Hide Glue, Gelatines & Co.

In violin making, the beneficial properties of animal glues have always been appreciated. In contrast to synthetic glue, glue with gelatine can be re-opened again at any time with the addition of heat and moisture without causing damage to the workpieces. Synthetic glue remains hard after curing, which can damage musical instruments or furniture. [GME (2/2021)]

History of animal based glues

Historically, animal glues have been used for a long time. “From 7000 BC, the advanced civilizations in Mesopotamia (Iraq) and Egypt extracted animal components (animal bones, skin, fish waste) through boiling. At the same time, gelatine glue was used in furniture production in Daedalus and Icarus (Near East). The first historical evidence comes from the ancient civilizations of the Mesopotamia between the Euphrates and Tigris, from the sunken city-states: Sumer - Akkad - Assur and Babylon (approx. 4000 BC). The Sumerians consciously and by hand developed an adhesive and wrote history through it. They boiled a kind of glutine glue out of animal skins and seem to have already used asphalt as a binding agent when building their houses and temples. It can also be assumed that tendon backings were used for hunting and war bows at this early point in time, but no archaeological evidence exists. This glue was later refined by the ancient Egyptians. A plaque of hide glue was found in the grave of Tutenchamun, which proves that veneer work was cemented with glue long before the birth of Christ. As early as 3500 BC, the Egyptians were able to produce glue based on protein, which they made by boiling animal hides. As early as 3300 BC hide glue was used in Egypt and secured by a hyroglyphic text. "[Ulmer, R. / Westebbe, P. (SS 2002) p.4]

The first historical evidence for the use of hide glue can be found in a rock chamber in the mortuary temple of Queen Hatshepsut in Egypt from 1500 BC. Here a fresco shows the use of hide glue and thus the gluing of different types of wood. With the fall of the Roman Empire, however, the art of gluing seems to have been forgotten and was only rediscovered in the 16th century. [Baumann, H. (1967)]

Until the late 17th century, glues and gelatine extracts were obtained by boiling bones and pieces of skiin. Between the late 17th and the beginning of the 19th century there were first attempts at production on a craft scale. [Babel, W. (1996) p.4]

Then in 1690 the first artisan glue factory for technical gelatine was founded in Holland. [Baumann, H. (1967)]

It was not until the beginning of the 19th century, as a result of the English sea blockade in France and the associated nutritional deficiencies, that industrial gelatine production was started in order to be able to supply the population with protein. With the advent of photography and its use as a binder for light-sensitive silver haloids [GME (1/2021)], the demand for gelatine increased rapidly, so that by the end of the 19th century, gelatine production on an industrial scale had developed in all industrialized countries. [Babel, W. (1996) p.4]

Terminilogy for animal glue and gelatine production

• Isinglass: cartilage, fish bones [Ottmeier, W. (2/2011) p.26]
• Rabbit glue: glue made from bones and skins of small animals [Ottmeier, W. (2/2011) p.26]

• Isinglass glue: swim bladders of fish [Ottmeier, W. (2/2011) p.26]

• Hide glue, leather glue: animal skins, tendons [Ottmeier, W. (2/2011) p.26]

• Bone glue: bone waste, cartilage, tendons [Ottmeier, W. (2/2011) p.26]

• Button glue: bone glue in the form of drops [Ottmeier, W. (2/2011) p.26]

• Carpenter's glue: generic term for hide and bone glue [Ottmeier, W. (2/2011) p.26]

• Glutine: Purified basic substance from bones, skins etc. and main component of gelatine [Babel, W. (1996) S3.]
• Collagen: protein family made up of long, cross-linked molecular chains, insoluble in water (area of ​​application: food supplements, beauty care products, etc.) [Babel, W. (1996) S3.]

• Gelatine: Long Mollekülketten, soluble in hot water, gelling (application area: gummy bears, glue photo film etc.) [Babel, W. (1996) S3.]

• Collagen peptides: relatively short molecular chains, soluble in hot and cold water, non-gelling (area of ​​application: food supplements, muscle building products, etc.) [Babel, W. (1996) S3.]

• Bloom / Bloom grade: Describes the gel strength according to the AOAC measurement in the gelled, not dried state. The key figure is the mass in grams that is required so that a punch with a diameter of 0.5 inches deforms the surface of a 6.67% gelatine / water mixture four millimetres deep without tearing it. The test takes place standardized at 10 ° C with a previous aging of the gelatine of 17 hours. [GPF (2021] No conclusions can be drawn about the adhesive strength in the dry state from the Bloom value. From my own experience, I can rather conclude the duration of the processing time, but this has not been proven.
 The AOAC measurement corresponds to the international standard Measurement of the bloom value. Max. Values ​​here are 300-330 bloom. Other measurement methods, which are especially for glue, come to different values.
• Water content: Air-dry gelatine usually contains between 8 and 12% water. With water contents that exceed 16%, there is always the risk of microbiological spoilage. For gluing, this means a reduction in the adhesive properties. [GPF (2021]

• Clarity: The clarity is indicated by the transmission. That means how much light gets through. E.g. LUT Universal: Transmission of 620nm greater than or equal to 85%, transmission of 450nm greater than or equal to 60%

• Viscosity: Here, according to DIN 53260, the flow time is determined that a 10% gelatine solution, at 60 ° C, from a pipette with calibrated capillary determination. This is interesting for the glue because this value relates to the handling temperature. [GPF (2021]

• pH value: In the case of gelatine, this value depends on the salt content and has an effect on the foamability of a gelatine solution. Completely pure gelatine has the pH value of the isoelectric point. The isoelectric point of gelatine is determined by its manufacturing process and is between pH 8.0 and 9.0 for acidic gelatine. In the case of alkaline digested gelatine, the isoelectric point is close to pH 5.0. [GPF (2021]

• Smell and taste: Gelatine has a very low taste or smell of its own. In the case of glue, the smell is caused by impurities or poorly cleaned glue, such as skin, bones, rabbit glue etc.


Figure 1 from Gelatine to Collagen

Animal glues

The different known types of animal glue refer to their historical production method. Nowadays they are often more an indicator of their purity. Bone glue is dark red, button glue is a little lighter than the bone glue and the hide glue is a little lighter still. Whereby the rabbit glue is the lightest. The name usually says little about their composition, properties, purity, quality or even original animal components.

However, they all have one thing in common: they owe their adhesive strength to the glutine, a main component of gelatine. The adhesive force of good gelatine can be up to 1.6 tons per square centimetre. [GME (2/2021)]

Everything that gives a "classic" glue (bone glue, hide glue, etc.) colour and the often unpleasant smell in use are impurities such as blood, fibres, skin, herdsman's or hair remains, etc. These impurities prevent the molecular chains from being crosslinked and thus impair the adhesive behaviour or adhesive strength. In addition to the negative influence on the adhesive properties, these impurities, as a perfect breeding ground for germs, promote mould formation. This in turn has negative effects on the shelf life and storability.

In the case of so-called bone glue, rabbit glue, etc., a constant quality cannot be guaranteed as the properties change with each batch. Both the degree of contamination and the composition of the gelatine, length and type of the molecular chains vary greatly. This in turn has a major impact on the adhesive strength.

Basically, gelatine consists of linearly linked individual amino acids whose starting material is collagen. Collagen in turn consists of one third glycerine, 22% imino compounds from proline and hydroxyproline. 45% from a further 17 different amino acids. With today's knowledge, the collagen protein family can be divided into 17 types of collagen. [Babel, W. (1996) p.3]

For the application as glue, the purest possible “technical” gelatine has emerged as the best solution. With such a gelatine, various parameters can be set at will, according to the application. Gel strength, clarity, tack, tack time, etc.

Our LUT Universal and Restoration Extra glues belong to the so-called, highly pure and adjusted, special gelatines (food grade) and are specially adjusted to the needs of violin making.

Manufacturing

The bones and skin of calves, cattle and pigs are used for the production of gelatine (and also bone, hide, rabbit glue etc.). These are secondary raw materials from both the meat or meat processing industry and the hides processing companies.

Two processes have emerged for converting the water-insoluble collagen obtained from the raw materials into water-soluble gelatine.

The acid process: This process was developed in the USA in the 1930s and is mainly suitable for processing pork skin into gelatine. Since the skin shows little collagen cross-linking due to the young age of the animals, acid pre-treatment is sufficient to later achieve good gelatine extractability. The gelatine obtained from it is a so-called type A gelatine.

The classic, alkaline process: This process is used to obtain gelatine from bovine hides and bones. In a complex process, the starting material is subjected to a calcium hydroxide suspension for 8-12 weeks with gentle mechanical movement. Called "liming". A one to two week “liming” with caustic soda applies to cattle hide. The type of gelatine obtained from this is type B gelatine. The raw materials pre-treated in this way are then washed free of alkali or acid. Gelatine can now be obtained in several steps. [Babel, W. (1996) p.7 f.] Graphic S5 Fig.1 Process steps in gelatine production [Babel, W. (1996) p.5]

Abb.2 Verfahrensschritte der Gelatineherstellung [Babel, W. (1996) S.5]

Gelatine properties

It is believed that the gelling process takes place in three steps as follows.

1. Accumulation of dissolved molecular chains.
2. Crystallite formation through association of two or three ordered segments.
3. Stabilization of the structure through local formation of hydrogen bonds.

“The firmness of the gel ((gel strength) not dry) depends heavily on the gel strength of the starting gelatine, the pH value, the temperature control during solidification, the measuring temperature and other additives such as salts or carbohydrates.

The gelatines available on the market melt between 25 ° C and 35 ° C, depending on the gelatine quality and concentration. Their freezing point is 2-5 ° C lower.

Depending on the addition of water and heat, the gel gradually reaches its melting point. The influencing parameters of viscosity are of interest for processing as glue. The viscosity is influenced by the gelatine quality and concentration, temperature, the pH value and the addition of possible additives. A higher concentration and lower temperatures work towards increasing the viscosity. " [Babel, W. (1996) p.7 f.]

Molecular chain lengths and their meaning

Very long molecular chains in the gelatine spectrum with a high bloom value have the property of being able to bridge large distances. However, their inner cohesion is stronger than in the short-chain geltaine. However, these gelatines have slightly redused adhesive forces. They are used in the ballistic field (shot attempts and flight path calculation of projectiles), cake icing, photo film etc.

In contrast, the very short molecular chains in the gelatine spectrum have a low Bloom value.

Due to the short chain, their inner cohesion is less. However, they have particualrly high adhesive forces, as many small chains allow several anchor points between the two sides, which have to gab. One of its areas of application is its use as a glue.

In between there is the entire spectrum of lengths and, depending on the application, in different mixing ratios. Gummy bears, food supplements, release agents, food adhesives, medical products, binders and much more. require very different properties, which can be adjusted with special gelatine.

Our glues, specially adjusted for violin making, are available in two versions that only look very similar at first glance. Both glues are a selected mixture of longer and short-chain molecular chains, highly pure (medical grade) and tested for their optical properties using test methods.

• The LUT Universal is a low-bloom gelatine with a longer processing time. The higher number of shorter molecular chains results in stronger adhesive forces. In addition to the high glue strength, the focus here is on a good processing time.
• Restoration Extra as a high-bloom gelatine with longer molecular chains, has a shorter processing time, but stronger cohesive forces. In addition to the extremely high glue strength, the focus here is also on low collour and clarity.

Utensils for perfect gluing

The Ultimate Glue Test

This is a little test where 8 different thicknesses of hide glue where mixed, 8 pairs of wood where joined, and brokenk.

The glues tested range from 11% to 39% glue by mass.

PLEASE NOTE: During the break test, the amounts read out as "by water" should be "total mass (glue + water)." The actual amounts used in the test are as according to the chart of glue ratios from the earlier section.

This is brought to you by students of the Chicago School of Violin Making.

• 0:30 Chart of glue ratios
• 1:31 Jointing procedure
• 2:25 The break test
• 5:25 Conclusions

Reverences

• Baumann, Hans: Leime und Kontaktaufkleber / Theoretische Grundlagen Eigenschaften — Anwendung, Springer-Verlag Berlin Heidelberg, 1967.
• Babel, Wilfried: Gelatine – ein vielseitiges Biopolymer / Herstellung, Anwendung und chemische Modifikation. Sonderdruck, Chemie in unserer Zeit / 30. Jahrg. 1996 / Nr. 2.
• Ottmeier, Wibke: Leime, Restaurator im Handwerk – Ausgabe 2/2011.
• Ulmer, Roland / Westebbe, Philipp H.: Werkstoffkunde Bindemittel, SS 2002, Prof. Erwin Emmerling / Seminararbeit Modifizeirte Glutinklebstoffe, TU München, Studiengang Restaurierung, Kunsttechnologie und Konservierungswissenschaft
• GME, Gelatine Manufacturers of Europe: Gelatine und ihre wichtige Rolle in der Fotografie und Kunst, https://www.gelatine.org/de/anwendungen/foto-und-kunst.html [Stand: 1/11.02.2021]
• GME, Gelatine Manufacturers of Europe: Gelatine und Kollagenpeptid in vielzähligen Anwendungen, https://www.gelatine.org/de/anwendungen/specials.html [Stand: 2/11.02.2021]
• GPF, Wichtige Kennzahlen von Gelatine, http://www.parmentier.de/gpfneu/gelatine/deutsch/kennzahl.php [Stand: 11.02.2021]