Q: I am doing some furniture repair and would value your opinion on the use of hide glue for this and for other construction. Also, what comments do you have about liquid hide glue and hot hide glue?
A: There is no doubt about it: Hide glues are excellent adhesives for many woods and for many uses. They have no VOCs, cure clear, cure at room temperature, and are quite strong. The hide glue joints can be easily taken apart; the adhesive softens with heat (over 120F and humidity (over 75 percent RH). When repairing old hide joints, the old adhesive can be left, as new hide glue will effectively attach to the old; almost all other adhesives require the old adhesive to be removed.
Here’s some background. Hide glue is derived from a protein molecule called collagen which is found in animal hides, as well as bone and connective tissues associated with the hides. That’s why hide glue manufacturing in the past was associated with the meat packing and tanning industries.
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Hide glue has a recent history of being used primarily for stringed instruments and for furniture repair, but 50 years ago or more (before PVAs were big), hide glue was the major adhesive for furniture manufacturing. In fact, the Egyptians were using hide glue 3,000 years ago.
Today, there are two types of hide glue: liquid and hot. Both types of glue create very strong joints. The liquid hide glues are ready to use and do not require any special tools or treatment. Hot hide glues require care in mixing with water, require the glue to be heated in a container (called a glue pot) to about 145 F (never go above 145F), and have a risk of bacterial and fungal growth after a day or so in the pot (so tools and the pot must be cleaned daily). Once mixed, excessive hot hide glue can be frozen and stored for long time periods. In short, hot hide glues do require specialized knowledge in how to use them properly.
Hot hide glues come in different strengths. A "gram strength" of 251 is commonly suggested for wood, but some folks like 315. (Be careful about using hide glues without a strength specification.) The higher the gram strength, the quicker the set. The higher the number, the stronger the adhesive. Proportions of glue and water when mixing will vary with the gram strength of the glue; never add more water than called for. (The old wives’ tale about hide glues being weak may reflect the situation when strengths under 251 were used. For many hide joints, the wood will fail before the glue does.)
Hot hide glue sets or cures initially by gelling as it cools (about 95F to 110F). Heated wood or hot air will extend the time before setting begins. Further hardening and strength development occur in both hide glues by evaporation. With both types of glue, the joints need to be clamped (excess glue squeezed out) and pieces held stationary before any curing begins. In some cases and formulations, curing may be delayed for 30 minutes. Even so, it is advisable to position the piece to be glued together within a few minutes with only slight movement (not opening the joint up to air) permitted until curing begins. If the glue has developed a "skin" on it, it is too late; remove the adhesive and start over.
As with almost all adhesives, the surfaces to be joined must be well-fitted (hide glues do bridge a gap a bit better than most other glues, however), clean and freshly prepared. Special note: When repairing a loose joint, it is possible to drill a small hole and inject glue into the joint rather than taking the joint completely apart first. This would also be the repair process for mortise and tenon joints that have only a few spots that are loose.
Hide glue joints are somewhat brittle and will break more easily on shock impact, compared to PVAs. Both these properties allow old joints in furniture, musical instruments, and the like to be totally disassembled, if necessary for repairing, without damaging the wood or finish. Small joints can be heated in an oven briefly to soften the adhesive.
Liquid hide glue or hot glue both have expiration dates that must be adhered to. Once mixed, hot glue also must be used promptly, unless stored in a freezer.
A stainable filler can be made by simply mixing the hide glue (both hot and liquid) with very fine sawdust, the finer the better) It is best to work with a light colored sawdust like maple as it is easier to color a fill darker that it is to lighten it.