Reversibility and quick tack make this traditional method worth the trouble – sometimes.
In my last story (April 2007, issue #161) I discussed regluing doweled chairs. The chair I used for illustration was made in the 1920s or early 30s. As with virtually all furniture made before white glue was introduced in the 1950s, this chair had been glued originally with hot animal hide glue.
Because hot hide glue is more difficult to use than modern adhesives, it’s not likely you use it for your projects. I’m not advocating you switch to hide glue, at least for new woodworking. The glue takes getting used to, which means using it regularly for a while. Hot hide glue is very useful in restoration, however.
Even if you don’t use hide glue, you may still be interested in it, just as you are probably interested in old methods of woodworking. Following is an introduction to hide glue.
What is Hide Glue?
Animal hide glue is made by decomposing the protein, or collagen, from animal hides. Almost any hide can be used, including horsehides (“take the horse to the glue factory”). In modern times, however, cowhides are universally used. In fact, hide glue factories are commonly located near tanneries.
The hides are washed and soaked in lime for up to a month to break down the collagen. After being neutralized with an acid, the hides are heated in water to extract the glue. The glue is then dried and ground into Grape-Nuts-sized granules, which is the form in which it is usually sold.
The heating process is done a number of times to extract all the glue. Each subsequent extraction produces a lower grade of glue. The grades are measured by how much weight, in grams, it takes to make a dent in a 12.5 percent solution of gelled glue a given amount.
Standard grades range from a high of 512 grams, called “512 gram-strength glue,” to a low of 85 grams. The most common glue used in woodworking and furniture restoration is 192-gram strength. The most common used by musical instrument makers is 251-gram strength. Higher gram strengths gel too quickly to be useful in woodworking.
Animal bones, or at least the sinew in the joints, can also be used to make glue. But “bone” glue is greasy, more odorous, and has less tacking strength than hide glue, so it is not a good choice. Contrary to what you often read, horns and hoofs are never used because they don’t contain collagen.
Hide glue has an unfortunate reputation of being weak and not water resistant.
As just explained, the strength of hide glue varies depending on the extraction, and also on what additives may be included (for example, in liquid hide glue, discussed on page 85). But all hide glues sold for woodworking create a bond stronger than the wood itself as long as the wood is clean and the parts fit snugly. No more strength than this is needed.
It’s true that hide glue isn’t very water resistant, but this is more an advantage than disadvantage because it allows repairs to be done more easily. Poor water resistance is rarely a problem with wooden objects intended for indoor use, anyway.
Proof that less strength and water resistance compared to most modern adhesives aren’t serious problems is the survival of so much furniture made before the 1950s.
(Actually, I think the bad reputation began in the 1950s when manufacturers claimed superior strength and water resistance for their white and yellow glues to get craftsmen to switch. These non-issue claims are still used today by suppliers of even stronger and more water-resistant adhesives.)
Disadvantages and Advantages
Nevertheless, hide glue still has a number of disadvantages compared to modern adhesives. Hot hide glue has to be prepared in advance and applied hot. It sets up too fast for relaxed assembly and has a relatively short pot life (several weeks at room temperature) before it begins rotting and losing strength. So there is usually a lot of waste. In addition, and not least important, hot animal hide glue has an aroma many find unpleasant.
But the glue has two unique advantages over all other adhesives: reversibility and quick tack. Both are far more useful for repairing old furniture glued originally with hide glue than for making new furniture.
I don’t buy the argument that you should use hide glue on new furniture so it can be repaired more easily decades from now. It’s rare that today’s repairmen recognize the glue, so what are the chances tomorrow’s will? So there’s little likelihood they will take advantage of hide glue’s easier repairability. The best argument for using hide glue is to stay true to the original when making reproductions.
Reversibility is the quality that makes redissolving possible after the glue has dried. It allows you to reglue loose joints without first having to remove the old glue (to create clean wood). Simply remove any loose or powdered glue, apply fresh hot hide glue, and reassemble. The moist heat of the new glue dissolves the old glue and the two combine to create a strong bond.
Though it’s fairly easy to remove white and yellow glues by soaking and scrubbing with hot water or vinegar, no freshly applied glue redissolves into these glues. Other adhesives, including polyurethane, epoxy, plastic-resin and cyanoacrylate, can’t be broken down. They have to be scraped off which can’t help but remove some of the wood, resulting in loose-fitting joints.
Reversibility also makes cleaning hands and removing glue seepage from newly assembled joints a relatively quick and easy task. Simply rinse or wipe with hot water. And it makes separation of sound glue joints possible by injecting hot water, steam, vinegar or denatured alcohol (which crystallizes old hide glue).
Unlike other adhesives, hot animal hide glue bonds in two steps. An initial tacking occurs when the glue cools from its application temperature of 140-150° Fahrenheit to about 95°F. The bond becomes complete when the water evaporates out of the glue.
The initial tack allows you to glue two pieces of wood together without clamps. Simply apply the glue to both surfaces and rub the pieces together to work out the excess glue. As the glue cools, it gels and the pieces begin to stick. Position them correctly and let the glue dry to complete the bond.
As long as the pieces are not forced apart while the glue is still in the gel state, the tack is strong enough to create a good bond. On the other hand, if you don’t like the positioning of the pieces, you can separate them for some time before the bond becomes too strong.
The glue blocks you see behind legs and on the inside corners of old case furniture were positioned with this rubbing technique, called a “rub joint.” It’s not uncommon to see glue blocks still sound after 200 years, especially if the grain of both parts runs parallel.
You can use a rub joint to replace broken pieces on carvings, small pieces of veneer, and other parts that would be difficult to clamp. You can also strengthen case furniture by regluing old glue blocks or by inserting new ones. Just be sure to remove chunks of old glue to create room for rubbing, and remember that the surfaces have to fit snugly for a good bond. You can’t glue air spaces.
Yellow glue offers some degree of tack, so you can sometimes perform these tasks with this glue. But the tack is much weaker, so the slightest pressure during drying will move the parts and destroy the bond.
The technique of “hammer veneering,” which is used to apply veneer using hot hide glue, is a form of rub joint. (For in-depth information on hammer veneering, see “Old School Veneering,” issue #162.)
Dealing With Rapid Tacking
Rapid tacking is an advantage when performing rub joints, but a problem when gluing up furniture with numerous joints – especially in a cold shop. The easy solution is to warm the wood beforehand to retard the gelling.
In the old days, cabinetmakers stacked furniture parts next to the shop stove to warm them. Factories made a “steam room” with temperatures as warm as 120°F for gluing up. With all parts together, the assembled pieces would be moved to a cooler room so the glue could harden.
Neither of these methods is feasible in most modern shops, but we have electric blow driers and heat lamps not available to our ancestors. I have used a combination of heating devices countless times when my shop was cold to create enough time to get all the parts together. Also helpful are laying the parts out logically and dividing complex objects, such as chairs and case furniture, into sections, assembling each separately before assembling the entire unit.
Keep in mind that gelling, which can cause a thick glue line on parts clamped edge to edge, is less of a problem in housed joints. As a tenon or dowel is slid into the mortise or hole, or as dovetails are assembled, the gelled “skin” on the surface of the glue is removed, exposing still-liquid glue underneath for some time.
Liquid Hide Glue
Franklin, the manufacturer of Titebond, makes a widely available product called “liquid hide glue.” A similar product labeled “Old Brown Glue” is available at wpatrickedwards. com. Both are the same as hot hide glue, but with gel depressants added to keep them liquid at room temperature and preservatives added to retard deterioration for about a year. These glues redissolve just like hot hide glue, but they don’t have an initial tack and don’t dissolve well into old hide glue unless applied hot.
On the other hand, these glues are ready to use and have a long “open” time, even longer than white glue. They may be a good choice when there are many parts to be glued simultaneously, such as on Windsor chairs. PW
Preparing Hide Glue
Most hide glue is sold in granular form. To prepare the granules for use, soak the amount you need in water for about 10 minutes, or until they soak up the water and become mushy (like Grape-Nuts in a bowl of milk). Then heat the glue to about 140-150°F so it becomes liquid.
If you use hide glue often, an electric glue pot is worth buying. Otherwise, you can place a jar of the glue in water placed over a heat source (creating a double boiler situation), or heat the glue in a microwave. I know people who use a Crock-Pot.
The ratio of water to glue depends on the gram strength of the glue. For 192-gram strength, I find the ratio of 2-2⁄3 cups of water to one pound of glue works well. You want the glue to be about the viscosity of yellow glue. You can always add water or more saturated granules to adjust the viscosity of the glue.
For your first batch, I suggest you use 1⁄4 lb. of glue and 2⁄3 cup of water so you have less waste. Or reduce both amounts, keeping to the same ratio, if your needs are less for the job at hand.
If you leave the glue heating for a while, a lighter colored skin will form at the surface due to contact with cooler air. The first time you heat the glue, this skin may contain some dirt or other foreign matter. It’s a good (but not absolutely necessary) practice to remove the majority of this skin from the glue pot, using a stirring stick or brush and throw it away.
From then on you can stir the skin back into the glue. Now and then, you’ll have to add water to replace evaporation. To slow evaporation and the thickening of the glue, keep the container covered.
Hide glue is organic material, so it rots in time. The aroma becomes stronger and the strength, or tack, of the glue weakens. Eventually, mold will form on the surface of the glue. To retard this natural deterioration, store the glue in a cool place, such as a refrigerator. Be sure to cover the glue tightly so it doesn’t dry out. — BF
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