilling petro- leum was kerosene. It was quickly found that this needed some purification in order to make it burn

satisfactorily in lamps with wicks. The first refining agents used for purifying kerosene distillates were lime and caustic alkali. These were not satisfactory, however, since kerosene refined in this manner causes a crust to form on the lamp wick and causes clogging. The value of treating kerosene with sulfuric acid followed by a caustic soda wash was acci- dently discovered in 1852 in an apothecary in Galicia, where two assistants named Lukasiewicz and Zeh treated some petroleum distillate in this manner and found that the treated oil burned well in a lamp. Prof. Benjamin Silliman, Jr., BS early as 1855, used sulfuric acid in refining the distillates which he obtained from a sample of Pennsylvania petroleum, and Professor Eichler is said to have introduced into Russia the use of sulfuric acid followed by neutralization with caustic soda. This method of refining kerosene was, there- fore, known fifty years ago and is still in general use today for treating both light distillates and lubricating oils.

Composition of Crude Petroleum

The action of sulfuric acid as used in the purification of petroleum distillates and residual oils is not fully known and must vary considerably with products from crudes of widely different origin and chemical composition. Many different types of chemical compounds are found in crude petroleum and more are produced as secondary products when oil is subjected to cracking processes. I n addition to saturated hydrocarbons, crude petroleums contain aro- matic and naphthenic ring hydrocarbons, both types of which may have side chains attached to the rings. Certain gasoline distillates contain more than 50 per cent of these two types of hydrocarbons and one crude petroleum has been encountered which gave a natural gasoline distillate containing approximately 70 per cent of naphthenes. Dis- tillates from cracking processes contain unsaturated hydro- carbons such as olefins and diolefins. Crude petroleum also contains sulfur compounds, the type and quantity of which vary greatly with the origin of the various crude oils. Mercaptans are common in most crudes, but in addition to these there may be found many other classes of sulfur

1 Received July 28, 1926.

compounds, some of which require special methods for their removal.

Pu'itrogen compounds are found in some crudes and in rare cases may be present to the extent of several per cent.

Oxygen is present in many crudes in the form of organic acids. The most common of these are the so-called naph- thenic acids, which exist in the naphthene base crude found in many parts of the world, and which are highly resistant to the action of sulfuric acid. While there may be some doubt as to the exact constitution of the naphthenic acids and while they vary widely in molecular weight and structure, they contain one or more carboxyl groups and can, therefore, form soaps with alkalies.

Refining Agents

SULFURIC ACID-The foregoing brief statement of some of the types of compounds in crude petroleum will serve to show how much work is expected of sulfuric acid when it is used as a general refining agent. Sulfuric acid acts as a purifying agent of petroleum products in the capacity of a solvent of certain impurities such as tars and by direct chemical action on different groups of compounds, among which are the olefins and aromatics. It also acts as an oxidant on some petroleum compounds, as evidenced by the sulfur dioxide which is evolved during the treating proc- ess. One of the main functions of the acid is to remove sulfur compounds from petroleum, Although new methods of treating petroleum distillates have been introduced during the last fifty years, there is no doubt that sulfuric acid is still the most important reagent in petroleum refining.

Certain crudes, such as the Lima of Ohio and some found in Canada, contain sulfur in such a combination that sulfuric acid is not effective for purification. For the treatment of these crudes Herman Frasch invented the Frasch process for the desulfurization of kerosene. This process consists in distilling the kerosene with large quantities of copper oxide. This procedure desulfurizes the oil from 0.75 to 0.02 per cent and removes the skunk odor which makes such oil unsalable. The introduction of the Frasch process increased the price of Lima crude from 14 cents to $1.00 per barrel and made it possible to produce kerosene of a Pennsylvania grade from this bad crude.

DOCTOR SoLuTIox-One of the most widely used refining