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Q. I have been told that old hide glue joints are reuseable. I took one apart with gentle steaming, but when I decided to just add moisture, heat, and put it back together again, it didn't hold. I had to make a new pot of glue just to fix that joint. Are there differences in hide glues to account for this, or is this just a common property?
A. This trick is useable only with new hot hide glue joints which have never reached their final phase of "fully cured bond." A full cure may take from several weeks or months, to several years. The amount of moisture, both in the wood itself when bonded, as well as the ability of the glue to rid itself of moisture is the determining factor here. The glue will reach full strength long before all its moisture has dried out, and it will seem to be as hard as a rock, but now comes the long term "mummification of the joint" (my term). So joints which are only a few weeks to a month old, and in wood with a higher moisture content are sometimes easily reglued with their own glue.
Joints that are (usually) 6 months old or older are seldom recoverable that way, but it just depends. In many cases, the wood has drawn up the glue into portions of its end grain, so the joint will hold the first time, but reheating and gelling again will still leave a weak joint. Bottom line: This is not an acceptable practice in good joinery. make your joints right the first time, and if you have to change them, plan on adding fresh glue. The excess rags off easily and wipes clean from finishes without staining.
Q. We have been reading that hot hide glue is slightly flexible. You say it doesn't move at all. But we tested a glue joint that we could reheat and move around, so that must mean that it isn't permanently rock hard after all. We have also seen hide glue gradually creep, just like PVAs. And an old woodworker has told us they will, too. So we agree with those who think it is thermoplastic, and that it creeps, just like any other glue would.
A. There are plenty of "old woodworkers" with strong opinions, anyway. Few woodworkers directly test things to see for themselves. They go by seat-of-the-pants experience, and the fewer facts some of them have, the stronger their opinion becomes. However, hot hide glue will "creep" until fully hard and dry. And you will be able to reheat a dry joint and move it again, because there is anywhere from 5-10% moisture in the wood. But what allows this is the reactivation of water into the glue itself. The glue must be "wet" again with water, so heating the joint steams and reactivates the moisture, and the combination of new dampness and heat then softens the joint.
Hot hide glue joints which are "old," and fully cured, without new moisture added first, do not melt in wood, so they will not move, until the temperature gets over 425 degrees F. At that point, it softens, but quickly turns black and then disintegrates completely. Also, re-adjusted joints, even when re-wet and heated, will have only half their original strength-- or less. This is usually still enough, because even at that, they are stronger than PVA under any circumstances, but if you want the full strength again, add new glue.
Regarding elasticity however, it is possible to make hot hide glue elastic by adding glycerin, but it also weakens the glue, as well. Occasionally though, you will need elasticity, as when assembling a tambour door. KCl and potash are two other chemicals that will prevent the glue from getting overly brittle and crazing over time, but to tell someone how much of these chemicals to add is a waste of time, because of the vast differences in gram strengths of glue. Experiment with small quantities first and measure by weight. Generally speaking, any additive that improves the flow-out time and slows the setting also softens the glue a bit in its final phase.
This authority on hot hide glue comes not from resellers, ghurus, or users, but from the technical director of the manufacturer of hot hide glue himself, Jacob Utzig, who has written a very good article on hide glue when he was technical director for Hudson Glue Corp, now Milligan and Higgins.
Q. Is it true you can make hot hide glue completely waterproof?
I don't know, from first-hand experience because I have never done it or had a need to. But Henley's Formulas For the Home Workshop contains several recipes for waterproofing hot hide glue, and claims that it renders the glue totally waterproof. One of the easier chemicals to experiment with would probably be alum, available from the grocery store spice shelf, if nothing else.
The warning is not to let the glue dry in the pot, because you will never get it out again.
This page was last revised on March 10, 2015 by John A. Tuttle.
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