Hot Hide Glue is what musical instruments where built with for centuries, although synthetic adhesives have greatly reduced its use recently. It is easier to take apart, bonds to other glues, is very stable, you can apply it over itself or any other glue, and smells awful (it’s made from boiled animal hide). This is not the liquid hide glue sold in bottles. See below.
Alternatives:
Titebond wood glues (I, II & III) are a little harder to take apart, have longer open time, but tend to “creep” under high loads (such as attaching a bridge) and only glues to wood, not existing glues (surfaces must be free of old glue). The 3 glues are very close in strength. “I” is for interior use only, 5 minute open time. “II” is exterior, 5 minute open time. “III” is exterior, 10 minute open time. All 3 glues are PVA glues, PolyVinyl Acetate, a type of thermoplastic.
Titebond Liquid Hide Glue. Titebond LHG is about the strength as their regular wood glues, interior use only, 10 minute open time. Titebond LHG is a “natural protein emulsion”. It has a shelf life of 12-18 months if kept in a cool dry place. It does have some of the properties of hot Hide Glue. It releases with heat, and is generally stronger than wood it bonds. But, while it releases with heat, you cannot apply more Titebond LHG without removing all of the old glue.
Fish Glue is similar to hot Hide Glue, with the advantage of longer open time, but can be a little harder to take apart and the squeeze out is very sticky and harder to clean up. It’s also less tolerant of high humidity.
Titebond and Fish Glue just need to be applied, the parts assembled and clamped, and clean up the squeeze out. Hot Hide Glue must be mixed properly and applied “HOT” for it to correctly bond.
Items Required:
Granular Hide Glue. I got mine from Woodcraft. Behlen B940-00255 https://www.woodcraft.com/products/behlen-ground-hide-glue-1-lb $17
A teaspoon. To measure Hide Glue granules.
Small double boiler or “Hot Pot”. To heat the Hide Glue solution. I use a Sylvania model WM-6105, which is no longer available. The Sunbeam BVSBWH1001 looks to be functionally the same for about $20.
An 8 ounce Mason jar. Kerr 501, 2.63” dia. under the rim, 3.95” (over cap) long.
An analog meat thermometer. To measure the temperature of the hot Hide Glue.
A ½” acid brush. To apply the hot Hide Glue.
A plastic irrigation syringe. To add water to the Hide Glue solution.
A ceramic mug. To hold extra hot water, to be added to the Hide Glue solution or for cleanup.
Paper towels. For cleanup.
Q-tips. For cleanup.
A hair dryer. To heat the parts being glued.
Hide Glue storage:
Granular Hide Glue comes in a tin to keep it fresh. To minimize the amount of moisture exposure to the Hide Glue in the tin, I keep some in a quart size zip lock bag with the teaspoon. This bag is rolled up and sealed, to eliminate any air inside, and placed inside a gallon size zip lock bag, which is rolled up and sealed.
Setting up the double boiler:
I modified my Hot Pot by using a hole saw, and a little sanding, to make a 2.64” diameter hole in the lid, allowing the mason jar to be suspended in the water.
Add hot water to a little under the MAX line. Check that the Mason jar, suspended in the lid, is in the water by about ½”.
Mixing the Hide Glue:
Hot Hide Glue needs to be mixed and heated correctly for it to have the proper strength.
Put a small amount (3 to 6 teaspoons, depending on the size of the job) in the small Mason jar and cover with hot water (just enough to cover the granules). Mix with the acid brush. Let the mixture sit for 10-15 minutes to allow the granules to absorb the water.
Heating the Hide Glue:
With the Hot Pot properly filled with water, the Hide Glue solution in the Mason jar, and the Mason jar suspended in the water, turn the Hot Pot heat control to between 1 and 2 o’clock. Put the meat thermometer in the Hide Glue solution, NOT the water. Mix the solution with the acid brush. Adjust the temperature to get the solution to 140 to 150 degrees. Do not overheat the mixture. Once this is fine-tuned it will probably never have to be adjusted again. Make a mark on the Hot Pot control with a black permanent marker in case the knob ever gets moved.
Heat the mixture and add water or more granules as necessary to achieve the desired consistency (determined by the task you are working). Start with medium pancake batter or room temperature honey. A thinner mixture won’t have as much strength but it will soak more into the wood, and is useful for getting into cracks. A thicker mixture won’t squeeze out easily.
BEWARE!!! The steam coming out of the pour spout will burn you if you get too close!! Avoid the steam!!!!
Preparing the parts to be glued:
One of the best properties of Hide Glue is its ability to bond to nearly anything, including other glues. As long as any old glues are firmly attached, don’t worry about sanding or scraping down to bare wood. A little sanding to rough up the surface is always a good idea, just be sure to remove all sanding dust before gluing.
The “worst” property of Hide Glue (other than the smell!) is it must remain hot to properly bond. When hot Hide Glue gets below 95 degrees it turns into a brown sticky Jell-O-like (gelatin) substance, which greatly reduces its bonding strength. Gelatin is also used in marshmallows and pharmaceutical capsules. So Hide Glue and Jell-O are basically the same thing, although Jell-O is food grade (more refined). Chew on that.
It’s best to heat surfaces of the parts to be glued to increase the glues open time. I use a hair dryer on its highest setting. The combination of the Hot Pot (1000 watts, or 8.3 amps) and the hair dryer (1500 watts, or 12.5 amps) will probably exceed the amperage available in your room. I heat the glue, unplug the Hot Pot, and use the hair dryer to heat the parts. Go back and forth between the 2 parts and heat them for a minute or 2 total, depending on the size of the parts. Thin or small parts won’t hold the heat for long.
BUT, before applying (or even mixing) the hot Hide Glue, practice your clamping. With the limited open time, you only have 1 shot to get it right. Otherwise you will have to take it apart and try again. Which isn’t a bad thing, just pull the joint apart, reheat the pieces, and apply a little more hot glue.
Applying the Hide Glue:
After heating the parts, immediately use the acid brush to apply hot Hide Glue to the surfaces. If you unplugged the Hot Pot, a skin will have formed over the glue. Quickly clamp the parts (You did practice your clamping??). Arrange the clamps so you can clean up any squeeze out with hot water on a paper towel or a Q-tip. This is much easier to do while it’s hot.
Cleanup:
Cleanup after the Hide Glue is hardened can be done with hot water on a paper towel or Q-tip, but you risk dissolving the already hardened glue, possibly weakening the joint.
Cleaning up the Mason jar and tools is done with hot water.
Pour the hot water from your ceramic mug into the Mason jar that is still in the Hot Pot.
Use the acid brush to mix the solution to dissolve the glue and attempt to remove the slightly hardened glue from the sides of the Mason jar.
Use hot water on a paper towel to scrub the slightly hardened glue off of the shaft of the meat thermometer.
Dump the liquid out of the Mason jar and refill with hot water. Rub the sides to remove any Hide Glue still attached to it.
Dump the hot water out of the Hot Pot.
Rinse the acid brush in the Mason jar and be sure there’s no glue left on it or in the bristles.
Once the Mason jar is free of attached Hide Glue, dump the hot water.
Wipe all tools dry with paper towels.
Storage:
I store all the tools, and the plastic bag with the Hide Glue granules, inside the Hot Pot, with the Mason jar in the hole.
NOTES:
Dry Hide Glue has an unlimited shelf life if protected from moisture.
The glue mixture can be kept for several weeks in the refrigerator, but discard it if you notice a foul odor, mold or any contaminants.
Excess Hide Glue should be washed off finished surfaces to avoid chipping the finish as the glue dries and shrinks.
Old Hide Glue joints can be restored by adding more hot Hide Glue. The heat and moisture will reactivate the old glue on the joint surfaces.
Hide Glue sets initially by gelling as it cools. This is useful for “rubbed” joints where initial maximum strength isn’t required. Cleats can be installed over cracks by hand, pressing hard and sliding to squeeze out the excess glue. While the glue cools, the cleat will be held in place by the gelled glue.
Hide Glue cures entirely by evaporation. Hide Glue is about 2/3 water. It is important to pay attention to the amount of water absorbed into the work piece. Joints can swell when glued and must be allowed to dry completely before assuming parts are in their final position. A neck glued on after a neck reset will need a week to dry before doing the final setup. Hide Glue shrinks as it dries, drawing the joint tighter together.