NOVEMBER/DECEMBER 1983, No. 43, $3.50 Making ... - Wood Tools

Which Glue Do You Use?
Chemical types, not brands, make the difference
by George Mustoe
T ike the alchemists' attempts to transmute base metal into
L gold, much human effott has gone into the search for the
perfea glue. This goal is probably as unrealistic as the dreams
of alchemy, but the inventors' struggles have not been without
reward: adhesives manufacruring is a big growth industry
in the United States, and per-capita consumption is about
40 lb. per year.
Not surprisingly, "What kind of glue did you use?" is a
frequent query heard whenever woodworkers gather. Unforrunately,
these exchanges generate some old wives' tales,
among them the colorful but incorrea assertion that cyanoacrylate
"superglue" is derived from barnacles (FWW #37).
Because wood is a relatively weak consttuction material,
most adhesives produce bonds that are stronger than the surrounding
lumber, so claims of extremely high sttength are
seldom meaningful to the woodworker. Instead, the most impottant
charaaeristics are setting rate, viscosiry, resistance to
As water, flexibiliry, color, sandabiliry, and gap-filling properties.
a
woodworker who happens also to be a chemist, I've
developed a keener than usual interest in the literally hundreds
of glues sold today, discovering in the course of my
research that only abour a dozen kinds are useful for woodworking.
Within each category, I've found that different
brands will usually perform equally, so the choice for a particular
project is best made by understanding the chemical
makeup and charaaeristics of the glues we use.
In this article, I'll cover those glues that are best suited to
general woodworking. In a second article, I'll talk about
epoxies, hot-melt glues, cyanoacrylates and contaa cements,
all specialry glues that are usually more expensive, though not
always better, than our old standbys.
Protein glues-The natural world abounds with examples of
sophisticated adhesives which display impressive tenaciry; barnacles
and mussels, for example, cement themselves to beach
rocks and ship bottoms with a substance that resists pro-
. longed immersion in salt water. Though the chemistry of
these natural adhesives is poorly understood, most sticky secretions
are combinations of various complex proteins. Thus it
is not surprising that early artisans discovered that the best
raw materials for glue were protein-rich animal products such
as skin, bone and blood.
Today, despite the advent of modern synthetic adhesives,
animal-protein glues are still common. They can be divided
into three rypes: hide and bone glue, fish glue, and blood
glue. Of the three, hide and bone glues are of the greatest
interest to the woodworker. The use of fish glue, which is
derived from the water-soluble proteins in fish skins, is limited
to industry, mainly for attaching labels to bottles and occasionally
as a tack-improving additive to white glue. Blood
glues, once developed as water-resistant adhesives for early
military aircraft, are made by dispersing beef or pig blood in
62
water, with wood dust, lime or sodium silicate added as
thickening. They're most often encountered in vintage plywood,
but are praaically impossible to buy today and have
no significant advantages over readily available synthetics.
can
Hide and bone glues, on the other hand, are far from obsolete.
Besides being widely used in industry for products
such as gummed paper tape, sandpaper and bookbindings,
hide glue finds plenry of uses in the woodshop. The setting
time and spreadabiliry can be varied, and the adhesive cures
into a colorless, nontoxic, sandable glueline which be undone
by the application of moist heat-a feature that is attractive
to luthiers, for instance, who may need to remove the
sound board of an insttument to repair it. Water also softens
hide glue, and some furniture conservators use a 50% vinegar
solution to speed the disassembly and repair of antiques.
Hide glue consists of protein derived from collagen, the
main ingredient of skin and conneaive tissue. The glue is
prepared by cooking animal hides, hooves and tendons into a
protein-rich broth which is then cooled to a gelatinous solid,
sliced, dried and ground into a coarse powder. In retail stores,
hide glue is commonly sold as a pre-mixed liquid, but it can
be bought in powder form, in which case it must be mixed
with hot water. For an explanation on how to mix hide glue,
see FWW #42, pp. 74-75. Liquid hide glues have two advantages
over mix-your-own: you don't need a heated glue
pot, and the slow setting rate may be valuable for complicated
assemblies. During the heydey of hide glue, it could be
bought in 18 grades, each with a different viscosiry and setting
time. Today, woodcraft suppliers usually offer only a single,
high-grade produa. Setting time can be slowed by adding
more water, but this leads to a slightly weaker bond.
Many other proteins have adhesive properties. Soybeanbased
glue is used in some interior plywood. Casein or milk
glue, which has been detected in medieval picture frames, is
made from skim milk, and is used today for laminating interior
beams and trusses. This glue is a light-colored powder
that must be mixed with cold water and allowed to stand
about ten minutes before use. Unlike the other protein glues,
casein sets both by evaporation and by chemical reaction,
forming calcium caseinate. The resulting neutral-colored bond
is highly moisture-resistant but not waterproof. Casein can be
used in cool weather and on woods containing up to 15%
moisture. It is particularly effeaive with oily woods such as
teak, yew and lemonwood. Powdered casein glue is available
from National Casein, 601 W. 80th St., Chicago, Ill. 60620.
Petrochemical resins-Casein glue is sometimes confused
with polyvinyl acetate (PV A) white glues. Part of this confusion
stems from the milky appearance of white glue and also
because dairy-related companies such as Borden, who once
marketed casein, now sell PV A glue. Developed during the
1940s, PV A glue is part of a family of synthetic resin glues