The hot dish on hide glue
Imagine a scrap heap of once-great ideas and advancements, now tossed aside to fade into oblivion. Cassette tapes are over there, rotary-dial telephones down in front, and on top: tube tires for cars, the telegraph, and hide glue. Not so fast! Don't throw out hide glue just yet.
Even though it's been holding wood together for more than 5,000 years with little change in its formula—yes, it's still made from the collagen in cattle hides—hide glue still has a place in 21st-century woodworking. And, it has a few advantages that Johnny-come-lately adhesives can't offer. But, in all fairness to glues that come ready to use, hot hide glue does require more prep time and labor.
Why seek hide?
Furniture built before the 1920s was typically joined with hide glue because that's all craftsmen had. And that furniture's longevity testifies to hide glue's ability to hold wood together. But modern glues hold well, too, so why should you use hide glue?
As you can see from the list below, hide glue has several advantages over synthetic yellow and white PVA glues and polyurethane glues. Should you ever need to take apart a joint—for example, loose chair legs and stretchers—simply soften the hide glue in the joints with warm water and lightly tap them apart with a mallet. For this reason, luthiers (makers of acoustic wooden musical instruments) use primarily hide glue when crafting their guitars and violins. Because of the stress and wear from vigorous playing, these instruments often need to be taken apart to repair internal components.
Here's another of hide glue's unique properties: It bonds to itself. You don't need to clean out old hide glue from a joint—which you must do with synthetics. Instead, the new hide glue you add to the joint dissolves the old glue and bonds with the wood. This proves especially helpful in hard-to-reach mortises and beneath buckled veneer.
Because hide glue comes in different grades (known as gram strengths), it also varies in open times. This allows you to pick the glue you want for each job, from almost instant tack (grade 379, for marquetry, intarsia, or hard-to-clamp pieces, where a minute of hand pressure does the trick) to as long as 3 minutes (135, for face frames or carcases with many parts). We suggest starting with grade 192, with about 2 minutes of open time, until you become familiar with hide glue and feel comfortable with quicker-setting grades.
And, a fully dried hide-glue joint packs the same strength and rack resistance as a similar joint bonded with synthetic glues. So there's no sacrifice in using hide glue.
The sticky truth about hide glue
■ It is reversible, allowing disassembly of joints without damaging the wood.
■ Bonds to old hide glue when regluing joints.
■ As strong as modern-day synthetic yellow, white, and polyurethane glues.
■ Multiple grades of glue provide progressive open times.
■ You can extend its open time by adding salt. (See "Preparing Hide Glue," below.)
■ Glued joints don't creep (move after the glue has dried).
■ Dry hide glue has unlimited shelf life; mixed glue can be reheated.
■ Cleans up with warm water.
■ Water-based finishes could soften hide-glue joints.
■ Must be mixed exactly to manufacturer specifications for different open times.
■ Preparation time required (mixing, heating) before glue-up.
■ High humidity can weaken joints made with liquid hide glue (not hot hide glue).
■ Temperatures of the glue (135°–145°F), your shop, and workpieces (above 50° for both, but warmer is better) need to be monitored for optimum glue-up conditions. Chilly boards suck the open time out of hot hide glue.
Preparing hot hide glue
Hide glue comes in three dry forms: ground, flakes, and pearls, as shown above. They're the same glue once melted, and they cost the same. The only discernible difference: Ground glue, our choice, needs less time to soak before heating (about 30 minutes), while flakes require about an hour. Pearls can take 8 to 10 hours.
Mix the dry glue with cold water according to the manufacturer's instructions, then allow it to soak. When it achieves a lumpy, gelatinous consistency, heat the mixture in a double-jacket glue pot, above. Or, put the mixture in a glass jar, then set it in a hot pot (with variable temperature controls) filled with water, below. With both methods, maintain the glue temperature at 135°–145°F. Anything hotter over several hours will reduce the glue's strength. Properly prepared hot hide glue should have the consistency of maple syrup.
Add table salt to increase the open time of a batch. Determining the right amount of salt depends on how much open time you want. An amount that equals 5% of the dry glue weight gives you an extra 3 to 4 minutes before the glue begins to set. Add up to 20% of the dry glue weight for 15 to 18 minutes of open time, and your hide glue will remain liquid at room temperature, just as with synthetic glues. If you're working with small batches of glue, just add a pinch of salt and try it.
Apply glue while it's hot
Hide glue must be applied in a liquid form, within the 135°–145° range. Spread it with any style of bristle brush wide enough to make quick work of it. Because your shop and workpieces will be about half that temperature, the glue will begin to gel quickly. Don't worry. Try it on a few test pieces first to get a feel for the set time.
Brush glue onto both surfaces to be joined, then rub them together, if possible, to work out any air bubbles and create a tight bond. Clamp the assembly, just as you would with synthetic glues, and then allow it to dry. Wipe off squeeze-out with a wet cloth.
Store leftover glue in an airtight container in your refrigerator, and it will be good to use again and again. Simply reheat it—don't add more water—and it will revert to a liquid. If stored glue becomes watery or moldy, throw it away and mix up a new batch.
Hide glue: Dry hide glue, $12–$18/lb or $50–$55/5 lbs plus shipping, Bjorn Industries, 704-953-2026.
Take a shortcut with liquid hide
Like the idea of hide glue but don't want to fuss with a hot pot and mixing ratios? Titebond's premixed Genuine Hide Glue comes ready to use, and works anywhere you'd use hot hide glue. It bonds with old hide glue to make furniture repairs, and it softens when wetted for disassembly. But with an open time of about 20 minutes, this glue won't give you the quick tack of high-grade hot hide glues. Bob Behnke of Franklin International (maker of Titebond) says its hide glue has a shelf life of 2 years and should be stored and used at room temperature.