I'll stick with hide glue. . .
Take a look at my controversial GLUE CHART
I'll stick with hide glue. . .
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.
(My current glue is Milligan & Higgins 192 gram strength high clarity. It's water-clear when mixed, and I useabout 1.9 parts water to 1 part glue by weight.)
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.
Hide glue is derived from the collagen found in animal hides. It is very similar to the gelatin we eat and is not toxic. In the U.S. edible gelatin is made from pork skins and hide glue from beef hides.
Glue is graded on a basis of its gel strength, a measure of how many grams of force it requires to depress a 1/2" plunger 4mm. into a 12.5% protein solution of the glue at 10° C. Glue is manufactured in standard grades from 32 to 512 grams. 192 gram strength is the most commonly used for woodworking; 251 is the highest normally used for instrument building; 135 is the lowest used for general woodwork.
The higher the gram strength, the stronger the cured glue, and the shorter the working or gel time. The lowest grades are considered strong enough for woodworking: "stronger than the wood itself." (Personally, I don't think any glue deserves that sort of comment. It may be that strong only for certain tests; it is not the same as the wood itself.) It is never a good idea to dilute too strong a glue to obtain lower viscosity or longer working time when it is possible to use a lower grade of glue. Glues of different gram strengths may be mixed to get an intermediate. I like 192 gram "high clarity" because it's transparent and doesn't gel too fast.
Now, here's an important note. The higher gram strength glues have higher molecular weight, which (I'm guessing here) may have slightly lower electrochemical adhesion, i.e., they may not stick as well. Because the very high gram strength glues require up to twice the water to reach workable consistency, there will be less glue actually in the joint after it is clamped and the water evaporates. This may be why the very high strength glues (300g. to 500g.) are often described in the literature as "too strong for woodworking." Additionally, they gel too fast for many applications.
Commercial liquid hide glues have gel suppressants & remain liquid at room temperature. They are not quite as useful for most instrument work: slightly greater tendency for the joint to "creep," lower moisture & heat resistance, short shelf life, no initial tack. Many instrument makers and repairers use liquid hide glue, but they are (or should be) careful to test each batch, in addition to watching the expiration date printed on the bottle.
Urea will extend the working time of the glue, and will not weaken the adhesion if it is mixed with the glue just before use. More than 5-10% urea (by dry weight) may increase the flexibility of the dried glue; it may allow glue joints to "creep" more, especially in heat stress. Most instrument builders and repairers try to avoid having to use urea.
When a hide glue joint is clamped, thick liquid glue should squeeze out all around. If the squeeze-out is "crumbly" when rubbed with a finger, or has the texture of cottage cheese, the joint may not have been clamped in time. Excess glue should be washed off finished surfaces to avoid chipping the finish as the glue dries and shrinks.
Hide glue cures entirely by evaporation. Hide glue is about 2/3 water, and it is important to pay attention to the amount of water absorbed into the work piece. Maple necks can take on quite a "back bow" as a result of water absorption. Joints will swell when glued and must be allowed to dry completely before leveling to avoid "sunken" areas around the glue joint. Old hide glue joints can be restored by adding more hot hide glue. The heat and moisture will reactivate the old glue on the joint surfaces, and the new joint may be nearly as strong as the original.
Hide glue sticks to surfaces by electrochemical attraction, or specific adhesion. Mechanical bonds, like little "fingers," may help modern adhesives such as epoxy because of their very high cohesive strength, but with hide glue's low cohesive strength, roughening joint surfaces will not help adhesion. It is better to be concerned about the electrochemical properties of the wood.
If you'd like a whole bunch more info about hide glue, its properties and uses, check out these FAQ pages about hide glue and player piano restoration:
player-care.com/hide_q-a.html
player-care.com/hideq-a2.html
Make sure the joint is clean, freshly prepared to avoid oxidation, well-fitted and clamped in time.
North America's only glue factory is Milligan & Higgins
PO Box 506, Johnstown, NY 12095 518-762-4638 (50 lb. min.)
5lb. quantities of various grades are available from
Bjorn Industries (best source for information, too!)