TLDR: Animal glue is a type of adhesive made by boiling animal connective tissue. It has been used for thousands of years for various purposes, including woodworking, painting, and bookbinding.

Animal glue is created by boiling animal connective tissue, such as skins, bones, tendons, and other tissues. This process is called rendering. The proteins in these tissues break down through hydrolysis, similar to gelatin. The resulting glue is a protein colloid that forms a strong bond with the object it is applied to.

Historically, animal glue has been used for a wide range of applications. In ancient times, it was used for wood furnishings, mural paintings, and even on the caskets of Egyptian Pharaohs. The Greeks and Romans used animal glue for veneering and marquetry, which involves bonding thin layers of wood. It was also used for repairing broken pottery and as a binder in paint media.

Different animals can be used to make animal glue, not just horses. Cattle, rabbits, and fish are also commonly used. The term "sent to the glue factory" is often associated with horses, but it's important to note that other animals are used as well.

The use of animal glue declined in Europe after the fall of the Roman Empire, but it reemerged in the sixteenth to eighteenth centuries with the rise of wooden furniture making. Today, animal glue is still used in specialty applications such as violin making, pipe organ building, piano repairs, and antique restoration. It is valued for its reversibility, creep-resistance, and ability to bond with various materials.

There are different types of animal glue, including hide glue, bone glue, rabbit-skin glue, and fish glue. Hide glue, made from animal hide, is commonly used in woodworking. It is dissolved in water and applied warm. Hide glue joints can be easily separated by applying heat and steam. Rabbit-skin glue is more flexible when dry and is used in oil painting, bookbinding, and as an adhesive component in gesso and compo. Fish glue, made from fish bones or tissues, was used in ancient times and is still used today in niche applications.

While animal glue has its advantages, such as reversibility and repairability, it also has limitations. It is water-soluble, which can be a disadvantage in certain applications. It can darken with age and shrink as it dries, potentially causing harm to wood, paper, or works of art. Additionally, the process of making animal glue is complex and requires fresh raw materials.

In modern times, animal glue has been largely replaced by synthetic adhesives and plastics. However, it is still valued for its unique properties and is used by artisans and craftsmen who appreciate its traditional qualities.