There are lots of tests for when your liquid hide glue has gone bad – the most common one that I know of is to put some glue between your index finger and thumb. Tap your finger and thumb repeatedly to see if the glue turns tacky and produces long stringy strands. If it does, then your glue is good.
I’ve done this test with glue during woodworking classes all over the world, and I once had the glue fail the finger-tapping test. And we didn’t have any other glue available at the school. Instead of freaking out (externally) I made a test joint with the “bad” glue to double-check my results. The “bad” glue set up nice and hard like it was supposed to.
Since that class about five years ago, I’ve been suspect of the finger-tapping test and have wanted a more reliable barometer.
The glue scientists at Franklin International gave me two tests – one slow and one fast. Both work brilliantly. I’ll share the slow one here. The fast one, which takes about 15 minutes at most – will be in an upcoming issue of Popular Woodworking Magazine (again, apologies for wanting to eat).
Here’s the “slow” test: Take three blocks of wood that each measure 3/4” x 3/4” x 2”. Glue them together in a staggered fashion as shown in the photo above. Clamp them normally and let the glue sit overnight. The next day, hit the center block with a hammer (or squeeze the assembly in a vise) until the thing breaks.
If the wood fails, the glue is good. If the glue fails, the glue is bad.
The above test was done with liquid hide glue that was one year past its expiration date. If you want to read about how to extend the shelf life of your liquid hide, go here.
— Christopher Schwarz