wood adhesive, is also a protein glue derived from the skins, bladders, and
other by-products of the processing offish for consumption. While the
collagen derived from fishis very similar to that obtained from horses and
other mammals, it tends to have a lower molecular weight and is there-
fore weaker and more easily soluble (Rose and von Endt 1984).
The protein glue used in the great majority of wooden artifacts
encountered by the author is animal-hide glue. Through a heated aqueous
extraction process, the protein collagen is removed from the hides, hooves,
and sinews of mammals, primarily horses and cows, and purified to form
gelatin or glue (Cummins 1986; Fernbach 1907; Perry 1944; Rose and von
Endt 1984; Rosser 1939). Because protein molecules are broken down by
heat, the temperature at which the collagen is extracted plays an impor-
tant role in the characteristics of the adhesive. Collagen extracted at lower
temperatures has a higher molecular weight and is stronger than collagen
obtained from processing at higher temperatures. This characteristic is
referred to as the gram-weight strength and is assigned by determining
the weight necessary to depress the surface of a “glue jelly” by a specific
amount according to a rigorously controlled protocol (DeBeaukelaer 1930;
Fernbach 1907; Rosser 1939). In general, the gram-weight strength of
glues normally used for woodworking is in the range of 200–300, although
the range available is much broader (<100–>400). The procedure for
preparing and using gelatin glues is based on the thermal and solubility
properties of collagen, which is thermoplastic and water soluble.
Modifications of animal glue include the addition of plasticizers
(usually gly cerin or sorbitol up to 50% by dry weight), for flexibility and
increased tack, and the addition offormaldehyde to yield a water-resistant,
thermoset adhesive.
Probably the most important reason that hide glues are so widely
used in the conservation of wooden artifacts is that they are almost com-
pletely reversible due to their water-soluble, thermoplastic nature. For
many fabricators of wooden objects, this reversibility is not a factor, and the
glue is used for other benefits, such as strength, ease of use, and availability.
For the conservator, r eversibility is a key consideration that becomes mani-
fest in two principal areas. The first is the treatment of damaged or disas-
sembled glue lines originally formed by hide glue. Manipulating, reforming,
or removing the original material may be possible, as it was thermoplastic
when applied and may remain so. The second benefit of this characteristic
is retreatability, which is discussed elsewhere in this article. The structure of
animal glues suggests a true chemical affinity for wood (von Endt 1986).
Thus, their adhesion to a wooden substrate is excellent.
Animal-hide glue is hygroscopic, and its stability and properties
are highly sensitive to environmental moisture. If the moisture level is too
low, the glue becomes extremely brittle and can be fractured with very
little applied stress, a circumstance that leads to failure of the bond line.
If the humidity is too high, the glue softens and is susceptible to plastic
deformation. In addition, an extremely high moisture level can result in an
attack of fungi on the surface of the glue. For panel paintings that remain
in—or are returned to—uncontrolled environments, this characteristic of
animal glues must be weighed carefully when their use in conservation
treatments of such panels is considered.80 Williams