Hide glue is a type of glue which is made from collagen, a protein found in animal hides and hooves. Until the 20th century, it was probably the most widespread glue in use, utilized in a wide range of fields to create strong joins between various objects. Today, a range of synthetic glue products are available in addition to hide glue; a hardware store or woodworking supplier is a good place to find this type of glue though, if you need some for a project.
In some fields, hide glue continues to be the glue of choice. People who restore antiques often work with it because they want to keep materials true to the time in which the antique was produced. It is also used by instrument makers, because it has a number of properties which make it ideally suited to the construction of fine instruments. It is also used in fine woodworking. These uses ensure that this glue remains readily available, along with the supplies needed to handle it, such as a glue pot for heating and working.
Typically, hide glue is sold in the form of sheets, flakes, or pellets. The dried glue must be mixed with water and heated to make it usable, and precise temperature control is extremely important, as the glue will degrade and become less effective if it is allowed to get too hot. Many people use a specialized glue pot for this purpose, because glue pots are designed to handle delicate temperate adjustments. If allowed to cool, the glue will become brittle and unworkable. Fortunately, the glue can be reheated multiple times; this trait can be extremely useful for fine-tuning and adjustments.
The bond created by hide glue is extremely strong, but it can be easily reversed with the application of heat, steam, or a very fine blade to the joint, depending on the type of joint involved. It is also fairly brittle, which can be a big advantage when a joint experiences extreme stress and breaks. Typically the break will be very clean, making it easy to repair, whereas synthetics tend to create ragged breaks which are challenging to fix.
Many people take advantage of the reheatable trait of hide glue. In instrument repair, for example, it can be gently melted to allow a luthier to pull an instrument apart for servicing, and then the glue can be heated again when the instrument is reassembled. Furniture makers also appreciate hide glue, as they can easily adjust the construction of a piece of furniture made with it, and it makes repairs very easy.