Hot Hide Glue
By Craig Brougher
|I have learned some things regarding hot hide glue recently that I thought I'd pass along.|
I buy my hide glue in 100# bulk from Milligan and Higgins. Up until now, I had assumed that the quality of all hide glue was generally mediocre, but usable. It never really passed the cold water test I have made on samples, assuming that was the best available. Here is what I would do:
I take a tall tumbler, as narrow as possible, and dump a small quantity of glue crystals into it. You can weigh them if you wish. I like to fill the bottom 1/5th with dry crystals. Then I add cold water to it and wait, overall, twelve or more hours. But within a few hours' time, the proteins and fats in the glue are beginning to separate. The poorer the quality of glue for woodworking, the weaker it will be. You tell this by both how fast it separates as well as the degree it ultimately separates. You will have a lot of "slime" above the glue lump in the bottom after twelve hours or more. Pour it all off, and weigh the lump in the bottom. The best glues will take up 3-5 times their weight in water and will be a solid and cohesive mass that has to be "scraped" out of the glass. [So it is better to weigh the dry glue and glass together to begin with, after you have first weighed the dry glass, and subtract.]
The lighter proteins in glue float at higher levels in the water. The lighter the protein, the weaker the glue. Very light colored glues, called "high clarity" or "technical gelatin" are usually weaker glues as well. The lighter color does not indicate a "pure form" of hide glue. So unless you are planning on flavoring it vanilla, I wouldn't look at crystal color as an indication. Granted, these glues are still strong enough for player work, but if you ever had to repair a broken Knabe grand lid, for example, I wouldn't recommend them.
The bad smell from glue comes from the putrefaction of fats still in these glues. You will find that the better the glue tests, the smaller the percentage of fats and the longer its pot life will be.
The stronger the glue, the more viscous or solid the batch becomes when equal amounts of water are added and heated up. As a matter of fact, their test of glue strength involves only a viscosity measurement because it is directly proportional. The number of grams required to depress a plunger down into a quantity of glue in a measured cylinder directly determines the percentage of heavy proteins it contains, and these determine the strength of the glue.
I had to test 6 different varieties of hot hide glue before I found one that came up to my specifications. Almost all of the commercially available hide glue crystals are less than satisfactory to me, now that I have found the particular batch of glue I really like. However, the stronger the glue, the more critical it is to use. It sets faster. It is the lightweight proteins that add the latitude which neophytes think they need. But I'll show you another way.
Hot hide glue can be made stronger than any other kind of wood glue, as a rule, although many varieties are not. It chemically and mechanically binds to wood, whereas the other glues, as a rule, are mechanical bonds. Once it drys, it does not "creep." So constant pressure against a joint will not cause it to give up in time. This is not true of carpenter glues.
It sets by gelling, thus you get a 100% joint coverage without voids when applied correctly. By adding salts such as urea or "preferably" potassium chloride to the glue, you can make it do about anything you want, and will increase its setting time, modify its brittleness if you wish, and even make it totally waterproof if you need it to be. Depending on the quantity of KCl you add. [KCl can be purchased from companies who sell it to put down on icy streets, cheap.] Other additives will cause it to bond with metals. It is a most versatile glue, in that you are able to add water to get the consistency you want. If you get a "cold joint" you just reheat the joint with an iron or heat gun and keep it tight until it gells again. You seldom require clamps with hot hide glue because of its "death grip" it gets on the parts once it sets. After that, it draws them together itself as it drys out.
So by adding, first of all, more water, you get the slower setting time again. And if you are gluing pneumatics, if you just size the shelves first, you will improve the strength and seizing characteristics. Just don't keep sliding the pneumatic around to position it. Get it about right and then weight it down quickly while the joint is warm.
If a hot glue joint is moved while it is gelling, it becomes a cold joint, just like solder. You aren't able to tell that the next day. It won't break for quite a while, but then it will break, finally, usually a few weeks later. However, the strong hide glue has less chance of that, and once you learn to be gentle and not disturb drying pneumatics, you will never have an instance of that.
The only way of making carpenter glues airtight is by filleting a bead of it around each pneumatic. By then, you have created something so impregnable you can never repair it again. It will have to be remade. It also looks sloppy. The era of Elmers is over. It is now the age of yellow glues which are truly and ghastly permanent.
The only glue, other than hot hide glue that you should use for wood bonding in players is liquid hide glue. And by sizing both pieces first before the bond is made, it should work pretty well. You can also make your own liquid hide glue, but that's another story.
The important thing to remember is that there are as many differences between hide glues as there are player pianos. They vary widely. Each has an optimum purpose, I suppose, but give me the strong stuff, and from there, whatever I need, I will make. Without the heavy proteins, nothing else you do to it will improve it much.
The present batch is 2XA hide glue, lot 9563 from Milligan and Higgins. Now, that's some real glue.
(Message sent Sat 20 Apr 1996, 14:18:26 GMT, from time zone GMT.)