Hide glue
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Hide glue, properly used, forms a strong and long-lasting bond. It was the most common woodworking glue for thousands of years until the advent of synthetic glues such as PVA and other resin glues in the 20th century. It has several advantages and disadvantages compared to these other glues, which can perhaps be best understood by describing its use.
[edit] Usage
Hide glue may be supplied as granules, flakes, or flat sheets, which have an indefinite shelf life if kept dry. It is dissolved in water, heated and applied warm, typically around 140°F. Warmer temperatures will quickly destroy the strength of most hide glues, so maintaining this temperature is critical. Commercial glue pots, simple water baths or double boilers may be used to keep the glue hot while in use. As hide glue cools, it gels quickly. At room temperature, prepared hide glue has the consistency of stiff gelatin, which is in fact a similar composition. Gelled hide glue does not have significant strength, so it is vital to apply the glue, fit the pieces, and hold them steady before the glue temperature drops much below 120°F. 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 often means having to heat the pieces to be glued, and gluing in a very warm room, though these steps can be dispensed with if the glue and clamp operation can be carried out quickly.
Where hide glue is in occasional use, 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.
A version that does not need to be heated, and has a long open time, known as liquid hide glue, is also available, but has a limited shelf life, and inferior strength compared to hot hide glue.
The open time of hide glue can be increased by the addition of various chemicals, urea being the most common choice. However, these additions usually decrease the strength of the glue. The open time of hide glue can also be increased by adding water, though adding too much water can cause a weak joint.
[edit] Properties
This significant disadvantage to using hide glue is offset by several advantages. Hide glue joints are reversible and repairable. Recently glued joints will release easily with the application of heat and steam. Hide glue sticks to itself, so the repairer can apply new hide glue to the joint and reclamp it. In contrast, PVA glues do not adhere to themselves once they are cured, so a successful repair requires removal of the old glue first - which usually requires removing some of the material being glued.
Hide glue creates a somewhat brittle joint, so a strong shock will often cause a very clean break along the joint. In contrast, a joint glued with PVA will usually break the surrounding material, creating an irregular, difficult to repair break. This brittleness is taken advantage of by instrument makers. For example, instruments in the violin family require periodic disassembly for repairs and maintenance. The top of a violin is easily removed by prying a palette knife between the top and ribs, and running it all around the joint. The brittleness allow the top to be removed without any damage to the wood. Regluing the top only requires applying new hot hide glue to the joint. If the violin top was glued on with PVA glue, removing the top would require heat and steam to disassemble the joint (causing damage to the varnish), then wood would have to be removed from the joint to ensure no cured PVA glue was remaining before regluing the top.
Hide glue also functions as its own clamp. Once the glue begins to gel, it pulls the joint together. Violin makers glue the 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 loads. PVA glues create plastic joints, which will creep over time if heavy loads are applied to them.
Hide glue is supplied in many different gram strengths, each better suited to specific applications. Instrument and cabinet builders will use a range from 120-200 gram strength. Glass artists take advantage of hide glue's ability to bond with glass, applying 600 gram strength hide glue to glass. As the glue hardens it shrinks, chipping the glass. Some hide glues are sold without the gram strength specified. Experienced users avoid this glue as the glue may be too weak or strong for the expected application.
