The connection between horses and glue is a deeply rooted cultural reference, often associated with the phrase “sent to the glue factory.” This saying refers to the historical practice of using deceased horses as a source material for animal glue, specifically known as hide glue. Horses were once a major industrial source due to their size and abundance as working animals, though other animals like cattle and pigs were also used. This traditional adhesive has been utilized for thousands of years, long before the advent of modern synthetic compounds.
The Specific Source Material
The part of the horse used to create this adhesive is not a single organ but rather a fibrous protein found throughout the body called collagen. Collagen is the primary structural protein in connective tissues, providing strength and elasticity. For glue production, this protein is harvested from tissues where it is most concentrated, namely the hide (skin), bones, tendons, and ligaments.
The hide is considered the most reliable source for producing high-quality hide glue, which gives the adhesive its name. Collagen is uniquely suited for this purpose because it is an insoluble protein that, when processed, converts into a water-soluble, sticky substance.
How Animal Parts Become Glue
The process of converting raw collagen-rich materials into a usable adhesive involves an industrial cooking process known as rendering. First, the raw materials, such as hide pieces or bones, are thoroughly cleaned and prepared, often involving soaking and liming to break down non-collagenous proteins and fats. This preparation ensures the purest collagen is ready for extraction.
The cleaned material is then subjected to prolonged heating in water, typically around 70°C (158°F). This gentle boiling process is a form of hydrolysis, where the insoluble collagen protein is gradually broken down into the soluble protein known as gelatin, which forms a liquid called “glue liquor.”
This liquid is drawn off, filtered to remove impurities, and concentrated by removing excess water through heating. The concentrated, semi-liquid glue is then allowed to cool and dry, forming a jelly-like mass cut into flakes, pellets, or sheets. The dried form can be stored indefinitely and later reconstituted with water and heat to restore its adhesive properties.
Traditional Applications and Modern Replacements
Before the 20th century, animal-based hide glue was the dominant adhesive used across various industries, particularly in fine crafts. Its non-toxic nature and capacity to form a strong, rigid bond made it a preferred material for high-end woodworking and furniture making. It remains a material of choice today for instrument makers, or luthiers, who build and repair stringed instruments like violins and guitars.
A significant property of hide glue is its reversibility, meaning the joint can be easily undone by applying heat or moisture without damaging the wood. This feature is invaluable in antique restoration and instrument repair, allowing for future maintenance and disassembly. Hide glue also sets quickly and dries hard, creating a joint that does not creep or shift over time.
Despite these advantages, animal glue was largely displaced for mass production by the rise of synthetic adhesives like polyvinyl acetate (PVA) and epoxy resins. These modern glues are easier to use, require no heating, and are cheaper to manufacture. However, hide glue continues to be utilized in niche markets where its traditional properties, such as reversibility and compatibility with historical materials, are essential.