Gelatine and glue have both been put to an immense variety of uses, and
the list is constantly extending. Indeed, no one who considers the
following account of their applications can doubt that gelatine and glue
have become a necessary part of our civilization.
Gelatine for edible purposes certainly forms a very considerable part of
the total used, and great pains are now taken to obtain a pure product.
Thus, a gelatine with more than 1.4 parts per million of arsenic, or
more than 30 parts per million of copper, is not considered good enough
for "pure food." The food value of gelatine, compared with other
proteids, is exceedingly low; its use in this connection has no
connection with the "calories" of heat energy it will yield. It is used
almost entirely because of its property of forming a gel. Table jellies
form, of course, one popular use of gelatin, but the manufacture of
sweets makes also a great demand upon the gelatine trade. Culinary
operations often require a little gelatine, especially is it used in
pies and soups. An extension of the same idea is found in its employment
for many manufactured foods, _e.g._ tinned meats, meat extracts, and the
concentrated foods. The use of gelatine in connection with the first of
these received a big impetus during the war period. In gelatine for any
of these purposes, the presence of excess of sulphurous acid is
objectionable, as its taste is easily noticed.
Gelatine for medicinal purposes finds an ever-growing number of
applications. Gelatine capsules for holding greasy liquids and solutions
of nauseous drugs are increasingly popular, for the dose may be
swallowed without unpleasantness. In making these capsules some sugar
is also used, and the finished article is often protected from
atmospheric moisture by treatment with a weak solution of alum. In a
similar way pills are often coated with a 33 per cent. gelatine sol.
Such pills are not only pleasanter to swallow, but are less liable,
after being dried, to stick together in the box. Alcohol solutions of
drugs (or essences, perfumes, etc.) may be suitably stored in gelatine
instead of metal tubes. Medicated wines are detannated by gelatine
before the addition of drugs which would have been precipitated by the
tannin. The British Pharmacopoeia specifies four kinds of "Lamellæ,"
which are small discs of gelatin and glycerin, each containing a minute
but definite dose of some powerful alkaloid. Glycerin jelly is a mixture
of gelatin glycerin with some water. It is used for chapped and rough
hands; the mixture is also used for glycerin suppositories, and for
mounting microscopic sections. The mixture also forms the basis of
gelato-glycerin, used in nasal bougies, and of glyco-gelatin for
medicated lozenges. Gelatine insolubilised by formalin (formo-gelatin)
has been used for making tabloids, wound dressings, and artificial silk.
Gelatine is in constant demand for bacteriological work, for which
purpose a high-grade product is desired. Nutrient media for the culture
of bacteria are solidified by 10-15 per cent. of gelatin, and the growth
of colonies of bacteria often show typical formations. By inoculating
into a melted and sterile quantity and setting quickly in a flat dish
after mixing, the number of bacteria in the volume introduced can be
judged from the number of colonies which develop. Bacteria are also
distinguished often as "liquefying" or "non-liquefying" according to
their type of culture on nutrient gelatine media. Gelatine for such work
should be neutral and of high clarity.
The gelatine required for photographic purposes is also a high-class
product. It should be neutral, colourless, and free from chlorides and
other mineral salts. Grease also is objectionable. Gelatine is used in
the numerous carbon processes, in which the principle is that gelatine
is made insoluble in water by the action of potassium dichromate under
the action of light. It is used also in Poiteoin process for copying
engineering drawings, which is based upon the power of a ferric salt to
render gelatine insoluble so long as it is not exposed to the actinic
rays.
Gelatine is used in the manufacture of the "crystalline glass" used for
decorative purposes. Advantage is taken of the immense contractile force
it exerts on drying. When ground glass is coated with gelatine, and the
latter dried, it tears away the surface of the glass itself, and leaves
peculiar fern-like patterns. Inorganic salts dissolved in the sol
influence the nature of the pattern obtained.
Gelatine is used also very largely in the textile trades, for finishing
coloured yarns and threads, for sizing woollen and worsted warps, and
for thickening the dyestuffs used in printing fabrics. It is also used
for finishing white straw hats; as a size in the manufacture of
high-class papers, and as a wax substitute for covering corks and bottle
necks.
Glue is used instead of gelatine in all cases where colour is not a
matter of much moment. The fact that it has not been bleached makes no
difference to its suitability in such a case, and the cost is
substantially reduced. Thus, for dark-coloured straw hats, textiles,
sweets, papers, and in all suitable woolwork, glue is used in place of
the more expensive article.
A very large quantity of glue is used in the manufacture of matches,
where it functions as the material binding the "head" to the stem. A
15-50 per cent. sol is used, containing nitrate or chlorate of potash as
oxidizing agent. The mixture is kept at 38° C. and the phosphorus
cautiously added, and when this is emulsified, the friction ingredients
(sand, glass, etc.) are also added. The glue acts also in preventing
premature oxidation. Glue is also used in making the match-boxes, and
similarly in making sand, emery, and glass papers and cloths.
There is a large consumption of glue by joiners, carpenters,
cabinet-makers, and all kinds of woodwork and fancy work. It is used in
the manufacture of furniture of all kinds, of pianos, organs, billiard
tables, panels, picture frames, and of toys and brushes. Mixed with
white lead, chalk, and sawdust, it forms a composition used for mirror
frames, rosettes, etc. Glue is used for veneering, for mosaics, plaques,
trays, fingerplates, leather wall coverings, and for staining floors.
There is also a considerable sale for glue in bookbinding, for which a
sweet, light-coloured, and strong product is required. It has been found
particularly suitable for leather bindings where the grain has been
artificially printed or embossed, and in finishing and gilding.
The compositions used for printing rollers all contain gelatine or glue
together with sugar or glycerin and possibly oil and soap. They are
often hardened with formalin. Similar mixtures are used for the beds of
hectographs.
Glue (together with waste leather) is used in the manufacture of
imitation leather and leather substitutes. Cotton and wool fibres are
often incorporated, and sometimes textile fabrics.
Much glue is converted into "size," which is a weak gel used as a
filling rather than as an adhesive agent. A low-grade glue is often
therefore preferred for such purposes, as having "body" rather than
"strength." Size is often sold in cake, but sometimes in the form of the
gel itself, in which case it may never have been evaporated. Indeed,
size is often overboiled glue, made by crude and out-of-date methods. It
is largely used in the paper trade, and for wallpapers, millboards,
papier-mache, paper and cardboard boxes, etc. Mixed with logwood and
iron, and possibly alum, it formed the "blue size" once largely used by
bootmakers as a foundation for blacking, and is similarly used in
currying. Size is also used in making oil paints and varnishes.
Distemper is a size with which is incorporated whiting or gypsum and
coloured pigments. In all applications of size, it is common to use
antiseptics. Salicylic acid has been widely used in this sense.
Low-grade glue is used for the manufacture of cheap brushes and for
fly-papers.
Innumerable patents have been taken out and mixtures invented for the
production of plastic materials, which frequently involve gelatine or
glue. Thus, gelatine and glue are used in making plaster casts, and for
imitation ivory, wood, stone, and rubber. Many of these inventions have
been investigated by Rideal, who points out the features common to most
of them. Usually a viscous sol is thickened by the addition of inert
fibres and powders, and with the object of making the preparation more
waterproof it is customary to incorporate oils, fats, waxes, tars, and
resins before the gel is set. The surface is hardened by "tanning" with
formalin or tannin solution, finally painted or varnished.
Equally innumerable are the inventions, recipes, and patents for making
glues that shall remain liquid. The convenience of this ideal is
obvious, but many of the suggestions are useless. It is quite easy to
incorporate into a gel substances which keep it liquid--any soluble
substances with a lyotrope influence of the iodide type will do
this--but these also prevent the glue setting when used. Even in small
quantity they will influence the tenacity of the joint. Other methods
depend upon a partial hydrolysis of the protein. Amongst the most
successful of these attempts are to dissolve 3 parts of glue either in
12-15 parts saccharate of lime, or in 9 parts of 33 per cent. acetic
acid.
Many special glues and cements are made from commercial glue, according
to the purpose required. "Marine glue" contains no glue; it is made from
shellac and rubber mixed with benzene or naphtha. Its advantage is
waterproofness.
REFERENCES.
"Glue and Glue Testing," S. Rideal, D.Sc., 2nd ed.
"Uses of Glue," chap. iii. p. 83.
"Uses of Gelatine," chap. iv. p. 100.
"Special Glues," p. 108.
"Liquid Glues," p. 119.
"Gelatine, Glue, and their Allied Products," T. Lambert.
"Uses of Glue and Gelatine," chap. ix. p. 80.
"Liquid Glues and Cements," chap. viii. p. 69.