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Facts About Hot Hide Glue
by Craig Brougher

There are certain basic things one must know if he is to use hot hide glue correctly-- for example, isn't it just unflavored jello? No.

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I have hired many different girls to help me in the years I have restored player pianos, and have discovered one fact: They all thought (initially) that they knew how to glue. The fact was, I knew I could teach them everything else, IF I could just teach them how to glue! That was the hardest part of my job.

Knox Gelatin is food grade gelatin, but is not made from animal hides at all. It is made from rendering cattle bones. That means, it is comprised of a particular group of very light proteins which are edible, and which, in turn, were never incorporated in good adhesives. Granted, it has been rendered "pure" edible grade gelatin and as such retains very few fats (greases). It is these fats which go rancid and cause the glue to stink as mold spores from the air begin to multiply in it.

All hot hide glue has some percentage of fat. It goes with the product-- even the so-called "technical grades." You just sprinkle a pinch of lye into your pot-- the fats are saponified and fall to the bottom, harmless, even when remixed. That takes care of the problem! Buy it at any grocery store. It is the proteins in the hides of animals which are the heavy, strong proteins required for glue!

Hot hide glue makes a chemical bond with (usually) natural fibers. However, there is a grade of glue that will even bond to glass. The weaker the glue's gram strength, the slower (usually) it tends to set up, and the wider tolerance it has for different materials and longer adjusting times. The particular grade used to stick to glass is still such a strong bond that as it dries and shrinks, it chips the glass it sticks to. This is how glass artists create the "chipped glass" effects found in art glass and fancy windows, yet today. Nothing else yet discovered can do the job as well.

Hot hide glues come in a variety of bond strengths, varying from perhaps 60 grams to about 600 gram. The variety that works best in player pianos varies between 120-180 gm. This gram strength is adjusted in the laboratory by relative viscosity measurements, and does not represent a physical test, directly. However, it does relate to a physical joint test perfectly.

The necessity of hot hide glue usage in player pianos is undebatable. Carpenter glues are totally unsuitable in player pianos for these fundamental three reasons:

1. Any glue which relies on the evaporation and absorption of moisture away from the joint to set and to eventually dry creates a "lacy pattern" in its joint structure, through which air is able to flow unless the joint is securely clamped for 24 hours. In which case, the air tightness will be better, but never perfect. It will rely strictly on a mechanical tightness and mechanical bond between the parts.

2. Carpenter glues cannot achieve maximum tightness by weighting the joint, unless the weight equals or exceeds a typical clamp! And clamping pneumatic joints cause them to "skate." Both the weight of the clamp on the side and the slight asymmetry of the joint area creates side forces which make perfectly aligned flat joints impossible. And because carpenter glues release their moisture into the flat, thin pneumatic boards being glued down-- long before the joint actually sets-- that moisture creates a slight warpage in many of the pneumatic leaves, which gradually warp and curl the wood, creating more skating and sliding around by the clamp pressure.

3. Carpenter glues are all "plastic" in that they have a property known as "creep" when used strictly for butt joints and flat joints. Unless the wood has been joined and the glue is used strictly to make the joinery permanent, carpenter glue is seldom permanent in any situation that exerts a constant shear force on the joint. Sooner or later, the joint "stretches" to the place that it will fail. For example, when valve blocks are glued with hot hide glue, they stay glued under any amount of spring tension used to clamp them down. But when the same blocks are glued by any carpenter glue, some of them will be found to have their pouch boards displaced from the body of the block about 1/16" in less than a year, simply because the block is pressed into the cork gasket and the pouch board, untouched by the spring, is sliding back from the compression on it by the cork.

There is a fourth reason you don't want to use carpenter glues, by the way. In the case of the yellow glues, they are difficult to get out of the wood. The joint, albeit a lousy one and a leaky one, will force you or the next person to build new parts! The advantage to these glues is obvious: Even an idiot can get as good a joint as an expert. The disadvantages are equally obvious: The joints are totally unsuitable, often inaccurate, cause warpage in some cases, and leak air. Except for those things and the fact that you cannot remove them, carpenter glues are just perfect-- for ruining player pianos.

There are 3 main reasons why hot hide glue is perfect, both for wood joinery and for butt and flat joints in player pianos and joints requiring perfect air-tightness.

1. Hot hide glue sets, not by water evaporation thresholds, but by cooling off a bit, and gelling! When this kind of glue sets, it gets a "death grip" on the parts, preventing them from sliding, so actually, only weighting is required, and just enough weight to squeeze excess glue out of the joint. The parts won't "skate." That allows one to find about a dozen lead weights of about 1 lb apiece (or somewhat less) and rotate them leapfrog through the entire range of pneumatics. Once the glue has set, the weight is no longer required.

2. Hot hide glue dries with a 100% chemical bond on the parts, creating a perfectly airtight seal, as long as the glue consistency (which is adjustable--another great advantage) is adequate and the joint in full contact when the parts were initially placed together. As with any glue, it is possible to create a leaky joint. But with hot hide glue, due to the gell setting factor, it is possible to make every joint as perfectly air tight as every other joint, while with carpenter glues, this is just not possible.

3. Hot hide glue becomes permanent and brittle. It has zero "creep." It exists in oriental furniture and chairs yet today, which still, after a thousand or more years of usage, have perfect joints. That, friends, is what anybody would call "great glue!" I have personally sat in a chair that was over 450 years old (at least) and was as solid as if it had been welded out of steel. It was built from carved teakwood, and glued up with animal hide hot glue. It was still as strong as the day it was made because its joint is a 100% filled chemical bond.

There is a fourth reason to use hot hide glue as well: It can be removed! Moisture or steam along with heat will eventually loosen animal glue, unless certain additives are used which are able to render animal glue waterproof! So, in the course of your rebuilding, you will very likely have to get back into some of your work. You should be comforted to know that with animal glue, you still can! With carpenter glues, you cannot.

To test your glue, your best test, of course, is to glue up 1" wide strips of wood with a 1" square flat joint between them. When fully dry, weight until the joint breaks, record your break strength. From that time on, you will know what you have with that particular batch of glue. Try thin and thick mixtures of it, too, and test to see if you are really getting the strength you need to have. Hot hide glue should be stronger than carpenter glues, as well. At least, it has the capacity to be stronger.

To test for glue quality before bonding, according to an article from the manufacturer:

use one ounce of glue* per pound of water**. Wait 12 hours for the glue to take up the water, fully. If the glue appears dissolved into the water, it is no good. If, when you pour off the water, the glue stays coherent to a degree, this is usable as glue. Now, weigh the mass of jelly glue. If it weighs 5 times or more its initial weight, it is excellent glue. And the solidity and coherency of the mass will tell you how strong it is going to be!
* One Ounce of Glue equals about 2 Tablespoons
** One Pound of Water equals about One Pint

You see, hot hide glue contains many different kinds of protein. The strongest and best proteins are also the heaviest and most cohesive, as you might expect. So when you start using your glue the next day, stir the pot, because your best and heaviest glue has settled to the bottom. Definitely not the stuff that Knox Gelatin is made of.

This glue article still leaves out information on the proper ways to use hot hide glue, but these are the basics.

Understand one very important fact about hot hide glue: It reminds one of using solder. The joint must be made hot, and after a quick, initial positioning, cannot be moved while the solder is going through its phase change to partially solidify, or you will have a "cold solder joint." It looks all right, but crystallizes and breaks loose.

So does hot hide glue, and that is why it takes a bit of getting used to. That is why I suggest cutting some scrap wood and playing with it. Write on the pieces you are doing, what it was that you did. Then break them off. If you cannot tell any difference between different methods, consistencies, and timing, you are not testing correctly.

As another test, be sure to include sizing a board with thinned down glue before gluing up. Sizing decreases the glue's latitude to be moved because it shortens the placement time before gelling and setting, so it greatly facilitates placing down pneumatics by speeding up the "death grip" if you have already a system for placing pneumatics accurately. Sizing also acts as a way of increasing the strength of your glue when used properly. So if you happen to have some cheap grade of hot hide glue, you can still use it if you size first.

One last note is, on any new wood in which end grain is to be glued to, as on pneumatic covers, BE SURE to size the end grain and let it dry before trying to glue anything to it.

Once you get accustomed to using hot hide glue, you will wonder how anybody could fall for bottled glues! That stuff is really a waste of good shelf space in a player shop.

Craig Brougher

Hot Hide Glue Questions & Answers
Hide Glue vs. Silicone for Rebuilding a Pianola by Ralph Nielsen, Ph.D. (polymer chemist)
E-Mail to: Craig Brougher
Phone No: 816-254-1693

Information supplied by The Smithsonian Institute:

Jane Woodall forwarded your question about hide glue to me. You should contact a conservator in the National Museum of American History's Preservation Services Office. They have done a lot of work with adhesives and can give you good advice. Conservation contacts are as follows:

Beth Richwine, Senior Objects Conservator, 202.357.1795,

Richard Barden, Senior Objects Conservator, 202.357.1795,

snail mail:
Smithsonian Institution
National Museum of American History
Preservation Services, Room ABo68
Washington, DC 20560-0642

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This page was last revised on March 15, 2014 by John A. Tuttle.
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