Animal Glues

Animal glues: a review of their key properties relevant to conservation Nanke C. Schellmann Schellmann in changing ambient environment, and ageing characteristics.

Abstract Collagen-based animal glues are widely used in the conservation of artefacts, serving as adhesives, binders and consolidants for organic and inorganic materials. With a variety of different animal glues on the market, such as hide and bone glues, fish glues, isinglass and gelatin, their individual properties need 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 respect to their specific physical, chemical and mechanical properties.

Types of commercially available animal glue Hide glues are primarily derived from bovine skins and those of smaller mammals, although connective tissue may also be used. Bone glues are predominantly prepared from fresh (‘green’) bones or sometimes extracted bones (degreased and demineralised, known as ossein) from cattle and pigs. Hide and bone glues are produced and sold as coarse powders, pearls, cubes, and cakes or plates, though the latter two appear to be increasingly rare [9]. Commercial gelatin, the purified active ingredient of any collagen-derived glue (pure denatured collagen), may be obtained from either skin or bone sources [9, 10] and is supplied in the form of thin sheets, plates or powder.

Introduction Animal glues are natural polymers derived from mammalian or fish collagen – the major structural protein constituent of skins, connective tissue, cartilage and bones. These glues may exhibit varied physical, chemical and mechanical properties depending on their origin and method of preparation. In the manufacture of objects and artefacts, an extensive traditional knowledge exists on which animal glues are most suitable for specific purposes. However, conservators sometimes lack the confidence to make informed choices between the different collagen-based glues available when conserving objects.

As the name suggests, rabbit skin glues should be produced purely from rabbit skins [10, 11], though collagenous waste from various small mammals may also be used [12]. Some suppliers sell rabbit skin glue that 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 accurate information by the manufacturers. It is generally assumed that most animal glues contain preservatives of some kind (e.g. sulphur dioxide) [9, 14]. Even rabbit skin compressed into cubes, a by-product from the felt industry commercially sold as a raw (and thus usually thought to be a pure) form of rabbit skin glue [10], has recently been found to contain preservatives [9]. Some traditional glues, such as the deer glue used in Japan as a binder for some inks, are now made from bovine or porcine gelatin manufactured to match the properties of the traditional genuine material [15].

The selection and preparation of glues are discussed in patent descriptions, woodworking and artists’ manuals, as well as conservation literature and product details from suppliers [1, 2]. There is also a large amount of technical research on the properties of collagen and gelatin published in scientific journals on polymerand bio-technology, medical science and the food and brewing industry. However, much of this literature is not readily accessible to conservators and it can be ambiguous or contradictory. This paper seeks to provide a review of the literature and to identify which properties of glue need to be considered when making decisions about conservation treatments.

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

The applications of collagen-based glue in the conservation field are diverse, ranging from its use as an adhesive, consolidant or binding medium for pigments and filler particles [3–8]. Generally, the following key properties need to be considered: •

chemical structure and denaturation of the protein molecules.

•

gelling properties: gelling temperature (Tgel), gel strength and setting times.

•

properties of the glue solution: viscosity, viscosity, surface surface tension and pH.

•

properties of the dried film: cohesion, adhesion and final bond strength, mechanical behaviour

A number of industrially manufactured cold liquid animal glues are available that have modified properties and a long shelf life. These glues usually contain additives that alter their natural behaviour, extending the working time at room temperature, or decreasing the propensity for biodeterioration and reducing the dried film’s sensitivity to moisture. However, the exact composition of industrially tailored collagen-derived glues and their overall performance may be difficult to judge, as manufacturers tend to keep their recipes 55

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secret. Furthermore, conservation requirements such as long-term stability and resolubility are unlikely to be a priority for commercial manufacturers. Given the range of additives that may be present in industrial glue formulations, minimally modified glues represent the safest option for conservation.

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low temperature yields gelatinous matrices containing protein fractions of long chain length and high molecular weight (MW) [38, 46]. As a general rule, gentle processing is appropriate for the hides of young mammals, 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 bonds that develop in older mammals. Furthermore, glues that are derived from fish cleave more easily on extensive heating than those of mammalian origin owing to their chemical structure [40, 46]. Conservators should thus be aware that when preparing a collagen-based solution, mild procedures should be employed [4, 46]. Preparation temperatures for collagen-based glues are generally recommended to be around 55–63°C. However, there is little loss of gel strength on heating at high temperatures (e.g. 80–90°C), even in the case of isinglass, but only if the solution is kept at these temperatures for no more than a few minutes [46, 47].

Chemical structure and properties Chemical structure and denaturation Collagen consists of long protein molecules composed of naturally occurring amino acids that are linked in a specific sequence by covalent peptide bonds. Due to the spatial conformation of some amino acid groups (notably proline and hydroxyproline) and the many ionisable and polar functional groups in the protein chain, the individual chains form triple-stranded helical coils that are generally believed to be internally stabilised by hydrogen-bonding [7, 25, 29–33]. Collagen is insoluble in cold water [30, 34] and is transformed into soluble gelatin by denaturation, a process of critical importance for the performance of the resulting glue. This is achieved by hot water extraction (hydrolytic breakdown) [9, 34–39]. Pretreatment (either acidic or basic) is necessary for most skin and bone collagen, but is not required for the extraction of isinglass from fish bladders, which contain less cross-linkage within the collagen. During extraction, the bonds (predominantly H-bonds) in the triple-helix structures of the collagen are broken so that it separates into disordered ‘random’ coils of single protein chains, thus completing the transition to gelatin [40]; in perfect conditions gelatin is pure denatured collagen. The temperature (Td) at which denaturation occurs is dependent on the chemical structure of the proteins in the particular collagen source, notably on the content of the amino acid derivatives proline (Pro) and hydroxyproline (Hyp). These are supposed to be largely responsible for the stabilising H-bonded water bridges in the triple helix [41] and are present more abundantly in mammalian collagen than in marine species [42]. Thus, adult mammalian collagen denatures at 40–41°C [31, 33], while isinglass and other fish collagens denature at lower temperatures. The Td of fish collagens ranges from approximately 15°C for deep cold 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 for commercial clarification of alcoholic beverages. There are also a few tropical fish species that reach Td levels of up to 36°C [31, 44].

Gelation and gelling temperature (T gel ) Although the process of denaturation, with the loss of the triple helix arrangement of the protein molecules, is irreversible, some helical structure can be restored during gelling and drying. On gelling the single random protein coils undergo partial rearrangement (renaturation) back into collagen-like triple helices [7, 26, 46, 48-50]. However, the misalignment of the single strands means that renaturation causes nodes (‘junction zones’) involving only part of certain strands. The remainder of these strands may form further nodes so that a continuous three-dimensional network structure emerges. The degree of renaturation is dependent on the chemical composition (Pro and Hyp content), the chain length of the molecules (molecular weight, MW), concentration in solution and temperature [42, 49, 51]. High Pro and Hyp content, high MW, high solution concentrations and slow drying at a low temperature promote a high degree of renaturation and the development of a highly ordered network structure [34, 37, 48, 52]. The number of nodes that are established by the formation of H-bonds (and probably also by electrostatic interaction [42]) within and between the molecules 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 be repeatedly reliquefied by reheating, is one of the unique properties of collagen-based glues. The temperature at which gelation of the glue solution occurs (Tgel) depends mainly on the collagen source, but is also affected by the degree of protein cleavage. Gelation temperatures decrease with lower denaturation temperature (Td) and also with increasing cleavage of the molecules. Mammalian gelatin gels at around 30–35ºC, and cold water fish gelatin remains liquid down to around 8ºC [14, 43, 53]. However, this temperature will be lowered if the preparation temperature of the glue is significantly exceeded.

The process of denaturation is necessary for collagen to convert to gelatin, which can be used as a glue. Cleavage of the single protein molecule may also occur during pre-treatment, extraction and dissolution, and will significantly affect the properties of the gelatinous glue. The more vigorous the extraction process (i.e. the more extreme the pH, the longer the treatment and the higher the temperature during extraction), the more bonds within the protein molecule are randomly cleaved, leading to ever decreasing molecular weights [34, 37, 39, 45]. Mild extraction at moderate pH and

Gel strength Gel strength is a measure of the gel rigidity of gelatinous glues, and is strongly influenced by the molecular weight 56

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of the constituent proteins [34, 54]. According to several authors [35, 39], the average molecular weight (AMW) of animal glues can range from around 20000 to 250000 g.mol-1. It is thought that permanent gelling does 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 values of well over 150000 g.mol-1 [4, 33, 46], while liquid fish glue has AMW values of around 60000 g.mol-1 [10, 14], placing it at the lower end of the range. For most other commercial collagen-based adhesives, information on AMW is not readily available.

than alkaline pre-treated collagen derivatives (type B gelatins), whose MW distribution is skewed towards lower MW fractions [9, 34, 46]. Open (gelling) time, tack and drying The setting time of animal glues depends primarily on Tgel and gel strength. The lower the Tgel and gel strength, the longer the open time of the solution (i.e. the longer it takes for the glue to gel). High Bloom hot hide glues tend to gel rapidly, as gelation occurs at comparatively high temperatures [10, 11, 14, 39, 59]. Gelatinous glues derived from fish, which have low Tgel due to their chemical structure [42, 43, 58], and cold-set liquid hide glues are convenient to use when long open times are required. Commercial fish glues usually contain preservatives [60] and, sometimes, small amounts of other additives such as colour brightener, deodorizing agents or fragrance [10]. Liquid hide glues generally have further additives to inhibit gelation at room temperature [17, 28]. These are typically salts (e.g. urea, thiourea) or phenols that extend the setting time by inhibiting renaturation of the gelatinous matrix [28, 52]. Some manufacturers claim that their liquid hide glue does not contain gelling inhibitors [17], in which case the gelatinous matrix must be considerably affected by molecular cleavage to achieve the comparatively low MW that is necessary for the glue to be in a liquid state.

Characterisation by AMW is only common for fish glues, which are liquid at room temperature. Most other gelatinous glues are usually characterised by their gel strength, as AMW does not describe the molecular weight distribution and therefore may not always correlate reliably with the physical and mechanical properties of a glue [34, p. 60] (Table 1). However, it would be expected that high AMW adhesives, such as skin glues, have higher gel strength and viscosity, gel more rapidly and produce stronger bonds. Gel strength is strongly influenced by AMW but also shows a linear correlation with the degree to which the protein solution renatures during gelation [55], i.e. the higher the degree of formation of helical structures, the higher the gel strength. The presence of salts also influences gel strength, which decreases with an increasing concentration of ions in solution [42, 56].

The ability of collagen-based glue to develop tack upon gelation is a unique property. In general, glues of higher Bloom strength develop tack faster than lower Bloom glues. The tack ‘strength’ of glue can be empirically tested by conservators between two fingertips. Isinglass solutions may appear to be less tacky than equivalent concentrations of mammalian gelatin or hide glue, as they take longer to set at room temperature, since their lower gelation temperature delays the development of tack.

Gel strength, also known as Bloom strength, is measured in grams (g), or Bloom grams (gB), and equals the force required to make a specified depression into a gel sample prepared under standard conditions [25, 35, 37, 39]. Manufacturers commonly distinguish between grades of glue by their Bloom strength, which usually covers a wide range, being as low as 30 g for weak bone glues and rigorously extracted hide glues, and up to around 500 g for very strong hide glue [10, 11, 35, 37, 57]. Gelatins derived from tropical fish have significantly lower Bloom values than mammalian gelatins [58], since the degree of stabilisation of the triple helix by H-bonding is lower. Gelatins extracted from cold water fish do not have specified gel strengths as they are liquid at room temperature [42].

Drying time generally depends on the ambient temperature and relative humidity (RH). After gelation, the glue matrix dries by evaporation of water and this process 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 molecular mobility after gelation and during drying encourages the development of highly ordered network structures [52]. This maximises the elasticity and strength (toughness) of the resulting glue film. Isinglass naturally develops highly stable and elastic films if dried at room temperature, being slightly above its Tgel [9].

As gel strength is dependent on the structural conformation of the gelatinous matrix, it is useful for estimating the toughness, strength and resilience of the resulting bond. Furthermore, Bloom strength also correlates with the water-sorption capacity of the glue (in gel and solid state), viscosity (at least to some degree), and gelling temperature (Tgel), which generally all increase with rising Bloom value. High Bloom glues require a lower solid content in solution than glues with a lower Bloom rating to be effective as an adhesive, as they offer many sites for intermolecular bonding in a given volume [35, 56]. Mammalian skin glues are usually considered to have the highest AMW and produce the strongest gels and films [10], particularly those extracted by acid pre-treatment. Generally, acid pre-treated glues (type A gelatins) contain larger fractions of high MW

Properties of gelatinous glue solutions Viscosity The viscosity of the glue solution is primarily dependent on the molecular weight distribution [51]; the greater the proportion of molecules of higher MW the higher the viscosity [2, 35]. For a given MW distribution, the viscosity increases with increasing solution concentration and decreasing temperature [39, 51, 61]. The degree to which collagen-like helices [62, p. 128] 57

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IN CONSERVATION

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ANIMAL

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m o r f ( ) m s s i r s e a w l g s d d n h a i s l s i i f b

d n a n h e ) i t n e s a i o g l f b a e g m , l i t n o i r h s r i f f ( k s a c

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and intermolecular bonds have developed within the network (gel/Bloom strength) further contributes to higher viscosity [63, 64]. Strongly denatured gelatinous solutions (such as bone glues) or those affected by a high degree of molecular cleavage will normally have a comparatively low viscosity. At a given Bloom strength, alkaline pre-treated (Type B) gelatins are generally more viscous than acid pre-treated (Type A) gelatins [56] (Table 1).

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and gelatins contain less than 1% fat because of modern manufacturing methods [9, 10, 54, 58, 69] and may require additives to reduce the surface tension. Ethanol is commonly added to lower the surface tension and 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 conservation of Boulle-marquetry, and was shown to improve the wetting properties leading to stronger joints between the wood and brass components [70]. However, alcohol may also raise the gelling temperature, speeding up the gelation and decreasing the time for which the glue is workable [28, p. 102, 110], and may also promote swelling of the substrate. Alternatively, surfactants can be added to lower the surface tension [3, 8, 28, 72, p. 123].

Viscosity is an important factor in the choice of adhesive for bonding or consolidation, as it will affect the degree of penetration into a substrate. If the viscosity is too low the glue may penetrate too far into the substrate, leaving a joint starved of adhesive. For consolidation of porous materials, high viscosity may prevent adequate penetration and cause stress to develop at the interface between consolidated and unconsolidated areas. Unfortunately, the viscosity values for animal glues given in the literature and by suppliers vary widely and are not easily compared. Measurements were often taken under different experimental conditions and at different degrees of cleavage in the protein molecules [4, 21, 35, 37, 46].

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 and weaken bonds [5, 59]. Natural fats or free fatty acids present in glues play a vital role in reducing foaming [5, 68, 73], although some authors still express some doubt that there is a direct correlation between fat content and 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 for gilding. Sauer and Aldinger [68] have demonstrated an unambiguous dependency of the degree of foaming on fat content, whereas no direct relationship could be established with surface tension. They also could not find any influence of protein degradation products on foaming, while pH was established to have an inconsistent effect.

Isinglass has a much higher viscosity than hide glue at an equivalent solution concentration and temperature (above Tgel), which can be explained by its comparatively high proportion of high molecular weight fractions (which, in the following paragraphs, will be referred to as high Molecular Weight Distribution (MWD)) [4, 22, 46, 65]. This is contrary to what is often stated in the literature and to the traditional beliefs about the handling properties of isinglass [20, 21, 61]. However, where low viscosity values have been obtained for isinglass, it is likely that the particular preparation procedure of the glue used for the tests resulted in greater cleavage of the protein molecules [46]. Despite isinglass having a large fraction of high MW compounds, its low gelling temperature compensates for this by allowing more time for the glue to penetrate porous substrates at room temperature, therefore improving its penetration ability in comparison to gelatin and rabbit skin glue of similar high MW fractions, which will gel faster [8, 66].

pH For conservation applications, the choice of adhesive may be dependent on the pH sensitivity of the substrate [71]. Collagen-based glues can display varying pH values that are difficult to predict purely on the basis of the glue type or treatment during manufacture. The assumption that glues which undergo alkaline pretreatment display a slightly alkaline pH and acid-treated ones have an acidic pH [39, p. 171] is incorrect. It is stated in the literature that hide and fish glue solutions often have a fairly neutral pH in the range of 6.5 to 7.4, although wider variations are possible [9–11, 16, 35, 68]. In general, bone glues tend to be slightly more acidic [5, 10, 39], with pH levels between 5 and just below 7 [35, 37]. Pure gelatins from mammals and fish range between pH 5.0–6.5 and 3.5–5.0 respectively [10, 53, 54, 58, 61]. Isinglass yields solutions with a pH in the neutral range [19, 61, 71]. Conservators should test the pH value of the chosen glue before use if sensitivity of the substrate is of potential concern.

In order to obtain glue solutions of low viscosity, it is not always advisable to dilute viscous high Bloom glues excessively. The use of an over-diluted glue may result in swelling, leaching or staining of the substrate if it is water sensitive [67]. In such cases, a glue with a lower gel strength would be preferable. Surface tension Slow gelation and lower viscosity promote uniform film formation as the glue is able to spread evenly, providing adequate wetting of the surface. Wetting is improved with 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 linked to the presence of fats. Free fatty acids and neutral fats are regarded as particularly effective in reducing surface tension even in small concentrations. With the exception of rabbit skin glue (which has comparatively high fat levels of around 5% [9, 11]), most animal glues

Apart from being a relevant aspect to consider in conjunction with the sensitivity of the substrate, pH values also have an influence on the properties of the glue, as the viscosity increases when the pH of the solution shifts away from its isoelectric point (pI) [1, 37, 61, 74]. Since proteins and amino acids are amphoteric in nature (i.e. containing both acidic and basic functional 60

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groups), they have an isoelectric point, which is the pH at which all positive and negative charges within the molecule are balanced and the molecule carries no net electrical charge. If the electrical potential of the ions 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 pretreatment (most hide and bone glues, type B gelatins) usually have a pI of approximately 4.5 to 5.5, whereas glues derived from acid pre-treated collagen sources commonly display pI values of between 7.0 to 9.0 [17, 30, 34, 35, 51, 56, 75]. For practical purposes, this means that glues having a pH near their pI value (such as bone glues and type B gelatins) will already be at the lowest possible viscosity, as opposed to those which have pH values different from their pI, where to achieve the lowest possible viscosity the pH would have to be modified to take it closer to the pI (Table 1). The effect of the pH of a glue solution on its surface tension is inconsistent [28, p. 75, 68].

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 gelatinbased glue films, the elasticity and stiffness are greatly dependent on their MW distribution [63], the degree to which helical structures reform on gelling and the intra-/intermolecular bonding [7, 26]. The stiffness of the glue (elastic modulus, known as Young’s modulus E, mathematically calculated from the ratio of stress to strain values) increases with a higher ratio of high MW fractions, higher solution concentrations and with a greater renaturation level in the network [26, 45, 55]. Stabilisation of the gel network by increased electrostatic bonding induced by pH levels above or below the pI also increases the stiffness [42]. Mammalian gelatin generally has a higher modulus and therefore greater stiffness than fish gelatin due to its higher network stabilisation by intra- and intermolecular bonding [26, 58]. Isinglass is also more elastic than mammalian gelatin [79].

Mechanical properties of the dried film

The moisture content of an animal glue has an important effect on the mechanical properties. Under normal ambient conditions (50% RH and room temperature) gelatin-based glue films contain 12–14% of structural water bound to the polar groups of the protein macromolecules [52, p. 654]. This water contributes to the stabilisation of the helical structures within the glue and a specific amount of water is needed to maintain structural stability. Above around 25% moisture content the glue turns from a glassy to a rubbery state at room temperature [52]. Excessive dehydration of gelatinous films below a moisture content of 0.2% leads to the development of covalent cross-links between the protein molecules, which ultimately renders the glue insoluble in water [80, p. 509].

Cohesion, adhesion and bond strength The cohesive strength of the gelatinous matrix of a glue is determined by its molecular structure and intermolecular bonding, as expressed by the Bloom value. To produce an animal glue film that is as strong as possible in the dried state, the same rules apply as for obtaining a high gel strength (i.e. high MW distribution/minimum cleavage of protein molecules, maximum renaturation/content of collagen-like triple-helices, high intra-/intermolecular stabilisation). The cohesion strength of animal glues can be improved by the addition of a suitable amount of an alcohol, such as ethanol or glycerine [28, p. 108]. To achieve strong bonding, chemical adhesion between the glue and the substrate is as important as high cohesion within the glue matrix.

In general, gelatinous glue films with a low moisture content are very brittle regardless of the collagen source and molecular structure [34, p. 63, 52]. Even at a normal (12–14%) water content, gelatinous films undergo brittle fracture under impact. Randomly coiled structures exhibit much lower resistance to impact (greater brittleness) than helical glue matrices in the glassy state. Glue recipes often contain additives such as sugar alcohols (e.g. glycerine, sorbitol) and polysaccharides (e.g. dextrins) to improve elasticity and toughness [28, 34, 81, 91]. One traditional recommendation for achieving elastic and resilient glue films is the addition of honey [4, 18, 21, 22, 61, 82]. Sugars are hygroscopic and so stabilise the protein molecules by introducing additional hydrogen bonds involving water [25, 83], inducing an increase in gel strength and viscosity. Although these additives do not actually plasticise the glue matrix, they are often referred to as plasticisers in the literature. A high proportion of fat also improves elasticity, although it simultaneously reduces the gel strength of the glue and final bond strength [23, 84]. A higher water content or an excess of hygroscopic additives generates a reduction in the glass transition temperature of the glue [61, 81], which can promote an unwanted tendency to creep (elongation with time).

Hide glues generally have greater cohesive strength than the bone glues with highly cleaved molecules, which display a lower tensile strength and are much more brittle (Table 1). The tensile strength of hide glues is typically around 39 megapascals (MPa) (5700 pounds per square inch, psi) [76]. Mammalian collagen tends to yield stronger glues than most aquatic sources, owing to the reduced number of stabilising inter- and intramolecular bonds in fish collagen [33, 49]. Cold water fish gelatins in particular have a lower propensity to reform helical structures due to their small proportion of the amino acid derivatives, Hyp and Pro, and therefore show a comparatively low tensile strength of around 22 MPa (3200 psi) [26, 27, 53]. This value is comparable to the strength of bovine bone gelatin [54]. A high tensile strength similar to that of hide glue has been reported for mildly prepared isinglass from sturgeon [4], making it a useful adhesive for bonding wooden joints. The literature confirms that isinglass has often been used for structural woodwork in the Far East [24, 77]. Although rabbit skin glue has a high gel strength, it has been stated as having lower cohesion and bonding strength than other hide glues [23, 39, 61

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for improving the glue film’s hardness and resistance to water are the addition of tanning agents, such as aluminium trisulphate (alum), disodium triborate (borax), sodium acetate or formaldehyde [9, 28, 35, 91]. These salts remove a certain amount of bound water from the proteinaceous matrix by covalently bonding to the hydrophilic sites in the glue, thus inducing the formation of numerous new cross-links between the protein molecules.

Drying of collagen-derived glue films leads to the development of high internal stress and tensile forces within the glue matrix, while increasing humidity generally causes progressive loss of tension [4, 85]. This behaviour is dependent on the physical and chemical structure of the glue. A high degree of collagen-like triple-helix arrangement in a gelatin film has been shown to result in a reduced tendency to swell [34, 55], but is also responsible for increasing stress values due to stronger cohesion. Isinglass from sturgeon, which contains a high proportion of helical structures (due to its high MWD, despite its lower Hyp and Pro content), develops particularly high stress levels, which it is suggested are twice as high as in hide glue [4].

Mechanical properties of animal glues used as gap fillers Although the excessive shrinkage and brittleness of animal glues at low RH [88] makes them inferior gap fillers on their own, modification with ‘plasticisers’ and bulking agents can alter their properties, improving their suitability for this application [39, 81]. Hard films with a minimum tendency to distort can be achieved by the addition of fillers such as magnesium sulphate or mineral clays together with sugars and dextrins [28, 35].

If kept under moderate relative humidity conditions over a long period of time, initial stresses within gelatinous films relax owing to the absence of covalent cross-links [86]. Under fluctuating environmental conditions, the mechanical properties of collagen-based glues are subject to continual change [85]. Considerable development of internal stresses will affect the glue’s elasticity, strength and physical stability and may lead to significant damage to the substrates [48, 85–87].

The addition of an inert filler dramatically changes the physical and mechanical performance of animal glues, depending on the proportion of glue present. A high pigment concentration significantly reduces intermolecular bonding within the glue medium [76, 86] and thus impedes dimensional changes of the matrix in response to relative humidity changes [76]. In addition, with the lack of chemical adhesion between a proteinaceous binder and inert filler particles, the glue is substantially weakened and this leads to low tensile strength [7]. Therefore high MW glues, with their long protein strands and ability to develop stabilising Hbonds, are appropriate for fillers and gesso with a high pigment concentration.

At high RH levels (above 85%) animal glue films undergo a continuing reformation of helical structures. This will result in new, higher stresses on subsequent drying and can lead to severe shrinkage due to contraction of the glue matrix [48, 85, 86]. Cycling of RH can cause further strain – for rabbit skin glue, non-permanent total dimensional changes of up to 6% have been reported as the result of a single RH cycle [76, 86], which are only partly recoverable. According to Zumbühl [48], contraction mechanisms compete with plastic relaxation processes above 65% RH. However, even at high RH levels plastic relaxation may not sufficiently compensate for these stresses and continuous cycling further reduces the ability for stress relaxation [48]. This will result in permanent shrinkage of the glue matrix (up to 5% for rabbit skin glue [76]) and, in this case, loss of tension is only possible by substrate deformation or mechanical destruction (embrittlement) of the glue film [48].

Ageing characteristics Whilst substantial research has been published on the behaviour of collagen-derived glues in a fluctuating environment, information on the ageing mechanisms and behaviour on exposure to light seems to be more limited. According to Michel et al., isinglass from sturgeon, of all animal glues, best retains its mechanical properties with thermal and ultraviolet (UV) light ageing and RH cycling [79, p. 271]. It shows markedly less change in strength and stiffness than pure mammalian gelatin. Mammalian gelatin increases in tensile strength but becomes stiffer and more brittle upon artificial ageing under UV light, fluctuating RH and temperature. Isinglass from sturgeon remains much tougher and more elastic than gelatin [79, p. 274]. It also develops the least permanent dimensional change, whereas gelatin films swell or creep slightly during ageing, and other animal glues show an even more marked effect.

At low RH levels, more randomly coiled gelatinous structures (such as bone glues), which have comparatively low tensile strength and low resistance to stress, induce relaxation at an early stage by developing cracks in the glue matrix, thereby preventing high stresses on the substrate. These glues also show a greater tendency to creep under stress at high RH levels [61, p. 14]. Although animal glues containing a high degree of helical structure exhibit comparatively high stress when exposed to extreme and fluctuating environmental conditions, they still display greater stability in their strength properties than more randomly coiled structures. The strength properties of hot hide glue have been shown to be less sensitive to fluctuating RH and temperature than those of cold liquid hide glue [6, 28]. Liquid fish glues are even less stable than cold liquid hide glues under fluctuating conditions [17]. It has also been suggested that a high fat content, such as in rabbit skin glue, accounts for better stability in moist conditions [5, 39]. Common methods

Resolubility Collagen-derived glues, unless they have been modified by the addition of tanning agents which causes them to become relatively resistant to water, generally swell readily when exposed to water and redissolve when heated, even after centuries [23, 39]. Neher [89] established that the Bloom strengths of hide and rabbit 62

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skin glues are not correlated to their water-resolubility and that all tested samples were completely and equally successfully reversible after one month of natural drying. Wooden joints bonded with fish glue or cold liquid hide glue have also been shown to be detachable with water after six months of natural ageing or RH and temperature cycling [17]. An effect of the tannic acids of oak wood and walnut on their resolubility could not be established in this study.

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

Conclusions This review of the different types of currently available animal glue has shown that collagen-derived adhesives vary in their chemical, physical and mechanical properties. Being a natural polymer, performance is partly dependent on the original collagen source, which determines the glue’s chemical composition, but is also strongly affected by the extraction and preparation procedures. Molecular weight distribution is an important factor which directly influences the protein solution viscosity and contributes to gel strength and Tgel. The degree of stabilisation of the protein matrix by hydrogen and other chemical bonding is determined by amino acid composition, preparation procedures and drying time. This has an even greater impact on the performance of the glue, and significantly affects its strength, mechanical behaviour, sensitivity to ambient environment and stability with age. Changes in pH and the addition of hygroscopic additives (plasticisers) and salts can alter many of these properties. However, manipulation of one individual factor cannot necessarily be realised without simultaneously changing a whole range of other properties. As most of the properties are dependent on each other, selection of the appropriate glue should be based on a correct balance rather than on individual properties.

The dependence of water-resolubility on original solution concentration has been demonstrated for aged and UV-irradiated hide and bone glues at concentrations of between 2.5 and 20% [90, p. 302]. This research showed that the lower the original concentration, the lower the resolubility of the glue film. Bone glues were more resoluble than hide glues, supposedly because of their more pronounced molecular cleavage in the protein matrix (Table 1). Przybylo tested isinglass from sturgeon obtained from different suppliers [23], and found that the source, origin and preparation temperature have no significant effect on the resolubility of the glue in water after natural and artificial ageing, as all the films in the test series remained resoluble. In contrast, Michel et al. [79] report that their artificially-aged sturgeon isinglass films were insoluble in water, even though no significant molecular changes within the protein were detected. The contradictory results of these two studies may be due to different preparation procedures and artificial ageing conditions, which varied in the type of light source as well as cycles of exposure time, temperature and RH.

It has become evident that much important data that would allow comparison of the properties of the different types of glues is still missing. Very few gelatinous glues have been prepared and tested under the same conditions, and insufficient characterisation of these glues makes it difficult to draw exact conclusions for a general glue type, as physical and mechanical properties can vary substantially. However, a summary of the data does reveal general qualitative trends that can be used by conservators to make well-informed decisions on the suitability of a particular collagen-based glue for a given application.

Resolubility of animal glues may be reduced in cases where the protein has come into contact with metal ions (e.g. metal foils, tools, pigments), or with certain organic pigments and tannins, either before, during or even after their application [12, 23, 78]. Resolubility of collagen-derived glue containing no additives is thus very much dependent on the environment to which it has been exposed, rather than being predetermined by the type of glue. Cold liquid hide and fish glues, the ingredients of which are often unknown to the supplier and end user, may already contain additives that promote cross-linking and, therefore, increase insolubility.

Acknowledgements The author would like to thank Shayne Rivers, Senior Furniture Conservator at the Victoria and Albert Museum, London, and Dr Ambrose C. Taylor, Imperial College, London, for their ongoing support in discussing this paper and their valuable advice.

Colour changes on ageing Hide and bone glues are generally much more strongly coloured (amber to brown) and less transparent than gelatin or isinglass because of their higher impurity content. Higher levels of denaturation and molecular cleavage also intensify the colour of gelatinous solutions [47]. This phenomenon may be responsible for the general observation that the higher the Bloom value, the less yellow the gelatin [56]. Gelatin and isinglass appear clear 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 fast and show hardly any discolouration or yellowing with age [8, 30, 79], which is why they are the only collagenderived glues suitable for pigment consolidation. Isinglass is particularly popular for this purpose, as its

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IN CONSERVATION

Djagny, K.B., Wang, Z., and Xu, S., ‘Conformational changes and some functional characteristics of gelatin esterified with fatty acid’, Journal of Agriculture and Food Chemistry 49 (2001) 2987–2991.

Author Nanke Schellmann trained as a violin maker in Mittenwald (Bavaria) before undertaking several years of internships in the conservation departments of the National Gallery (Frames) and the Wallace Collection in London, the Bavarian National Museum, Munich and the Germanic National Museum, Nuremberg. In 2003, she received an MA in Furniture Conservation from the Royal College of Art/Victoria and Albert Museum (RCA/V&A) Joint Conservation Programme, London, UK. On finishing, she joined the workshop of Clemens von Schoeler, Munich as a conservator for furniture and historic wooden interiors. Since 2005 she has attended additional courses in natural sciences at the Ludwig-Maximilians-University, Munich and is currently undertaking a PhD at the University of Fine Arts Dresden, together with the V&A Mazarin Chest Project and Imperial College, London, in the field of oriental lacquer conservation.

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2007

Correspondence can be sent to: Nanke Schellmann Mazarin Chest Project Furniture, Textiles and Frames Conservation Section Victoria and Albert Museum South Kensington London SW7 2RL UK Email: [email protected]

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