Hide glue is an adhesive made from animal connective tissue. Technically it’s a protein colloid glue formed by boiling down the collagen from skins, bones, tendons and other tissues. The result is similar to gelatin. Animals commonly used include horses. The expression: “sent to the glue factory” was used to describe a horse to be put down.
The word "collagen" is derived from the Greek ‘kolla’: glue. As an adhesive, the proteins form a molecular bond with the glued object.
Glues derived from animal remains have specialty uses:
- Hide Glue – favored in luthierie
- Hoof glue - woodworking: specifically cabinetry.
- Rabbit-skin glue - sizing or priming of oil painters' canvases.
- Bone glue
- Fish glue
Animal glue was the adhesive of choice for thousands of years before the discovery (or development) of synthetic glues like polyvinyl acetate (a PVA brand name is ‘Elmer’s Glue’). Ironically, ‘Elmer’s’ carries some of that same barnyard reference to animal management.
There is evidence that the Egyptians used hide glue for furniture nearly 5,000 years ago. Stone carvings remain depicting the process of gluing different types of woods. There is also evidence that the Sumerians used hide glue even before the Egyptians.
Today it is primarily used for specialty applications like luthierie, pipe organ building and antique restoration. Specialty Glass artists take advantage of hide glue's ability to bond with glass. As the glue hardens it shrinks, chipping the glass in a controlled fashion.
There are several properties that make hide glue an ideal adhesive for luthierie: Instruments are built to last many lifetimes. It is assumed that they will need to be worked upon or restored at some time. When they are made, they are designed to be disassembled, adjusted, modified, fixed or restored.
- Most animal glues are soluble in water, useful for joints, which may at some time need to be separated.
- Alcohol can be applied to joints to dehydrate the glue and make it brittle and easier to crack apart.
- Hide glue sticks to itself, so it can be reapplied to a joint and reclamped.
- Hide glue creates a brittle joint, so a strong shock can cause a very clean break along the joint.
- Hide glue can function as its own clamp: Once the glue begins to gel, it pulls the joint together. Violin makers may glue the center seams of top and back plates together using a rubbed joint rather than using clamps. This technique involves coating half of the joint with hot hide glue, and then rubbing the other half against the joint until the hide glue starts to gel, at which point the glue becomes tacky. At this point the plate is set aside without clamps, and the hide glue pulls the joint together as it hardens.
- Hide glue regains its working properties after cooling if it is reheated. This property can be used when the glue's open time does not allow the joint to be glued normally. For example, a cello maker may not be able to glue and clamp a top to the instrument's ribs in the short one-minute open time available. Instead, the builder will lay a bead of glue along the ribs, and allow it to cool. The top is then clamped to the ribs. Moving a few inches at a time, the maker inserts a heated palette knife into the joint, heating the glue. When the glue is liquefied, the palette knife is removed, and the glue cools, creating a bond. A similar process can be used to glue veneers to a substrate. The veneer and/or the substrate is coated with hot hide glue. Once the glue is cold, the veneer is positioned on the substrate. A hot object, such as a clothes iron is applied to the veneer, liquefying the underlying glue. When the iron is removed, the glue cools, bonding the veneer to the substrate.
- Hide glue joints do not creep under load.
The fact that hide glue becomes brittle and pulls materials together as it shrinks appears to benefit the sound qualities of an instrument. PVAs create a plastic joint that creeps under load. By definition, a PVA joint is not as hard as a hide glue joint. In theory, a PVA joint can absorb some of the vibration transfer between brace and soundboard. Hide glue, on the other hand, comes very close to a perfectly rigid joint between materials – optimum energy transfer implies better sound. That’s the theory anyway.
Hide glue may be supplied as granules, flakes or flat sheets. As long as the product is kept dry, it has an indefinite shelf life. It is dissolved in water, heated and applied warm by brush or spatula, typically around 140°F (60°C). Warmer temperatures destroy the strength of hide glue. Commercial glue pots, simple water baths or double boilers can be used to keep the glue hot while in use. As hide glue cools, it gels. At room temperature, prepared hide glue has the consistency of stiff gelatin (in fact, a similar composition). Gelled hide glue does not have much strength, the pieces need to be fitted and clamped before the glue temperature drops much below 120°F (50°C). All glues have an open time; the amount of time the glue remains liquid and workable. Joining parts after the open time is expired results in a weak bond. Hide glue's open time is usually a minute or less. In practice, this can mean heating the pieces to be glued, and gluing in a very warm room. These steps are unnecessary if the glue and clamp operation is done quickly.
Where hide glue is used only occationally; excess glue may be held in a freezer to prevent spoilage from the growth of microorganisms. Joints to be glued with hide glue must fit perfectly, as it has poor gap-filling properties. This is an asset in Luthierie.
It is also possible to keep hide glue liquid at room temerature by adding urea. An example of this type of mixture is Old Brown Glue, created by W. Patrick Edwards, the director of the American School of French Marquetry. In stress tests performed by Mark Schofield of Fine Woodworking Magazine, liquid hide glue compared favorably to hot hide glue in average strength of bond. "However, any liquid hide glue over six months old can be suspect because the urea eventually hydrolyzes the protein structure of the glue and weakens it— even though the product was 'protected' with various bactericides and fungicides during manufacture."
Hide glue is supplied in many different gram strengths, each suited to specific applications. Instrument and cabinet builders will use a range from 120-200 gram strength.
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