Glue Fight: Hide vs. PU

There is a lot of talk about hide glue and the great feeling it provides. I’m always skeptical of this kind of claims so I decided to test this myself by building two exact blades, with the only variable being the glue used. First I will give a brief description of these two glues:

Hide Glue: This is an animal protein based glue, which is water soluble. Because of this it is vastly used in instrument building and repair. The ease of reversibility allows luthiers to perform repairs without damaging the instrument by simply injecting moisture into the joint. The glue doesn’t penetrate the wood, instead it forms a layer over the material and bonds to it. This glue is hard to work with because it has to be applied while still hot. Because of this it has a very short open time, and if the layers aren’t clamped within this time frame a poor bond can be formed.

PU Glue: Polyurethane glue is a polymer based adhesive with great strength. It is waterproof once cured and it penetrates the pores of the wood forming a great bond. It is easier to work with because it has a relatively high open time.

Building procedure:

I decided to perform this test with a 5 ply composition similar to the perhaps most famous hide glue blade out there, the Nittaku Acoustic. But instead of using a Limba medial ply, I chose to use a harder but thinner Sycamore Maple layer. So, the composition is Limba / Sycamore / Kiri / Sycamore / Limba.

All the plies were carefully selected so that the weight before the glue was applied was the same. The method for applying the glue was the same on both blades, a layer of glue on both sides to be glued, and the blades were clamped at the same time, using the same pressure.

The weight difference after the plies were glued and the shape was 0.6g, with the PU blade being the heavier. It wasn’t possible to keep the handles with the same exact weight but the difference was very small. The final weight of the blades was 89.5g for both.

Results:

These were my findings after testing both blades. The first difference I noticed was on the pitch while bouncing a ball on the bare blades. The Hide glue blade possessed a lower pitch, which was corroborated by the measured peak frequency. The Hide glue blade measured 1205 Hz while the PU measured 1227 Hz, a small but noticeable difference. This immediately told me the PU blade would be a little harder and stiffer. These were also the impressions I got during play. The Hide glue blade provides a slightly softer feeling, while the PU feels just a little harder and faster.

Conclusion:

So, is there a difference? Short answer: Yes! But is one better than the other? Not necessarily…

I have a preference for softer blades so in this case my vote goes for the Hide glue blade, but I’m sure that a lot of people would prefer the more solid and direct feeling of the PU blade. This decision is very subjective and would depend on the player. Keep in mind that these differences were very small, and in a blind test some might not even notice the difference. There is also the matter of the amount of glue used. With hide glue a layer on each side is needed to have a perfect bond, but with PU glue I normally just need one layer to have a strong bond. So in the case of this test I used more glue than needed, which exacerbated the feeling. In retrospect, I should’ve built a third blade using this method...