I was thinking about the glues people use to sinew bows. Hide glue is widely regarded as the best glue for it, and there are two main reasons usually given:
1. It's made of basically the same stuff as sinew, so it combines with it and forms a strong matrix of cohesive material.
2. It shrinks when it dries and pulls the bow in to reflex, pretensioning the back.
But a number of people have tried with something like Titebond III for its waterproof qualities, and most seem to report that it works fine and they get a good bow out of it.
If Titebond III soaks in and adheres well enough, why does the cohesive matrix matter? And if you manually add reflex while you're gluing up, why does the shrinking induced reflex matter?
I hypothesized there is a 3rd reason hide glue is the best. So I decided to do an experiment. I used Knox gelatin glue instead of hide glue, as its easier for me to obtain, but the results should be about the same. It's all fairly unscientific, but I think you guys might find the results interesting.
I got 2 equal lengths of flax string. Soaked them in water, squeezed out the water, soaked one in Titebond III and the other in Knox glue, then squeezed out the excess glue. I hung them up to dry and twisted them up with themselves.
The dark one is Knox glue, the light one is Titebond III (the color difference is only from the wet glues).
They both took 2 days to dry and to stop losing weight. I waited 5 days just to be sure. They had hardened and stiffened nicely, and were both a lot like long twigs. Before the glue, each string weighed 3g. After the glue dried, the Titebond III string weighed 8g, and the Knox string weighed 11g. So Knox is considerably heavier, which is bad.
However... here are some bend tests. The first bend in the video is the first bend ever. Hide glue is on top.
https://imgur.com/a/JaBN0i5I was just eyeballing the amount of bend, trying to bend them each equally progressively more each time. I hope the difference in spring back comes through in the video, it was quite evident in person. But what you can't see is how much stiffer the Knox string was. It took considerably more force to bend. And note the difference in the set they took. In light of this, I will be using hide glue for any backing with which the glue soaks in and makes up a significant part of its structure (sinew, paper, cloth, cordage, plant fibers). Mechanically, using Titebond III seems rather like using perpetually green wood as a backing.
