Series editors' preface - Wood Tools

Plastics and polymers, coatings and binding media, adhesives and consolidants 171
hydrolysing the initial collagen in a strong acid
or alkali, breaking the polymer chains into
smaller extractable units by the action of water
at an elevated temperature to produce gelatin.
Whilst the collagen must be treated in order to
break it down to produce the adhesive, the
longer it is treated, and the more extreme the
pH, the more protein is broken down. Bones
require extended treatment whilst adhesive can
be extracted from skin with much less aggressive
preparation. Acid processed animal glue
will be slightly acidic whilst that prepared
using alkaline processing will be slightly alkaline.
Collagen molecules are characterized by a
triple stranded helix with frequently repeating
glycine-proline-hydroxyproline amino acid
units. The overall structure is maintained by
strong hydrogen bonding between the
hydroxyl group of the hydroxyproline and the
amino hydrogens of adjacent glycine units.
Gelatin results from the separation of the three
strands as a consequence of the scission of the
hydrogen bonds between them and their
replacement with hydrogen bonds to solvent
water. As the solution cools the coils cannot reform
exactly but become misaligned resulting
in a gel formation.
The bulk of animal glue is made from cattle
hides (scraps from tanneries being a readily
available supply) and is called hide glue. Other
starting stock such as bones (bone glue) fish
skins and rabbit skins are also used to produce
distinctive adhesives. The glue stock (starting
material) is first prepared by washing. Fats and
oils are removed by saponification in a lime
solution, and acid solutions are used to neutralize
the lime and to remove unwanted
mucus-type proteins and polysaccharides. The
glue stock is then steeped in water at controlled
temperatures and a series of broths
removed by pressing and draining the stock as
the desired protein concentration is reached.
Because the collagen will continue to break
down by hydrolysis as the temperature rises
and the steeping time increases, a series of
broths of progressively lower molecular weight
(shorter chain) proteins can be extracted at
progressively higher temperatures until the
glue stock is exhausted. Below a molecular
weight of about 20 000, adhesive properties of
the extract are lost. The highest quality glue
comes from the first draining of the glue-broth.
The broth is dehydrated by low temperature
boiling in a vacuum (to prevent further damage
by high temperatures) until it gels and is
then air-dried to the final product (Cummins,
1986).
Hide glues possess unique properties that
have made them useful in a wide range of
applications. A small amount of glue can tie up
large quantities of water into a hydro-colloidal
gel structure. The flexible and rubbery gels
consist of expanded open networks of protein
polymer chains which hold water in the structure
by hydrogen bonding and other forces.
This gel state is temperature dependent being
fluid when moderately hot but gelled and rubbery
at room temperature. When animal glue is
heated in water, the coiled strands dissolve
into single strands. Animal glue is applied hot
and fully fluid in a water solution that can
readily wet a polar substrate such as wood. On
cooling, hydrogen bonding allows animal glue
molecules to bond to each other and to appropriate
substrates. Gelling occurs when the glue
gels upon cooling into a rubbery elastomeric
state. The gel then dehydrates and contracts
until it is a set, hard and tough solid (Von Endt
and Baker, 1991).
Animal glues were among the earliest adhesives
and continued to be virtually the only
adhesives employed in furniture construction
until relatively recently. In addition to their use
as woodworking adhesives they are also used
as binding media for gesso and bole in gilding,
as components of fillers and moulding materials
and as a simple but surprisingly durable
paint medium. Animal glues have proven
extremely durable under ideal circumstances
and intact glue joints can be found that are
centuries old. Optimum animal glue joints are
stronger than the wood itself but even under
less ideal circumstances offer adequate
strength. They do not stain wood or impede
the application of stains and coatings, are nontoxic
and easily cleaned from areas where they
are unwanted. Disadvantages of animal glue
include limited working time at room temperatures,
poor gap filling abilities, and biodeterioration
under some circumstances. As
water-soluble adhesives, animal glues are
moisture-sensitive. This makes them unsuitable
for some applications but this ease of
reversibility continues to recommend them for
furniture conservation. Because they are most