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, its 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 hydroscopic 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 hydroscopic 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 mortice and tenon joints on a
large piece if only a few joints are loose and can not be disassemble