1. HOT HIDE GLUE
Hot hide glue is one of the earliest adhesives used by mankind. It was used, among other
things, for gluing wood together, and there are many ancient examples of this that have
stood the test of time. Surprisingly enough, these examples also have the ability to
reconstitute back into liquid form by soaking for a time in water! That is one of the
distinctions of this glue. Although I have heard that other wood glues can be reversed,
and have no doubt they can be, they are not as readily, cleanly and easily reversible as hot
hide is. But there is another important difference here. With other glues, residue must be
thoroughly cleaned from the wood surfaces before attempting to re-glue, or you will risk
a poor glue joint. Not so with hot hide. After initially wetting the old glue (preferably
with hot water), it has the ability to reconstitute the old glue with itself, and as long as the
old glue does not interfere with the surfaces from properly mating, will provide a good
strong joint again.
Bonding Forces
Hot hide mechanically bonds the materials to be joined. This means that the glue
penetrates into the joined pieces of wood for an airtight bond. It is not necessary to
roughen up surfaces if you are using aliphatic resin glues, or some other chemically
produced glue such as Elmer’s white glue. Roughing up the surfaces when using hide
glue however, is recommended and will improve bonding from 5% to 30% (Hide Glue by
Stephen Shepherd pp. 12) and is a primary reason why bonding occurs.
Bonding is also accomplished by diffusion of the materials being glued at a
macromolecular level. This is important because it means that the materials being glued
must be soluble with the glue on that macromolecular level. This will explain why hide
glue doesn’t adhere very well to plastics or metals.
While yellow and white glues depend mainly on chemical bonding for adhesion, there is
also a chemical bonding which takes place with hide glue, namely, between the hydrogen
and oxygen atoms of the materials being glued and the hide glue. This naturally adds to
the strength of the hide glue joint.
Polymers, or chains of protein molecules, are the strength of the glue. The longer the
chains are, the stronger the glue is. Subsequent re-heatings of the glue, little by little,
break down these chains and thus weaken its strength. Therefore, after several re-
heatings, the glue may be too weak for certain applications. The ingredient in cold hide
glue that keeps it liquid at room temperature does the same thing to those chains over
time. That is why there is a 1 year shelf life to cold hide. However, keeping it cool or
freezing it will extend its shelf life.
General Principles of Hide Glue Usage
Hide glue must not be heated to over 145 degrees F, or those chains mentioned above
will be shortened, and thus the glue spoiled. The usual way hide glue gets heated is by
some kind of a water bath set up. I use a glue pot which I got at one of the supply houses,
fill the sleeve part way with water and place a glass jar or a squeeze bottle applicator

2. filled with glue, in the water, but you can get on www.spurlocktools.com to find out how
you can make your own hot pot to save some scratch. The temperature of the water
would be somewhat hotter than the glue, usually 15 to 20 degrees more, so if, for
example, you want your glue to be 140 degrees, then the water would measure 155 to 160
degrees.
Now that you are set up to use hot hide glue, there are certain other principles to which
you must adhere. You must not let your glue gel before applying clamps to your work.
Once the glue gels, it is too late for clamping (or the caul, or whatever you are using to
clamp). The items will not bond unless they are already together and immobile when the
gelling begins. It is the gelling action that starts the bonding. There are a few reasons
that the gelling will start too early. One, the work on which you are applying the glue is
cold, and the glue is immediately beginning to gel. Two, you are taking too much time
applying the glue, or, you have waited too long putting the glued items together, or the
area you are covering with glue is too large (taking too much time). Three, the room
itself may be too cold. Each of these problems can be reasonably overcome.
If strength is not a primary concern, you can add a little salt or urea to slow down the
gelling time (5% to 15% by weight). These additives will affect the glue’s strength, but it
may not matter, depending on what you are gluing. Adding a little cold hide to the hot
hide will also slow gel time. Experiment a little to find out what will happen. You can
also heat up the parts, or pre-wet them with hot water. These two latter actions will slow
gelling somewhat; heating will not affect glue strength, but pre-wetting will slightly
weaken it, but probably not as much as the additives. Another idea is to heat up the
room. Back when hot hide glue was used exclusively to build pianos, they had ‘glue
rooms’ which were heated to over 95 degrees, and this is where they glued larger items. I
don’t mean to say crank up the heat to 95, but to at least 70 degrees. That at least will
help some.
Uses in Piano Work
I like to use hot hide glue for hammer hanging, any felt to wood application (particularly
damper felt, back rail felt, and key bushings), trapwork felt and leather, and glue sizing
various holes (such as key balance rail holes, and key frame front and balance rail holes,
when replacing pins, etc.), to name a few. I find fish glue to be very useful in gluing
understring felt to the plate, and re-gluing felt such as butt felt squares. Fish glue has an
immediate tack characteristic that is conducive to such re-gluing tasks such as butt felt
squares, and it seems to glue things to metal fairly well. I carry a small bottle of fish
glue, which you can get at Lee Valley (look it up on the internet) and Old Brown Glue
(which is a cold hide glue) in my repair case, and have used those exclusively now for
about a year. If you google “Old Brown Glue”, you will find out how to obtain it.