The Major Advantages of Hide Glue
The Major Advantages of Hide Glue
Soluble in water
Forms a strong and long-lasting bond
Non-hazardous, non-toxic, biodegradable. Eco-adhesives like hide glue begin to break down within weeks of being recycled into the natural environment.
Can be modified for open time, tack, viscosity and pH.
Animal glue was the most common woodworking glue for thousands of years until the advent of synthetic glues such as polyvinyl acetate (PVA) and other resin glues in the 20th century. Today it is used primarily in specialty applications such as lutherie, pipe organ building, piano repairs, and antique restoration.
It has several advantages and disadvantages compared to other glues. The glue is applied hot, typically with a brush or spatula. Glue is kept hot in a glue pot, which may be an electric unit built for the purpose, a double boiler, or simply a saucepan or crock pot to provide a warm water bath for the container of glue.
Most animal glues are soluble in water, useful for joints which may at some time need to be separated.[17] Alcohol is sometimes applied to such joints to dehydrate the glue, making it more brittle and easier to crack apart.
It 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 60°C (140°F). Warmer temperatures quickly destroy the strength of hide glue.[18] 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 stiffgelatin, 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 50°C (120°F). All glues have anopen 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,[19] 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. Hide glue has some gap filling properties,[20] although modern gap-filling adhesives such as epoxy resin are better in this regard.
The significant disadvantages of hide glue – its thermal limitations, short open time, and vulnerability to micro-organisms – are 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 allows the top to be removed, often without significant damage to the wood. Regluing the top only requires applying new hot hide glue to the joint.
Hide glue also functions as its own clamp. Once the glue begins to gel, it pulls the joint together.
Hide glue regains its working properties after cooling if it is reheated.
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 suited to specific applications. Instrument and cabinet builders will use a range from 120 to 200 gram strength.
Bone and hide glue also offer high strength, good low-pressure gap-filling ability, high initial tack, and fast cure.
Next, place the gelatinous glue mixture into a water bath. Heat the water to 140°F. Be sure to use a thermometer, as it's important to maintain this temperature. If the water gets too hot, it will overheat the glue and cause it to lose strength. It's a good idea to stir the glue mixture occasionally to help work out any lumps but it's not absolutely necessary. After a few minutes, the pellets will be melted and the glue will be ready to use. A properly prepared and heated batch of glue forms a steady thread as it drips from a brush back into the glue pot. If the glue drips from the brush in droplets, it's too thin. To thicken it, stir the glue while it's being heated until sufficient water has evaporated from the mixture. If the glue forms thick clumps on the brush, dilute it with hot water until the desired consistency is achieved.
Hide glue is sold in many different "gram strengths." The higher the gram strength, the tackier and stronger the glue. The stronger the glue is, the less "open time" you have between when the glue is applied and when it sets and immobilizes the item being glued.
Hide glue is the oldest and most traditional of all the glues used in woodworking. Hide glue has some good and bad features: on the minus side, it must be used hot (click below for information about glue pots); it can spoil; and cheap glue smells bad. But its positive features are numerous: it is easily reversible; easy to repair; easy to sand; essentially non-toxic; the resulting joint will not creep; and best of all, it is transparent to most finishes so you don’t get visible glue blotches near a joint.
The 192 gram strength is a good general purpose glue, and it’s the least expensive hide glue around. That’s why it’s the most common. Its real application is veneering, although you can use it for regular gluing in a pinch. A lot of people consider it the best all-around glue for general woodworking (including veneering), because it also has the longest open time. If you are new to hide glue, this is the grade you should get.
The 251 gram strength glue is traditionally the most appropriate for regular cabinetwork. Its higher strength means that you can do rub joints more easily, and clamped joints will have less time to creep. But it’s not optimal for veneering, when you’d want the maximal amount of time for squeezing out the excess glue.
The 192 gram strength "high clarity" is more expensive than its cousins because it is especially refined for maximum transparency. It’s a good all-around glue that’s slightly more tacky then the regular 192 glue. This is the glue to use if you are worried about visible glue lines.
The 315 gram strength is a special purpose glue for very high stress applications. It is favored primarily by instrument makers for situations where a joint will be under constant force. Of the glues the 315 has the shortest open time.
Hide glue has the best characteristics for stringed instrument work, BUT it requires care and practice to use properly. This is serious business, you really do have to learn to work with it.
Mix dry glue with cold water and allow water to be fully absorbed (about an hour.) Heat in glue pot or double boiler 145°F and stir gently until mixture is smooth and clear. Glue must be heated (145°) for use, but may be kept for several weeks covered in refrigerator. Discard if you notice foul odor, mold or any other contamination. To avoid loss of strength, do not heat glue for long periods, or above 145°.
Proportions of glue and water will vary with the gram strength of the glue. Practice with each new shipment of dry glue to obtain the ideal mixture, and mix by weight, not volume for the most consistent results. Most of us like the glue thin enough to just run off the brush when dipped into the heated mixture.
Joints must be clamped before glue begins to gel (around 95°.) It may help to warm work pieces slightly or to raise temperature in workroom to prolong gel time. Addition of urea (5-10% of the dry weight of glue) will extend gel time without seriously decreasing strength, although it may cause a slight increase in flexibility.
Hide glue sets initially by gelling as it cools. Further hardening occurs entirely by evaporation; this process is entirely reversible. Wetting and reheating the glue will bring it back to its original usefulness, although it is not practical in most cases once applied to wood. If the joint doesn't go together in time, you can wash off gelled glue with water and start over. When glue forms a “skin” or reaches the consistency of tapioca it is too late to apply clamping pressure.
Because it sets as it gels on cooling, hide glue can be used for “rubbed” joints where maximum strength is not required. Patches or “cleats” used to reinforce repaired cracks may be held in place by hand, pressing hard and sliding to squeeze out excess glue. When the glue cools, the cleat will be held in place by the gelled glue. Hide glue shrinks as it dries, and the cleat will be drawn tighter and will have sufficient strength in most cases.
Thinning glue excessively with water will reduce strength of the glue joint. Wood surfaces must be well-fitted and clean. Ideally, gluing surfaces are prepared the same day to avoid effects of oxidation. Old surfaces should be planed, scraped or block sanded to remove oxidation. “No-load” stearate coated abrasive paper may contaminate the surface.
Dry hide glue has an unlimited shelf life if protected from moisture.
Here's a method for preparing hide glue for quick use, without a glue pot:
1. Weigh and mix glue with cold water; allow water to be absorbed;
heat & blend as usual. Store covered in refrigerator.
2. When you're ready to use the glue, prepare the joint, and assemble
all the clamps, cauls, and fixtures you'll need.
3. Heat a 500 ml beaker or other handy container full of water (my
microwave takes 55 seconds) to 160°. (It'll cool as you go.)
4. Place a small chunk of cold glue in 1-2 oz. disposable plastic cup
(like take-out hot sauce cup) and float or clip inside beaker.
5. Use glue when it melts. After use, discard plastic cup, applicator
and leftover glue. Applicator can be cheap brush, Q-tip, etc.