Testing Liquid Hide Glue

by Bill Tindall

Liquid hide glue is a superb woodworking adhesive, but it decomposes over time, either by attack from bacteria and fungus or by chemical decomposition of the protein. A test to determine if liquid hide glue is still good to use is as follows: Rub some glue between your thumb and index finger until it begins to dry and becomes tacky, then pull thumb and finger apart. Good glue will form thin strings resembling spider webs connecting thumb and finger as they separate. What is happening is that glue fibers are being spun from the dissolved protein polymer. Spiders would be impressed.

For these stringy fibers to develop, the fiber molecules -- protein in this case -- must be tangled up with one another. As the glue is pulled apart, the tangled-up molecules stuck to your thumb and finger pull along the molecules that were in between, creating fibers connecting the thumb and finger.

To illustrate this concept, suppose you had a bucket full of long strands of yarn all tangled together. If you grab a few strands and pull them out of the bucket, the rest of the strands would follow because they're tangled together. However, if the strands of yarn were short, they would break loose from the rest.

This example is analogous to pulling a fiber made of long polymer molecules. As some of the molecules start to be pulled, they connect to the rest of the molecules by entanglement and the rest are pulled along making a fiber. Similarly, when you crack an egg and the egg white hangs in a long strand from the broken shell, it is being held together by the entangled long molecules of egg white protein. If it were composed of short molecules like water, it would just separate into drops. Tough glue has long, entangled molecules. Breaking the glue requires breaking these entangled molecules apart, and that takes a lot of force. So, any test of long molecules will show whether the glue is good or if it has decomposed into shorter, useless molecules.

Franklin suggests making a film from the hide glue and letting it dry. When dry the glue is good if the film is tough. If it is brittle, its molecules are too short and the glue is breaking between the molecules, and not breaking apart the entangled molecules themselves. The spinning fiber test is quicker to perform.

When you get a new bottle of glue, spin some fibers to get calibrated as to how good glue performs. Test periodically and if the fibers between the thumb and finger break prematurely, buy new glue.