Hide GlueComments (0)
This article is from Issue 9 of Woodcraft Magazine.
One of the oldest adhesives known to man, hide glue is still widely available – for plenty of good reasons.
My neighbor is an interesting guy. He has kept a lot of stuff from when he was growing up in the years before World War II, such as a side table he built in his high school shop class. Of course, being a woodworker, I had to get on my knees and take a look underneath it to see how it was built. During my inspection, I was surprised to find evidence of something I had never seen before - hide glue. In today’s age of ready-to-use yellow glue and ultra-strong epoxy and polyurethane glues, I started to wonder – is there a good reason why hide glue is still around?
You bet there is, and this old-fashioned glue still has plenty of applications in today’s shop.
What is this stuff?
Hide glue is one of the oldest adhesives known to man. Evidence of hide glue can be found on furniture retrieved from the Egyptian pyramids, and very early hunters discovered that it was good for binding spear points and arrowheads to shafts for the hunt.
Hide glue is, in effect, very strong gelatin which is cooked out of animal hides and hooves. Since my second-favorite hobby is cooking, the process is easy for me to understand. To make a really good pot of chicken stock, all you have to do is simmer chicken bones in water for most of a day. After straining and cooling, the cold stock has a jellylike consistency.
What causes that? When animal bones or hides are cooked for a long time on low heat in a moist environment, the protein collagen found in the bones and connective tissue is converted into gelatin during the cooking process. It’s this gelatin that gives a rich feel to a bowl of chicken soup, or, if it’s really concentrated, can be used as glue.
Of course, ease of use is the hallmark of modern adhesives, and the knock on hide glue is that it is fussy to use. Nothing could be further from the truth.
Let's whip up a batch
It will take some time, some heat and a little bit of know-how before you can use hide glue. First, get yourself a supply of granular or pearl hide glue, which comes in the form of hard little pellets. This is the dried form of the glue, and the first thing you’ll have to do is soak it. Your best bet is to find a jar that can be used for canning; they can take the heat, and they have screw-on lids for storing your melted glue. A dozen of these can be yours for less than $10 at most grocery stores, or you can find old ones at yard sales.
Place an equal amount of hide glue pellets and cold water in the jar. Screw on the lid and let the mix sit overnight. When you come to the shop the next morning, you’ll be greeted by a sticky, gloppy mess. That’s good. It means that the glue has rehydrated, soaking up all the water you put in with the pellets. Now, to the heat.
Taking the heat
You’ll have to have some way to get the glue temperature up to about 140°. There are electric glue pots for sale that can keep glue at the proper working temperature all day long, but they can be a pricey investment up front – especially if you are just starting out.
For me, a hot plate and a pot of water act as a double boiler, providing even heating for my jar of glue. I carefully monitor the water temperature using an instant-read thermometer to ensure it doesn’t exceed 150°. Too much heat can break down the protein in the glue, making it useless. You can tell whether this has happened by rubbing a small amount between your index finger and thumb. Properly prepared glue will stick tenaciously after a few seconds of rubbing, and when you pull your fingers apart, you will see many protein “threads” form. However, you won’t feel the same tack from overcooked glue, and those threads will be a lot less impressive.
A few minutes on the heat and a few stirs, and the sticky mess becomes a thin syrup-like consistency that runs off your stirrer. Now it is ready to use right out of the jar to glue up your projects. If you can’t use your glue right away, you can keep it on the heat all day, or you can pour it off into a resealable plastic container and refrigerate it. If you do, the glue will stiffen up into a rubbery mass. From there, you can simply slice a piece off and melt it again in your double boiler.
The traditional way to apply hide glue is to brush it onto a surface. But if you want even more convenience, transfer it to a plastic squeeze bottle with a nozzle.
The first use I found for hide glue in my shop was applying a biscuit-joined drop molding onto some shelves. The water in the glue swelled the biscuits quickly, making the joint very tight before the clamps went on. The glue easily allowed a five-minute open time. And, had the glue gelled up, it wouldn’t have been a problem at all. Unlike yellow glues, you can simply apply more hot glue to reactivate the old stuff.
After an hour, the glue was cured enough to allow me to take the shelves out of the clamps – about the same amount of time yellow glue would need.
What's hide glue good for?
Well, besides general joinery, chair makers swear by the stuff. If a round stretcher glue joint fails, all that’s needed is a hot, damp towel, a few mallet whacks, and an application of new hot glue to reactivate the old stuff. If the chair was glued with any other type of adhesive, it would require a lot of sanding and scraping to remove the old residue before a repair could be attempted.
Hide glue also sands off projects very quickly. I’ve been known to accidentally touch a gluey hand to part of a project I am assembling, only discovering my misstep during finishing. Yellow glue is very hard to remove, while hide glue sands away without any residue. Once you stain or apply any type of finish, you’ll be pleasantly surprised just how easy it is to deal with glue squeeze-out.
When you apply hide glue, try rubbing the two pieces of wood together, and you will be surprised how quickly the glue will fix them to each other. This can be a great help if you are gluing together small parts. In fact, you can pretty much work without clamps on small glue-ups, unlike the clamping pressure you need to keep a slippery yellow-glue joint in place. This gripping quality is perfect for veneering, and, unlike some other more noxious glues such as contact cement, hide glue is non-toxic and doesn’t require ventilation and respirators.
Hide glue isn’t subject to creep. If you have ever peeled some dried yellow glue off the nozzle of a bottle, you’ll notice it can be bent and flexed. This plastic state is what allows more springback on cold bent laminations. Dried hide glue, on the other hand, is very brittle, making it better for freezing a bent project in place.
Of course, hide glue isn’t a miracle adhesive. It does have its drawbacks. Don’t use hide glue anywhere there will be moisture, as water can weaken the glue joint.
Like food, hide glue can go bad relatively quickly. If you just leave the glue out on the counter in the shop, it will begin to grow mold, which will lessen its adhesive qualities. You can get a few weeks if you refrigerate your batch, and maybe squeeze a few months out of it if you freeze. Be sure to reheat only small batches at a time instead of your entire pot. Excessive reheating can also reduce the strength of the glue.
Also, the stuff has a certain aroma. No, unlike the horror stories you might have heard about hide glue, it won’t stink you out of your shop. Think more along the lines of dirty socks than a skunk. Contact cements and some epoxies have much more pungent odors.
Now, will I throw away my yellow glue? No. But hide glue is something every woodworker should try at least once. Besides being a useful adhesive, it’s another way to hearken back to the techniques used by woodworkers who came before us.
— Tom Iovino is a woodworker from Largo, Fla.
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