Animal Glues

8/18/2019 Animal Glues

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Abstract

Collagen-based animal glues are widely used inthe conservation of artefacts, serving as adhesives,binders and consolidants for organic and inorganicmaterials. With a variety of different animal glues onthe market, such as hide and bone glues, fish glues,isinglass and gelatin, their individual propertiesneed to be well understood in order to choose a glue fit for a specific purpose. This paper reviews a wide range of publications on currently available animal glues, with respect to their specific physical,

chemical and mechanical properties.

Introduction

Animal glues are natural polymers derived frommammalian or fish collagen – the major structuralprotein constituent of skins, connective tissue, cartilageand bones. These glues may exhibit varied physical,chemical and mechanical properties depending on theirorigin and method of preparation. In the manufacture ofobjects and artefacts, an extensive traditional knowledgeexists on which animal glues are most suitable forspecific purposes. However, conservators sometimes

lack the confidence to make informed choices betweenthe different collagen-based glues available whenconserving objects.

The selection and preparation of glues are discussed inpatent descriptions, woodworking and artists’ manuals,as well as conservation literature and product detailsfrom suppliers [1, 2]. There is also a large amount oftechnical research on the properties of collagen andgelatin published in scientific journals on polymer-and bio-technology, medical science and the food andbrewing industry. However, much of this literatureis not readily accessible to conservators and it can

be ambiguous or contradictory. This paper seeks toprovide a review of the literature and to identify whichproperties of glue need to be considered when makingdecisions about conservation treatments.

The applications of collagen-based glue in theconservation field are diverse, ranging from its use as anadhesive, consolidant or binding medium for pigmentsand filler particles [3–8]. Generally, the following keyproperties need to be considered:

• chemical structure and denaturation of the proteinmolecules.

• gelling properties: gelling temperature (Tgel), gel

strength and setting times.

• properties of the glue solution: viscosity, surfacetension and pH.

• properties of the dried film: cohesion, adhesionand final bond strength, mechanical behaviour

in changing ambient environment, and ageingcharacteristics.

Types of commercially available animal glue

Hide glues are primarily derived from bovine skinsand those of smaller mammals, although connectivetissue may also be used. Bone glues are predominantlyprepared from fresh (‘green’) bones or sometimesextracted bones (degreased and demineralised, knownas ossein) from cattle and pigs. Hide and bone glues areproduced and sold as coarse powders, pearls, cubes,and cakes or plates, though the latter two appear to be

increasingly rare [9]. Commercial gelatin, the purifiedactive ingredient of any collagen-derived glue (puredenatured collagen), may be obtained from either skinor bone sources [9, 10] and is supplied in the form ofthin sheets, plates or powder.

As the name suggests, rabbit skin glues should beproduced purely from rabbit skins [10, 11], thoughcollagenous waste from various small mammals mayalso be used [12]. Some suppliers sell rabbit skin gluethat is mixed with bovine hide glue to alter its properties[13]. The information on the source, pre-treatment,or additives provided by suppliers may not always

be reliable, as they may not have been given accurateinformation by the manufacturers. It is generallyassumed that most animal glues contain preservativesof some kind (e.g. sulphur dioxide) [9, 14]. Even rabbitskin compressed into cubes, a by-product from the feltindustry commercially sold as a raw (and thus usuallythought to be a pure) form of rabbit skin glue [10], hasrecently been found to contain preservatives [9]. Sometraditional glues, such as the deer glue used in Japan asa binder for some inks, are now made from bovine orporcine gelatin manufactured to match the properties ofthe traditional genuine material [15].

The skins of non-oily types of fish [16, 17], as well astheir bones [10, 12, 18, 19], are used to manufacture fishglues which are sold in liquid form. The swim bladdersof various species are the source for isinglass [20–24],which is available either in the form of complete driedbladders or membranes, thin plates or fine strips.In recent years, fish skin and bone gelatin has alsobecome available in the food industry as a substitute formammalian gelatin [25–27].

A number of industrially manufactured cold liquidanimal glues are available that have modified propertiesand a long shelf life. These glues usually containadditives that alter their natural behaviour, extending

the working time at room temperature, or decreasingthe propensity for biodeterioration and reducing thedried film’s sensitivity to moisture. However, the exactcomposition of industrially tailored collagen-derivedglues and their overall performance may be difficultto judge, as manufacturers tend to keep their recipes

Animal glues: a review of their key properties relevant to conservation

Nanke C. Schellmann


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REVIEWS IN CONSERVATION NUMBER 8 2007

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secret. Furthermore, conservation requirements suchas long-term stability and resolubility are unlikely tobe a priority for commercial manufacturers. Given therange of additives that may be present in industrial glueformulations, minimally modified glues represent thesafest option for conservation.

Chemical structure and properties

Chemical structure and denaturation

Collagen consists of long protein molecules composedof naturally occurring amino acids that are linked in aspecific sequence by covalent peptide bonds. Due tothe spatial conformation of some amino acid groups(notably proline and hydroxyproline) and the manyionisable and polar functional groups in the proteinchain, the individual chains form triple-stranded helicalcoils that are generally believed to be internally stabilisedby hydrogen-bonding [7, 25, 29–33].

Collagen is insoluble in cold water [30, 34] and istransformed into soluble gelatin by denaturation, aprocess of critical importance for the performanceof the resulting glue. This is achieved by hot waterextraction (hydrolytic breakdown) [9, 34–39]. Pre-treatment (either acidic or basic) is necessary formost skin and bone collagen, but is not required forthe extraction of isinglass from fish bladders, whichcontain less cross-linkage within the collagen. Duringextraction, the bonds (predominantly H-bonds) in thetriple-helix structures of the collagen are broken sothat it separates into disordered ‘random’ coils of single

protein chains, thus completing the transition to gelatin[40]; in perfect conditions gelatin is pure denaturedcollagen. The temperature (Td) at which denaturationoccurs is dependent on the chemical structure of theproteins in the particular collagen source, notably onthe content of the amino acid derivatives proline (Pro)and hydroxyproline (Hyp). These are supposed to belargely responsible for the stabilising H-bonded waterbridges in the triple helix [41] and are present moreabundantly in mammalian collagen than in marinespecies [42]. Thus, adult mammalian collagen denaturesat 40–41°C [31, 33], while isinglass and other fishcollagens denature at lower temperatures. The Td offish collagens ranges from approximately 15°C for deepcold water fish (such as cod used for fish glue) [10, 43]up to 29°C for most warm water species [31, 33, 44],which are the preferred source of isinglass produced forcommercial clarification of alcoholic beverages. Thereare also a few tropical fish species that reach Td levels ofup to 36°C [31, 44].

The process of denaturation is necessary for collagento convert to gelatin, which can be used as a glue.Cleavage of the single protein molecule may also occurduring pre-treatment, extraction and dissolution, andwill significantly affect the properties of the gelatinous

glue. The more vigorous the extraction process (i.e.the more extreme the pH, the longer the treatmentand the higher the temperature during extraction), themore bonds within the protein molecule are randomlycleaved, leading to ever decreasing molecular weights[34, 37, 39, 45]. Mild extraction at moderate pH and

low temperature yields gelatinous matrices containingprotein fractions of long chain length and highmolecular weight (MW) [38, 46]. As a general rule,gentle processing is appropriate for the hides of youngmammals, as well as all fish skin and swim bladders,because they are rich in collagen and the collagen is not

so strongly stabilised by the additional chemical bondsthat develop in older mammals. Furthermore, glues thatare derived from fish cleave more easily on extensiveheating than those of mammalian origin owing to theirchemical structure [40, 46]. Conservators should thusbe aware that when preparing a collagen-based solution,mild procedures should be employed [4, 46]. Preparationtemperatures for collagen-based glues are generallyrecommended to be around 55–63°C. However, there islittle loss of gel strength on heating at high temperatures(e.g. 80–90°C), even in the case of isinglass, but only ifthe solution is kept at these temperatures for no morethan a few minutes [46, 47].

Gelation and gelling temperature (T gel )

Although the process of denaturation, with the loss ofthe triple helix arrangement of the protein molecules,is irreversible, some helical structure can be restoredduring gelling and drying. On gelling the singlerandom protein coils undergo partial rearrangement(renaturation) back into collagen-like triple helices [7,26, 46, 48-50]. However, the misalignment of the singlestrands means that renaturation causes nodes (‘junctionzones’) involving only part of certain strands. Theremainder of these strands may form further nodes so

that a continuous three-dimensional network structureemerges. The degree of renaturation is dependent onthe chemical composition (Pro and Hyp content), thechain length of the molecules (molecular weight, MW),concentration in solution and temperature [42, 49, 51].High Pro and Hyp content, high MW, high solutionconcentrations and slow drying at a low temperaturepromote a high degree of renaturation and thedevelopment of a highly ordered network structure [34,37, 48, 52]. The number of nodes that are establishedby the formation of H-bonds (and probably also byelectrostatic interaction [42]) within and between themolecules determines gel strength and the rigidity and

elasticity of the glue matrix [7, 46, 51].The ability to form a rigid gel on cooling, which can berepeatedly reliquefied by reheating, is one of the uniqueproperties of collagen-based glues. The temperature atwhich gelation of the glue solution occurs (Tgel) dependsmainly on the collagen source, but is also affected bythe degree of protein cleavage. Gelation temperaturesdecrease with lower denaturation temperature (Td) and also with increasing cleavage of the molecules.Mammalian gelatin gels at around 30–35ºC, and coldwater fish gelatin remains liquid down to around 8ºC[14, 43, 53]. However, this temperature will be loweredif the preparation temperature of the glue is significantlyexceeded.

Gel strength

Gel strength is a measure of the gel rigidity of gelatinousglues, and is strongly influenced by the molecular weight


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of the constituent proteins [34, 54]. According toseveral authors [35, 39], the average molecular weight(AMW) of animal glues can range from around 20000 to250000 g.mol-1. It is thought that permanent gellingdoes not take place below an AMW of 20000 g.mol-1 [38, p. 43]. Isinglass from sturgeon, if prepared under

mild conditions, reaches average molecular weight valuesof well over 150000 g.mol-1 [4, 33, 46], while liquid fishglue has AMW values of around 60000 g.mol -1 [10, 14],placing it at the lower end of the range. For most othercommercial collagen-based adhesives, information on AMW is not readily available.

Characterisation by AMW is only common for fishglues, which are liquid at room temperature. Mostother gelatinous glues are usually characterised by theirgel strength, as AMW does not describe the molecularweight distribution and therefore may not alwayscorrelate reliably with the physical and mechanical

properties of a glue [34, p. 60] (Table 1). However, itwould be expected that high AMW adhesives, such asskin glues, have higher gel strength and viscosity, gelmore rapidly and produce stronger bonds.

Gel strength is strongly influenced by AMW but alsoshows a linear correlation with the degree to whichthe protein solution renatures during gelation [55],i.e. the higher the degree of formation of helicalstructures, the higher the gel strength. The presence ofsalts also influences gel strength, which decreases withan increasing concentration of ions in solution [42,56].

Gel strength, also known as Bloom strength, is measuredin grams (g), or Bloom grams (gB), and equals the forcerequired to make a specified depression into a gel sampleprepared under standard conditions [25, 35, 37, 39].Manufacturers commonly distinguish between gradesof glue by their Bloom strength, which usually coversa wide range, being as low as 30 g for weak bone gluesand rigorously extracted hide glues, and up to around500 g for very strong hide glue [10, 11, 35, 37, 57].Gelatins derived from tropical fish have significantlylower Bloom values than mammalian gelatins [58],since the degree of stabilisation of the triple helix byH-bonding is lower. Gelatins extracted from cold water

fish do not have specified gel strengths as they are liquidat room temperature [42].

As gel strength is dependent on the structuralconformation of the gelatinous matrix, it is usefulfor estimating the toughness, strength and resilienceof the resulting bond. Furthermore, Bloom strengthalso correlates with the water-sorption capacity of theglue (in gel and solid state), viscosity (at least to somedegree), and gelling temperature (Tgel), which generallyall increase with rising Bloom value. High Bloom gluesrequire a lower solid content in solution than glues witha lower Bloom rating to be effective as an adhesive, as

they offer many sites for intermolecular bonding in agiven volume [35, 56]. Mammalian skin glues are usuallyconsidered to have the highest AMW and produce thestrongest gels and films [10], particularly those extractedby acid pre-treatment. Generally, acid pre-treated glues(type A gelatins) contain larger fractions of high MW

than alkaline pre-treated collagen derivatives (type Bgelatins), whose MW distribution is skewed towardslower MW fractions [9, 34, 46].

Open (gelling) time, tack and drying

The setting time of animal glues depends primarily onTgel and gel strength. The lower the Tgel and gel strength,the longer the open time of the solution (i.e. the longerit takes for the glue to gel). High Bloom hot hide gluestend to gel rapidly, as gelation occurs at comparativelyhigh temperatures [10, 11, 14, 39, 59]. Gelatinousglues derived from fish, which have low Tgel due totheir chemical structure [42, 43, 58], and cold-set liquidhide glues are convenient to use when long open timesare required. Commercial fish glues usually containpreservatives [60] and, sometimes, small amounts ofother additives such as colour brightener, deodorizingagents or fragrance [10]. Liquid hide glues generally

have further additives to inhibit gelation at roomtemperature [17, 28]. These are typically salts (e.g.urea, thiourea) or phenols that extend the setting timeby inhibiting renaturation of the gelatinous matrix [28,52]. Some manufacturers claim that their liquid hideglue does not contain gelling inhibitors [17], in whichcase the gelatinous matrix must be considerably affectedby molecular cleavage to achieve the comparativelylow MW that is necessary for the glue to be in a liquidstate.

The ability of collagen-based glue to develop tack upongelation is a unique property. In general, glues of higherBloom strength develop tack faster than lower Bloomglues. The tack ‘strength’ of glue can be empiricallytested by conservators between two fingertips. Isinglasssolutions may appear to be less tacky than equivalentconcentrations of mammalian gelatin or hide glue, asthey take longer to set at room temperature, since theirlower gelation temperature delays the development oftack.

Drying time generally depends on the ambienttemperature and relative humidity (RH). After gelation,the glue matrix dries by evaporation of water and thisprocess can be accelerated by elevating the temperature.However, collagen-based adhesives should be allowed to

dry as slowly as possible, as a longer period of molecularmobility after gelation and during drying encourages thedevelopment of highly ordered network structures [52].This maximises the elasticity and strength (toughness) ofthe resulting glue film. Isinglass naturally develops highlystable and elastic films if dried at room temperature,being slightly above its Tgel [9].

Properties of gelatinous glue solutions

Viscosity

The viscosity of the glue solution is primarily dependent

on the molecular weight distribution [51]; the greaterthe proportion of molecules of higher MW the higherthe viscosity [2, 35]. For a given MW distribution,the viscosity increases with increasing solutionconcentration and decreasing temperature [39, 51, 61].The degree to which collagen-like helices [62, p. 128]


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T a b l e 1

C o m p a r i s o n o f t h e p r o p e r t i e s o f d i f f e r e n t g l u e t y p e s . T h e g l u e s a r e q u a l i t a t i v e l y r a n k e d r e l a t i v e t o o n e a n o t h e r f o r e a c h p r o p e r t y , i . e . w i t h i n e a c h i n d i v i d u a l c o l u m n . N u

m e r i c a l d a t a i s o n l y

r e f e r r e d t o i n t h o

s e c a s e s w h e r e i n f o r m a t i o n w a s c o n s i s t e n

t i n t h e l i t e r a t u r e

P R O P E R T Y

m o l e c u l a r w e i g h t ( M W )

[ R e f . ]

g e

l / B l o o m s t r e n g t h [ g B ]

[ R e f . ]

d e g

r e e o f h e l i c i t y

[ R e f . ]

v i s c o s i t y

[ R e f . ]

p H ( a p p r o x i m a t e v a l u e s )

[ R e f . ]

F a c t o r s

i n f l u e n c i n g

p r o p e r t y

d e c r e a s e s w i t h r i g o r o u s p r e t r e a t m e n t

a n d w i t h e x c e s s i v e / p r o l o n g e d h e a t i n g

i n

c r e a s e s w i t h h i g h e r M W a n d

i n

c r e a s i n g h e l i c i t y

i n c r e a s e s w i t h h i g h e r M W ,

h i g h e r P r o a n d H y p c o n t e n t

a n d

i n c r e a s i n g s o l u t i o n

c o n

c e n t r a t i o n

i n c r e a s e s w i t h i n c r e a s i n g B l o o m ,

d e p e n d e n t o n i s o e l e c t r i c p o i n t

( p I ) a n d p H

i n f l u e n c e s t h e v i s c

o s i t y

G L U E T Y P E S

b o n e g l u e

l o w t o m e d i u m

l o

w t o m e d i u m ( d o w n t o

5 0 g B )

[ 9 – 1 1 ,

1 4 , 3 5 ,

3 7 , 5 7 ,

6 9 ]

l o w

t o m e d i u m

l o w t o m e d i u m ( m i n .

v i s c o s i t y a r o

u n d p H

4 . 5 – 5 . 5 )

[ 1 0 , 1 4 ,

6 9 ]

5 – 7

[ 5 , 1 0 ,

3 5 , 3 7 ,

3 9 ]

h i d e g l u e

h i g h

[ 1 0 ]

h i g h ( u p t o 5 0 0 g B , h i d e

g l u e p e a r l s p r o d u c e l o w e r

B l o o m v a l u e s t h a n h i d e

g l u e g r a i n s )

[ 9 – 1 1 ,

1 4 , 3 5 ,

3 7 , 5 7 ,

6 9 ]

m e d i u m t o v e r y

h i g h

m e d i u m t o h i g h ( m i n .

v i s c o s i t y o f

a l k a l i n e

p r e t r e a t e d g

l u e a t

a r o u n d p H

4 . 5 – 5 . 5 ,

a n d o f o f a c

i d

p r e t r e a t e d g

l u e a t p H

7 . 0 – 9 . 0 )

[ 1 0 , 1 4 ,

1 7 , 3 0 ,

3 4 , 3 5 ,

5 1 , 5 6 ,

7 5 ]

6 . 5 – 7 . 4 ( w i d e r v a r i a t i o n s

a r e p o s s i b l e )

[ 9 – 1 1 ,

1 6 , 3 5 ,

6 8 ]

r a b b i t s k i n g l u e h i g h

[ 1 0 ]

h i g h ( u p t o 5 0 0 g B )

[ 9 , 1 0 ,

1 4 ]

h i g h t o v e r y h i g h

h i g h ( m i n . v

i s c o s i t y a t

a r o u n d p H

7 . 0 – 9 . 0 )

[ 9 , 1 0 ,

1 4 ]

5 . 0 – 7 . 5 ( w i d e r v a r i a t i o n s

a r e p o s s i b l e )

[ 9 – 1 1 ,

6 1 ]

m a m m a l i a n

g e l a t i n

1 1 0 0 0 0 – 1 6 8 0 0 0 ( t y p e A

g e l a t i n a c h i e v e s h i g h e r

v a l u e s t h a n t y p e B

g e l a t i n )

[ 9 , 2 6 ,

3 4 , 4 5 ,

4 6 , 5 4 ]

m

e d i u m t o h i g h ( b u t c a n

b e p r o d u c e d t o a c h i e v e

B l o o m v a l u e s a s l o w a s

7 5 g B )

m e d i u m t o h i g h


( t y p e B g e l a

t i n

c o m p a r a t i v e l y m o r e

v i s c o u s t h a n

t y p e A

g e l a t i n )

[ 5 6 , 6 1 ]

5 . 0 – 6 . 5

[ 1 0 , 6 1 ]

i s i n g l a s s ( f r o m

f i s h s w i m

b l a d d e r s )

c . 1 5 0 0 0 0 a n d h i g h e r u p

t o 3 0 0 0 0 0

[ 4 , 3 3 ,

4 6 ]

m

e d i u m t o h i g h


h i g h e s t

[ 4 , 2 2 ,

4 6 , 6 5 ]

6 . 0 – 7 . 5

[ 1 9 , 6 1 ,

7 1 , 7 9 ]

f i s h g e l a t i n

( f r o m f i s h

s k i n , b o n e a n d

c a r t i l a g e )

9 6 0 0 0 – 1 9 6 0 0 0

[ 2 6 , 4 5 ,

5 4 ]

l o

w t o m e d i u m

[ 5 8 ]

m e d i u m

m e d i u m t o h i g h ( m i n .

v i s c o s i t y b e t w e e n p H

7 – 9 )

[ 4 3 , 5 4 ]

3 . 5 – 5 . 0

[ 1 0 , 4 3 ,

5 4 , 5 8 ]

l i q u i d f i s h g l u e

6 0 0 0 0

[ 1 4 , 5 7 ]

–

l o w

t o m e d i u m

h i g h ( 4 0 0 0 –

6 0 0 0 m P a . s

a t

m a n u f a c t u r e d

c o n c e n t r a t i o n )

[ 1 0 , 1 4 ,

5 3 ]

4 . 0 – 6 . 0 ( h i g h e r p H v a l u e s

m a y b e p o s s i b l e )

[ 1 0 , 1 4 ]

c o l d l i q u i d

h i d e g l u e

n . a .

–

m e d i u m

h i g h ( 4 0 0 0 m P a . s

a t m a n u f a c t

u r e d

c o n c e n t r a t i o n )

[ 1 0 ]

6 . 5

[ 1 0 ]

n . a . d a t a n o t a v a i l a b l e


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and intermolecular bonds have developed within thenetwork (gel/Bloom strength) further contributes tohigher viscosity [63, 64]. Strongly denatured gelatinoussolutions (such as bone glues) or those affected by ahigh degree of molecular cleavage will normally have acomparatively low viscosity. At a given Bloom strength,

alkaline pre-treated (Type B) gelatins are generally moreviscous than acid pre-treated (Type A) gelatins [56](Table 1).

Viscosity is an important factor in the choice of adhesivefor bonding or consolidation, as it will affect the degreeof penetration into a substrate. If the viscosity is toolow the glue may penetrate too far into the substrate,leaving a joint starved of adhesive. For consolidation ofporous materials, high viscosity may prevent adequatepenetration and cause stress to develop at the interfacebetween consolidated and unconsolidated areas.Unfortunately, the viscosity values for animal glues givenin the literature and by suppliers vary widely and arenot easily compared. Measurements were often takenunder different experimental conditions and at differentdegrees of cleavage in the protein molecules [4, 21, 35,37, 46].

Isinglass has a much higher viscosity than hide glue atan equivalent solution concentration and temperature(above Tgel), which can be explained by its comparativelyhigh proportion of high molecular weight fractions(which, in the following paragraphs, will be referred toas high Molecular Weight Distribution (MWD)) [4, 22,46, 65]. This is contrary to what is often stated in theliterature and to the traditional beliefs about the handling

properties of isinglass [20, 21, 61]. However, where lowviscosity values have been obtained for isinglass, it islikely that the particular preparation procedure of theglue used for the tests resulted in greater cleavage ofthe protein molecules [46]. Despite isinglass having alarge fraction of high MW compounds, its low gellingtemperature compensates for this by allowing moretime for the glue to penetrate porous substrates at roomtemperature, therefore improving its penetration abilityin comparison to gelatin and rabbit skin glue of similarhigh MW fractions, which will gel faster [8, 66].

In order to obtain glue solutions of low viscosity, it is

not always advisable to dilute viscous high Bloom gluesexcessively. The use of an over-diluted glue may resultin swelling, leaching or staining of the substrate if it iswater sensitive [67]. In such cases, a glue with a lowergel strength would be preferable.

Surface tension

Slow gelation and lower viscosity promote uniform filmformation as the glue is able to spread evenly, providingadequate wetting of the surface. Wetting is improvedwith a decrease in the surface tension of the glue solution.Sauer and Aldinger [68] confirm that a decrease in

surface tension of a gelatinous solution is directly linkedto the presence of fats. Free fatty acids and neutralfats are regarded as particularly effective in reducingsurface tension even in small concentrations. With theexception of rabbit skin glue (which has comparativelyhigh fat levels of around 5% [9, 11]), most animal glues

and gelatins contain less than 1% fat because of modernmanufacturing methods [9, 10, 54, 58, 69] and mayrequire additives to reduce the surface tension.

Ethanol is commonly added to lower the surface tensionand improve the wetting abilities of collagen-based glues[21, 70, 71]. In one case beer containing 9% alcohol

was added to fish glue that was used in the conservationof Boulle-marquetry, and was shown to improve thewetting properties leading to stronger joints betweenthe wood and brass components [70]. However, alcoholmay also raise the gelling temperature, speeding up thegelation and decreasing the time for which the glueis workable [28, p. 102, 110], and may also promoteswelling of the substrate. Alternatively, surfactants canbe added to lower the surface tension [3, 8, 28, 72,p. 123].

Animal glues can have an undesirable tendency to foam,developing small air bubbles in the glue matrix which

can disrupt the uniformity of the dried glue film andweaken bonds [5, 59]. Natural fats or free fatty acidspresent in glues play a vital role in reducing foaming [5,68, 73], although some authors still express some doubtthat there is a direct correlation between fat contentand tendency to foam [9]. Nevertheless, Skans [73,p. 66] suggested that a natural fat content of above 5%would inhibit the development of pinholes in gesso forgilding. Sauer and Aldinger [68] have demonstrated anunambiguous dependency of the degree of foaming onfat content, whereas no direct relationship could beestablished with surface tension. They also could notfind any influence of protein degradation products

on foaming, while pH was established to have aninconsistent effect.

pH

For conservation applications, the choice of adhesivemay be dependent on the pH sensitivity of the substrate[71]. Collagen-based glues can display varying pHvalues that are difficult to predict purely on the basisof the glue type or treatment during manufacture. Theassumption that glues which undergo alkaline pre-treatment display a slightly alkaline pH and acid-treatedones have an acidic pH [39, p. 171] is incorrect. It isstated in the literature that hide and fish glue solutionsoften have a fairly neutral pH in the range of 6.5 to7.4, although wider variations are possible [9–11, 16,35, 68]. In general, bone glues tend to be slightly moreacidic [5, 10, 39], with pH levels between 5 and justbelow 7 [35, 37]. Pure gelatins from mammals and fishrange between pH 5.0–6.5 and 3.5–5.0 respectively [10,53, 54, 58, 61]. Isinglass yields solutions with a pH inthe neutral range [19, 61, 71]. Conservators should testthe pH value of the chosen glue before use if sensitivityof the substrate is of potential concern.

Apart from being a relevant aspect to consider in

conjunction with the sensitivity of the substrate, pHvalues also have an influence on the properties of theglue, as the viscosity increases when the pH of thesolution shifts away from its isoelectric point (pI) [1, 37,61, 74]. Since proteins and amino acids are amphotericin nature (i.e. containing both acidic and basic functional


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groups), they have an isoelectric point, which is thepH at which all positive and negative charges withinthe molecule are balanced and the molecule carries nonet electrical charge. If the electrical potential of theions is unbalanced, solution viscosity and Tgel increase,as well as the capacity for water-sorption and swelling

ability, while gel strength decreases [9, 46, 52, 61, 62].Commercial animal glues extracted by alkaline pre-treatment (most hide and bone glues, type B gelatins)usually have a pI of approximately 4.5 to 5.5, whereasglues derived from acid pre-treated collagen sourcescommonly display pI values of between 7.0 to 9.0 [17,30, 34, 35, 51, 56, 75]. For practical purposes, thismeans that glues having a pH near their pI value (suchas bone glues and type B gelatins) will already be at thelowest possible viscosity, as opposed to those whichhave pH values different from their pI, where to achievethe lowest possible viscosity the pH would have to bemodified to take it closer to the pI (Table 1). The effect

of the pH of a glue solution on its surface tension isinconsistent [28, p. 75, 68].

Mechanical properties of the dried film

Cohesion, adhesion and bond strength

The cohesive strength of the gelatinous matrix of a glue isdetermined by its molecular structure and intermolecularbonding, as expressed by the Bloom value. To produce ananimal glue film that is as strong as possible in the driedstate, the same rules apply as for obtaining a high gelstrength (i.e. high MW distribution/minimum cleavage

of protein molecules, maximum renaturation/content ofcollagen-like triple-helices, high intra-/intermolecularstabilisation). The cohesion strength of animal glues canbe improved by the addition of a suitable amount of analcohol, such as ethanol or glycerine [28, p. 108]. Toachieve strong bonding, chemical adhesion between theglue and the substrate is as important as high cohesionwithin the glue matrix.

Hide glues generally have greater cohesive strength thanthe bone glues with highly cleaved molecules, whichdisplay a lower tensile strength and are much morebrittle (Table 1). The tensile strength of hide glues istypically around 39 megapascals (MPa) (5700 poundsper square inch, psi) [76]. Mammalian collagen tendsto yield stronger glues than most aquatic sources, owingto the reduced number of stabilising inter- and intra-molecular bonds in fish collagen [33, 49]. Cold waterfish gelatins in particular have a lower propensity toreform helical structures due to their small proportion ofthe amino acid derivatives, Hyp and Pro, and thereforeshow a comparatively low tensile strength of around 22MPa (3200 psi) [26, 27, 53]. This value is comparableto the strength of bovine bone gelatin [54].

A high tensile strength similar to that of hide gluehas been reported for mildly prepared isinglass from

sturgeon [4], making it a useful adhesive for bondingwooden joints. The literature confirms that isinglasshas often been used for structural woodwork in the FarEast [24, 77]. Although rabbit skin glue has a high gelstrength, it has been stated as having lower cohesionand bonding strength than other hide glues [23, 39,

78]. This is thought to be due to its high fat content[9, 23].

Elasticity, resistance to impact (toughness) and creep

As for many of the other physical properties of gelatin-based glue films, the elasticity and stiffness are greatlydependent on their MW distribution [63], the degreeto which helical structures reform on gelling and theintra-/intermolecular bonding [7, 26]. The stiffness ofthe glue (elastic modulus, known as Young’s modulus E, mathematically calculated from the ratio of stress tostrain values) increases with a higher ratio of high MWfractions, higher solution concentrations and with agreater renaturation level in the network [26, 45, 55].Stabilisation of the gel network by increased electrostaticbonding induced by pH levels above or below the pIalso increases the stiffness [42]. Mammalian gelatingenerally has a higher modulus and therefore greaterstiffness than fish gelatin due to its higher networkstabilisation by intra- and intermolecular bonding [26,58]. Isinglass is also more elastic than mammaliangelatin [79].

The moisture content of an animal glue has an importanteffect on the mechanical properties. Under normalambient conditions (50% RH and room temperature)gelatin-based glue films contain 12–14% of structuralwater bound to the polar groups of the proteinmacromolecules [52, p. 654]. This water contributes tothe stabilisation of the helical structures within the glueand a specific amount of water is needed to maintainstructural stability. Above around 25% moisture content

the glue turns from a glassy to a rubbery state at roomtemperature [52]. Excessive dehydration of gelatinousfilms below a moisture content of 0.2% leads to thedevelopment of covalent cross-links between the proteinmolecules, which ultimately renders the glue insolublein water [80, p. 509].

In general, gelatinous glue films with a low moisturecontent are very brittle regardless of the collagensource and molecular structure [34, p. 63, 52]. Evenat a normal (12–14%) water content, gelatinous filmsundergo brittle fracture under impact. Randomly coiledstructures exhibit much lower resistance to impact

(greater brittleness) than helical glue matrices in the glassystate. Glue recipes often contain additives such as sugaralcohols (e.g. glycerine, sorbitol) and polysaccharides(e.g. dextrins) to improve elasticity and toughness[28, 34, 81, 91]. One traditional recommendation forachieving elastic and resilient glue films is the additionof honey [4, 18, 21, 22, 61, 82]. Sugars are hygroscopicand so stabilise the protein molecules by introducingadditional hydrogen bonds involving water [25, 83],inducing an increase in gel strength and viscosity. Although these additives do not actually plasticise theglue matrix, they are often referred to as plasticisers inthe literature. A high proportion of fat also improves

elasticity, although it simultaneously reduces the gelstrength of the glue and final bond strength [23, 84]. A higher water content or an excess of hygroscopicadditives generates a reduction in the glass transitiontemperature of the glue [61, 81], which can promote anunwanted tendency to creep (elongation with time).


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Stability in ambient environment and sensitivity to fluctuating levels of moisture and heat

Drying of collagen-derived glue films leads to thedevelopment of high internal stress and tensile forceswithin the glue matrix, while increasing humiditygenerally causes progressive loss of tension [4, 85]. This

behaviour is dependent on the physical and chemicalstructure of the glue. A high degree of collagen-liketriple-helix arrangement in a gelatin film has beenshown to result in a reduced tendency to swell [34, 55],but is also responsible for increasing stress values dueto stronger cohesion. Isinglass from sturgeon, whichcontains a high proportion of helical structures (due toits high MWD, despite its lower Hyp and Pro content),develops particularly high stress levels, which it issuggested are twice as high as in hide glue [4].

If kept under moderate relative humidity conditions overa long period of time, initial stresses within gelatinous

films relax owing to the absence of covalent cross-links[86]. Under fluctuating environmental conditions, themechanical properties of collagen-based glues are subjectto continual change [85]. Considerable development ofinternal stresses will affect the glue’s elasticity, strengthand physical stability and may lead to significant damageto the substrates [48, 85–87].

At high RH levels (above 85%) animal glue films undergoa continuing reformation of helical structures. Thiswill result in new, higher stresses on subsequent dryingand can lead to severe shrinkage due to contraction ofthe glue matrix [48, 85, 86]. Cycling of RH can causefurther strain – for rabbit skin glue, non-permanent total

dimensional changes of up to 6% have been reportedas the result of a single RH cycle [76, 86], which areonly partly recoverable. According to Zumbühl [48],contraction mechanisms compete with plastic relaxationprocesses above 65% RH. However, even at high RHlevels plastic relaxation may not sufficiently compensatefor these stresses and continuous cycling further reducesthe ability for stress relaxation [48]. This will result inpermanent shrinkage of the glue matrix (up to 5% forrabbit skin glue [76]) and, in this case, loss of tension isonly possible by substrate deformation or mechanicaldestruction (embrittlement) of the glue film [48].

At low RH levels, more randomly coiled gelatinousstructures (such as bone glues), which have comparativelylow tensile strength and low resistance to stress, inducerelaxation at an early stage by developing cracks in theglue matrix, thereby preventing high stresses on thesubstrate. These glues also show a greater tendency tocreep under stress at high RH levels [61, p. 14]. Althoughanimal glues containing a high degree of helical structureexhibit comparatively high stress when exposed toextreme and fluctuating environmental conditions, theystill display greater stability in their strength propertiesthan more randomly coiled structures. The strengthproperties of hot hide glue have been shown to be less

sensitive to fluctuating RH and temperature than thoseof cold liquid hide glue [6, 28]. Liquid fish glues are evenless stable than cold liquid hide glues under fluctuatingconditions [17]. It has also been suggested that a high fatcontent, such as in rabbit skin glue, accounts for betterstability in moist conditions [5, 39]. Common methods

for improving the glue film’s hardness and resistanceto water are the addition of tanning agents, such asaluminium trisulphate (alum), disodium triborate(borax), sodium acetate or formaldehyde [9, 28, 35, 91].These salts remove a certain amount of bound waterfrom the proteinaceous matrix by covalently bonding

to the hydrophilic sites in the glue, thus inducing theformation of numerous new cross-links between theprotein molecules.

Mechanical properties of animal glues used as gap fillers

Although the excessive shrinkage and brittleness ofanimal glues at low RH [88] makes them inferior gapfillers on their own, modification with ‘plasticisers’ andbulking agents can alter their properties, improvingtheir suitability for this application [39, 81]. Hard filmswith a minimum tendency to distort can be achievedby the addition of fillers such as magnesium sulphate

or mineral clays together with sugars and dextrins [28,35].

The addition of an inert filler dramatically changesthe physical and mechanical performance of animalglues, depending on the proportion of glue present. A high pigment concentration significantly reducesintermolecular bonding within the glue medium [76,86] and thus impedes dimensional changes of thematrix in response to relative humidity changes [76]. Inaddition, with the lack of chemical adhesion between aproteinaceous binder and inert filler particles, the glueis substantially weakened and this leads to low tensilestrength [7]. Therefore high MW glues, with their long

protein strands and ability to develop stabilising H-bonds, are appropriate for fillers and gesso with a highpigment concentration.

Ageing characteristics

Whilst substantial research has been published on thebehaviour of collagen-derived glues in a fluctuatingenvironment, information on the ageing mechanismsand behaviour on exposure to light seems to be morelimited. According to Michel et al., isinglass fromsturgeon, of all animal glues, best retains its mechanicalproperties with thermal and ultraviolet (UV) light ageing

and RH cycling [79, p. 271]. It shows markedly lesschange in strength and stiffness than pure mammaliangelatin. Mammalian gelatin increases in tensile strengthbut becomes stiffer and more brittle upon artificialageing under UV light, fluctuating RH and temperature.Isinglass from sturgeon remains much tougher and moreelastic than gelatin [79, p. 274]. It also develops the leastpermanent dimensional change, whereas gelatin filmsswell or creep slightly during ageing, and other animalglues show an even more marked effect.

Resolubility

Collagen-derived glues, unless they have been modifiedby the addition of tanning agents which causes themto become relatively resistant to water, generally swellreadily when exposed to water and redissolve whenheated, even after centuries [23, 39]. Neher [89]established that the Bloom strengths of hide and rabbit


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skin glues are not correlated to their water-resolubilityand that all tested samples were completely and equallysuccessfully reversible after one month of naturaldrying. Wooden joints bonded with fish glue or coldliquid hide glue have also been shown to be detachablewith water after six months of natural ageing or RH and

temperature cycling [17]. An effect of the tannic acids ofoak wood and walnut on their resolubility could not beestablished in this study.

The dependence of water-resolubility on originalsolution concentration has been demonstrated for agedand UV-irradiated hide and bone glues at concentrationsof between 2.5 and 20% [90, p. 302]. This researchshowed that the lower the original concentration, thelower the resolubility of the glue film. Bone glues weremore resoluble than hide glues, supposedly becauseof their more pronounced molecular cleavage in theprotein matrix (Table 1).

Przybylo tested isinglass from sturgeon obtained fromdifferent suppliers [23], and found that the source,origin and preparation temperature have no significanteffect on the resolubility of the glue in water afternatural and artificial ageing, as all the films in the testseries remained resoluble. In contrast, Michel et al. [79]report that their artificially-aged sturgeon isinglass filmswere insoluble in water, even though no significantmolecular changes within the protein were detected.The contradictory results of these two studies may bedue to different preparation procedures and artificialageing conditions, which varied in the type of lightsource as well as cycles of exposure time, temperatureand RH.

Resolubility of animal glues may be reduced in caseswhere the protein has come into contact with metalions (e.g. metal foils, tools, pigments), or with certainorganic pigments and tannins, either before, during oreven after their application [12, 23, 78]. Resolubilityof collagen-derived glue containing no additives is thusvery much dependent on the environment to which ithas been exposed, rather than being predetermined bythe type of glue. Cold liquid hide and fish glues, theingredients of which are often unknown to the supplierand end user, may already contain additives that promote

cross-linking and, therefore, increase insolubility.

Colour changes on ageing

Hide and bone glues are generally much more stronglycoloured (amber to brown) and less transparent thangelatin or isinglass because of their higher impuritycontent. Higher levels of denaturation and molecularcleavage also intensify the colour of gelatinous solutions[47]. This phenomenon may be responsible for thegeneral observation that the higher the Bloom value, theless yellow the gelatin [56]. Gelatin and isinglass appearclear and virtually colourless if dried to thin films, even

though they yield slightly yellow or whitish solutions[10, 14, 43, 56, 58, 61]. They are also very light fastand show hardly any discolouration or yellowing withage [8, 30, 79], which is why they are the only collagen-derived glues suitable for pigment consolidation.Isinglass is particularly popular for this purpose, as its

low refractive index, when compared with mammaliangelatin, causes the least change in appearance of thepigments after drying [71, 79].

Conclusions

This review of the different types of currently availableanimal glue has shown that collagen-derived adhesivesvary in their chemical, physical and mechanicalproperties. Being a natural polymer, performance ispartly dependent on the original collagen source, whichdetermines the glue’s chemical composition, but is alsostrongly affected by the extraction and preparationprocedures. Molecular weight distribution is animportant factor which directly influences the proteinsolution viscosity and contributes to gel strength andTgel. The degree of stabilisation of the protein matrixby hydrogen and other chemical bonding is determinedby amino acid composition, preparation procedures

and drying time. This has an even greater impact onthe performance of the glue, and significantly affects itsstrength, mechanical behaviour, sensitivity to ambientenvironment and stability with age. Changes in pHand the addition of hygroscopic additives (plasticisers)and salts can alter many of these properties. However,manipulation of one individual factor cannot necessarilybe realised without simultaneously changing a wholerange of other properties. As most of the properties aredependent on each other, selection of the appropriateglue should be based on a correct balance rather than onindividual properties.

It has become evident that much important data thatwould allow comparison of the properties of the differenttypes of glues is still missing. Very few gelatinousglues have been prepared and tested under the sameconditions, and insufficient characterisation of theseglues makes it difficult to draw exact conclusions for ageneral glue type, as physical and mechanical propertiescan vary substantially. However, a summary of the datadoes reveal general qualitative trends that can be usedby conservators to make well-informed decisions on thesuitability of a particular collagen-based glue for a givenapplication.

AcknowledgementsThe author would like to thank Shayne Rivers, SeniorFurniture Conservator at the Victoria and AlbertMuseum, London, and Dr Ambrose C. Taylor, ImperialCollege, London, for their ongoing support in discussingthis paper and their valuable advice.

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Author

Nanke Schellmann trained as a violin maker inMittenwald (Bavaria) before undertaking several yearsof internships in the conservation departments of theNational Gallery (Frames) and the Wallace Collectionin London, the Bavarian National Museum, Munich

and the Germanic National Museum, Nuremberg. In2003, she received an MA in Furniture Conservationfrom the Royal College of Art/Victoria and AlbertMuseum (RCA/V&A) Joint Conservation Programme,London, UK. On finishing, she joined the workshopof Clemens von Schoeler, Munich as a conservator forfurniture and historic wooden interiors. Since 2005she has attended additional courses in natural sciencesat the Ludwig-Maximilians-University, Munich and iscurrently undertaking a PhD at the University of Fine Arts Dresden, together with the V&A Mazarin ChestProject and Imperial College, London, in the field oforiental lacquer conservation.

Correspondence can be sent to:

Nanke SchellmannMazarin Chest ProjectFurniture, Textiles and Frames ConservationSectionVictoria and Albert MuseumSouth KensingtonLondon SW7 2RLUKEmail: [email protected]

Documents

Animal Glues