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Study of the mechanical properties of selected animal glues and their implication when  designing conservation strategies

A. Melià-Angulo, L. Fuster-López et A.Vicente- Escuder


Des colles animales ont joué un rôle dans beaucoup de traitements de conservation depuis les débuts de la restauration d'oeuvres d'art. Le voilage, la consolidation, le rentoilage, le masticage, etc. sont tous des exemples de traitements où les colles animales ont été largement utilisées comme adhésif ou comme liant assurant l'adhésion et la cohésion de la peinture. Dans cette recherche, les propriétés mécaniques de plusieurs colles animales sont présentées et des paramètres pour une sélection critique pour des utilisations spécifiques (c'est-à-dire composition, indice de Bloom, etc. ) seront exposés.

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This research was possible thanks to the financial support from the Polytechnical University of Valencia (PAID-UPV-06-10-2429) and the Valencian Regional Government (GV-2011/082). The authors are deeply indebted to Dr. M.F. Mecklenburg and the Museum Conservation Institute at the Smithsonian Institution (USA) for the equipment donated for tensile testing.



1In this project the study of selected Kremer®, OGI® and Lefranc&Bourgeois® glues and gelatins is presented. As seen in Table 1, the selection of materials is representative of glues and gelatins corresponding to different composition, Bloom’s strengths and pH.

Table 1: Glues and gelatins tested in this study

Table 1: Glues and gelatins tested in this study

#A, #B,#C indicates different batches for a same given glue.


Preparation of samples

210% (w/w) solutions in distilled water of each glue/gelatin was heated at 60ºC until complete dissolution and cast on polyester sheets.  Samples dried at room conditions for 24 h and free glue films were obtained. Strips measuring 150 mm x 10 mm x  0’057 mm (length x width x thickness) were obtained after drying. Samples presenting microfissures or air bubbles were rejected for testing.


3pH measurements were taken with a portable Hanna® pH meter connected to a contact probe. For each sample, 3 measurements were taken at room conditions.

Tensile testing

4Static uniaxial tensile testing was carried out with equipment donated by the Smithsonian Museum Conservation Institute (Washington,D.C.). The equipment consists of a rectangular methacrylate box that contains several tensile testers. This box acts as a climatic chamber where relative humidity (RH) and temperature (T) can be controlled. The tensile testers are located in the upper part of the chamber where as the light bulb and the tray are located on the lower level. A small fan circulates the air within the chamber.The fan accelerates the humidity absorption of the silica gel and creates stable and homogenous environmental conditions inside the chamber. A high precision dew point hygrometer is connected to the chamber and a small hygrometer is also located inside it. Samples were clamped in the testing gauge sand then allowed to come to equilibrium in the chamber for 24 h at 50º.0% (±3 % RH) and 19ºC(±2ºC) prior to testing. Strain increments of 0.0625 mm were applied progressively at 30-second intervals. Three tests were run for each type of sample in order to obtain consistent results. The tests were performed at a displacement rate of 3.75 mm/min and at 50.0% (±2 %) RH and 23ºC (±1º C). Such crosshead speed represents the speed at which induced strains might occur on works of art according to the specialised literature. Data analysis was undertaken using a customised Microsoft Excel spreadsheet.

5The results obtained from tensile tests were recorded and stress-strain curves were obtained. The elastic modulus was calculated considering the slope of the linear section of the curve in the elastic region (data processing by Microsoft Excel 2007).

6A micrometer (Outside Micrometer Mitutoyo, model: M317-25; measuring range 0–25 mm; graduation 0.01 mm) was used to measure sample thickness.

Results and discussion


7Results indicate that most of the glues tested present a pH slightly acid or even close to neutrality (table 1). Higher pH corresponds in all brands to higher Bloom’s strengths. However, it is relevant to note the following: On the one hand, different batches for a same given glue seem to correspond to different materials. In the case of K6310, batch #B presented a pH 5,71 whereas batches #A and #C were around 6,2. One the other hand, Lefranc&Bourgeois® glue presented an acid pH (4,7). Such an acidic value could be an issue considering the range of applications of hide glues in conservation treatments and the susceptibility to degradation by oxidation and hydrolysis of many of the organic materials present in our collections.

Tensile testing

Fig.1 Stress strain curves

Fig.1 Stress strain curves

Stress strain curves for all the glues and gelatins tested

8Illustration 1 shows the stress strain curves for all the glues and gelatins tested at room conditions. As it can be seen in all 9 cases, the ultimate tensile strength (UTS) is above 20 MPa which is a good indication of the high cohesive strength of these materials. A more detailed look to Kremer® glues indicates that different Bloom’s strengths determine different strength and elongation at break but not change in the stiffness is presented. K63020 (BS: 230-280) is twice stronger and almost even elongating twice as much than K63010 (BS:240 ±20). It is also interesting to note that slight changes in the mechanical properties of different batches corresponding to a same glue can be observed, which is an indication of the lack of consistence in the processing and manufacture of these products.

9Looking at the different OGI® samples, again, changes in Bloom’s strength imply different strength and elongation at break, but the flexibility of all three samples stays the same. This is interesting since conservators might tailor their glue solutions as a function of individual but complementary parameters such as flexibility, cohesive strength and ability to elongate according to their needs.

10Finally, the mechanical properties of Lefranc&Bourgeois® can also be observed in figure 1. The technical specifications of this glue are unknown and no information was obtained from the manufacturer. Nevertheless, correlating this glue with those of Kremer® and OGI®, it could be stated that L&B's Bloom’s strength could be similar to that of K63010 and OGI250, being, however, a more flexible material, given its lower elastic modulus observed.

Table 2  

Table 2  

Average UTS, elongation at break and E-modulus calculated


11This poster shows the earliest results of a research aimed to show the different mechanical behaviour presented by different gelatins and animal glues typically used in conservation. In this research it has been shown that for both Kremer® and OGI® glues, a change in Bloom’s strength means significant changes in the ultimate tensile strength and deformation at break, whereas the modulus of elasticity remains invariable. In other words, the glues studied present the same stiffness regardless their bloom strength. It is important to note that despite Lefranc&Bourgeois® hide glue presents appropriate mechanical properties, research has evidenced a rather low pH that could compromise the stability of the work of art. The study shows that animal glue represents a group of materials of different composition, pH and mechanical properties, and that a more exhaustive study is needed to find out how such properties fit in conservation practice and therefore what could be the critical parameters to select the appropriate glue to be used in each situation depending on the adhesive and cohesive needs.

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Document annexe

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Table des illustrations

Titre Table 1: Glues and gelatins tested in this study
Légende #A, #B,#C indicates different batches for a same given glue.
Fichier image/png, 64k
Titre Fig.1 Stress strain curves
Légende Stress strain curves for all the glues and gelatins tested
Fichier image/png, 41k
Titre Table 2  
Légende Average UTS, elongation at break and E-modulus calculated
Fichier image/png, 24k
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Référence électronique

A. Melià-Angulo, L. Fuster-López et A.Vicente- Escuder, « Study of the mechanical properties of selected animal glues and their implication when  designing conservation strategies », CeROArt [En ligne], HS | 2017, mis en ligne le 02 juin 2017, consulté le 20 septembre 2017. URL :

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A. Melià-Angulo

L. Fuster-López

A.Vicente- Escuder

Departamento de Conservación y Restauración de Bienes Culturales. Universidad Politécnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain; Instituto Universitario de Restauración del Patrimonio, Universidad Politécnica de Valencia, Camino de Vera s/n, 46022 Valencia (España); Instituto Tecnología de los Materiales. Universidad Politécnica de Valencia Universidad Politécnica de Valencia, Camino de Vera s/n, 46022 Valencia (España)

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Licence Creative Commons
CeROArt – Conservation, exposition, restauration d'objets d'arts est mis à disposition selon les termes de la licence Creative Commons Attribution - Pas d'Utilisation Commerciale - Pas de Modification 4.0 International.

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