What is Hide Glue and how is it used in furniture?
Animal (hide) glue is derived from "collagen" which is a protein constituent of hide materials, bone and connective tissues associated with the hides. These materials originate with the packing and tanning industries and are principally of cattle origin. Woodworking functions such as veneering and inlaying are pictured in Egyptian tombs and furniture removed from the tombs reveal that joints were dovetailed and glued with animal glue.
How do you mix hide glue? The general rule of thumb for mixing glue is one cup of dry glue to one cup water. The amount of water can vary slightly depending on the desired consistency. To make liquid (room temperature) glue add approximately 5 table spoons of urea (or 3 tablespoons of salt) for each cup of dry glue. Add the glue to the water and allow to soak for 30 minutes. Heat gently in a commercial glue pot or a double boiler (water jacket preferred) to 140 degrees F. , stirring occasionally. A microwave may be used set at half power, then kept warm in the double boiler or glue pot. DO NOT "cook" the glue as it will break down the protein chain which is what gives it its strength. The glue is strained through an old nylon stocking, then placed in the refrigerator to gel. A cup of glue will make enough to last a while, so it can be cut into pieces once gelled and kept in the refrigerator until needed. The pieces will keep for several weeks in the refrigerator.
How long can you keep it heated? Generally, the glue is kept in the glue pot for an entire day, then returned to the refrigerator at night. (I keep mine in a baby food jar, and fill with cut up pieces as I need it). The heat-refrigerate-heat cycle can be numerous, but if you have doubts about the length of time you have reheated the glue, toss out. I have seen cabinetmakers keep the glue in an open pot without refrigeration for several days, but I do not recommend it. Glue held at 140-150 degrees F. for 2-3 days will lose some of its viscosity and gelatin time, after a week throw it out. (put it in the garden, it's high in nitrogen)
How long can you store it? Dry glue has an indefinite shelf life if stored in a cool, dry place. Mixed glue can keep in the refrigerator for several weeks. Liquid glue (with urea) is good for several months, longer if refrigerated. Keep the lid tightly capped. Mixed glue can be frozen to last a very long time. Remember, glue is a gelatin and should be treated like a food, it can spoil and "go bad" if left out. It is used as pill capsules and marshmallows. (Don't forget "Jell-O") Once the glue is dried, as in a joint, it will last "forever", but bear in mind that hide glue hygroscopic and will react to the relative humidity. If the RH dips too low (10% and lower), for an extended period of time, the moisture content of the glue joint will lower making the glue brittle.
How do you disassemble a hide glue joint? Since hide glue is hygroscopic and thermoplastic, gentle warming or warm water will release the joint. Unlike modern plastic glues (PVA's and aliphatics), hide glue can be reused, meaning that a disassembled hide glue joint does not have to have all of the glue removed in order to be reglued. Smaller pieces can even be placed in a microwave at half power, to soften the glue enough to separate. We will often drill a small hole into a joint and inject warm water (don't use hot water as the joint may split) to facilitate the process. In fact, if the type of construction allows it, a joint does not have to be disassembled at all. A small hole is drilled into the joint, warm water is injected to soften the existing hide glue, and fresh hide glue can then be injected, be sure to clamp it correctly. This is particularly useful when you have a chair with a rush seat and it can not be disassembled without destroying the rushing. This works with mortise and tenon joints on a large piece if only a few joints are loose and can not be disassembled. There has been talk about using handheld microwave devices on large pieces, but I have yet to experiment with one.
What do you do with hide glue squeeze-out? Generally a toothbrush with warm water will do the trick, or you can wait a few minutes and allow the glue to gel, then roll it off with your fingers. Unlike modern plastic glues, any hide glue residue will not cause a permanent spot where stain will not take.
How do you make a hide glue filler? A stainable filler can be made by simply mixing the hide glue (both hot and liquid) with very fine sawdust, the finer the better) It is best to work with a light colored sawdust like maple as it is easier to color a fill darker that it is to lighten it.
What are the gap filling properties of hide glue? Hot hide glue has better gap-filling properties than liquid glue, but not on the level of the modern glues like epoxy.
How does hide glue react to finishes? Hide glue will not react with a surface coating, either under or over it, but it is wise to remove all excess.
What are the basics of hammer veneering? Hammer veneering is the quickest way to veneer a substrate. Using hot hide glue, both surfaces are brushed (or rollered) with glue and placed together. The top surface is also coated with glue to act as a lubricant. The "hammer" is then pushed across the surface from the center to force out the excess glue. The glue will begin to cool and gel immediately and hold the veneer in place. If the surface is large, then make sure the surfaces are warm and having a heat source, (heat lamp, etc) will keep the working time longer. It is not necessary to buy a commercial veneer hammer, a block of wood with a smooth edge will work.
Can you use a crock pot for a glue pot? Yes, the important thing is constant heat at 140 degrees. A $5 baby bottle warmer works too.
How does hide glue sand? Will it damage cutting edges? Normally hide glue will dry to a harder film with a higher tensile strength than PVA, so it won't clog sandpaper. Because of it high tensile strength, it may dull cutting edges.
Can hide glue be made waterproof? Yes, by adding 1/2-1% aluminum sulfate. Exposure to formaldehyde fumes will render dry hide glue insoluble.
Can hide glue be made flexible? Yes, by adding 5% (based on the weight of glue) glycerin. Increase the amount by increments of 2-3% of the glycerin to achieve greater flexibility. This works well when gluing canvas to the back of a tambour door.
Are PVA glues stronger? No, a good hide glue joint is one of the strongest possible with any adhesive. And it will not creep like PVA.
Are there different types of hide glue? Yes, hide glue is graded in terms of "gram strength". 85, 108, 135, 164 192, 222 ,251, 283, 315, 347, & 379. The higher the gram strength, the quicker the set. Generally it is recommended to use #251 gram strength in woodworking. There is also bone glue and fish glue.
What other hide glue tidbits are there? Hot hide glue has the advantage of creating a "rubbed" joint. By simply using hot glue, you can place a block in a difficult place to reach by rubbing the block back and forth to remove the excess glue and holding in place for a few moments. The glue will begin to cool and gel immediately eliminating the need for a clamp. The " wetting" properties of hide glue can be increased by adding approximately 1% vinegar.
Doesn't hide glue "wear out"? No, remember that all of the furniture created for the last 3000 years were made out of hide glue. Go to a museum and see for your self. We find 200-300 year old joints in our furniture conservation work that are extremely sound.
Where is the best place to purchase hide glue? Eugene Thorndahl of Bjorn Industries in North Carolina has the most experience with hide glue and has the widest variety of gram strengths (most woodworking catalogs have no concept). He can be reached at 704.364.1186.
Craig Deller is the President of the Chicago Area Conservation Group and is also on the faculty of the Masters in Historic Preservation Program at the School of the Art Institute of Chicago. You can visit his web site, http://www.deller.com/ to see examples of his work.
Copyright © 1998 The Deller Conservation Group, Ltd. Reprinted by permission of the author.
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