Animal-based glues are among the oldest known adhesives, with evidence of their use dating back thousands of years to ancient civilizations like the Egyptians and Greeks. These natural products served as the primary bonding agents for craftsmen and builders for millennia, long before the invention of modern synthetic glues. Their widespread use demonstrated the ingenuity of early humans in transforming biological by-products into a powerful, functional material. The effectiveness of these adhesives relies entirely on naturally occurring structural proteins derived from animal remains.
Collagen: The Core Component
The fundamental basis for all traditional animal glues is the protein collagen, which is a major component of connective tissues like skin, bones, and tendons. Collagen is naturally insoluble in water, so it must undergo a chemical transformation to become an adhesive. This process is called hydrolysis, which involves prolonged heating or boiling of the raw animal material in water. The heat breaks down the strong, triple-helix structure of the collagen protein into a simpler, water-soluble form known as gelatin, which is the actual glue substance.
Different parts of the animal require slightly different preparation methods, such as pre-soaking in lime or acid to remove impurities and prepare the collagen. The resulting gelatin solution, often called ‘glue liquor,’ is then concentrated and dried into flakes, granules, or cakes for storage. When needed, the dried glue is simply mixed with water and heated to a working temperature, typically around 140–150°F (60–66°C), to return it to a liquid, adhesive state. The quality and strength of the final glue product are directly related to the purity and concentration of the extracted gelatin.
Sources from Terrestrial Mammals
The most common and historically significant sources for animal glues come from the by-products of terrestrial mammals, particularly cattle, horses, and rabbits. Hide glue, often considered a high-quality adhesive, is made from the skins or hides of these animals, especially cowhide, which is universally used in modern production. The raw hides are cleaned, pre-treated with lime to break down the collagen structure, and then cooked to extract the gelatin. This glue is prized in woodworking for its excellent strength and ability to form rigid bonds.
Another major type is bone glue, which is extracted from the bones of animals, primarily cattle and horses. Bone glue is generally less pure than hide glue, often yielding a darker, more brittle final product, but it was historically a cheaper and more readily available option. The bones are degreased and then cooked under pressure to extract the collagen. Rabbit-skin glue is a specialized variant, known for being more flexible and elastic than standard hide or bone glues when dry. It is most frequently used in the fine arts for sizing or priming canvas before painting.
Sources from Aquatic Life and Milk
Adhesives have also been successfully created from non-mammalian sources, notably from aquatic life and milk.
Fish Glue
Standard fish glue is made from the heads, bones, scales, and skins of various fish species, following a process similar to mammalian glue production. This type of glue tends to be thinner and less sticky than hide glue, but it is often used for applications where a non-heating, liquid adhesive is desired. Isinglass is a specialized, high-grade fish glue derived exclusively from the dried swim bladders of certain fish, such as sturgeon, hake, or cod. Swim bladders are nearly 80% pure collagen, and isinglass produces a transparent, very strong adhesive used historically for specialized repairs and as a fining agent to clarify wine and beer.
Casein Glue
Casein glue, while also an animal-derived protein, is distinct because it comes from the casein protein found in milk. This glue is formed by separating the casein from skim milk and mixing it with an alkaline substance like lime or borax. Casein glues were historically important in construction, including for early wooden aircraft and water-resistant plywood. They are valued for their impressive strength and increased moisture resistance compared to traditional hot animal glues.
Modern Uses and Advantages
Despite the prevalence of synthetic adhesives, traditional animal glues persist in niche modern applications because of their unique performance characteristics. A primary advantage is their reversibility; a joint glued with hide glue can be easily undone by applying heat and moisture, which is invaluable for antique restoration, conservation, and repair. This reversibility allows conservators to take apart and reassemble wooden objects without damaging the original materials.
In the construction of musical instruments, such as violins and guitars, luthiers favor hide glue because its rigidity and lack of plastic creep enhance the instrument’s acoustic properties. The glue dries hard and brittle, contributing to sound transmission rather than dampening it like some flexible synthetic glues. Furthermore, animal glues are environmentally friendly, being non-toxic, biodegradable, and utilizing by-products from other industries, making them a sustainable choice for specialized woodworking, bookbinding, and fine art applications.