A high degree of transparency is a favorable sign as regards the purity of a glue, substances inducing putrefaction having been eliminated. Hence this property should be worthy of recommendation, but the consumer having been disappointed in the use of thin glass-clear glues, distrusts also the thick-cut transparent product, preferring a turbid, translucent or opaque article. For this reason the manufacturer is forced to render glass-clear glue turbid with coloring matter.
The color of the glue is also a means of judging it. To be sure, chemically pure glutin is a colorless substance, but glue is always colored more or less dark brown. Although this coloration does not in the least impair the adhesive power, the manufacturer endeavors to produce a product of as light a color as possible, and this is best effected by bleaching with sulphurous acid. By this means the glue not only acquires a lighter color, but also becomes more stable, substances inducing putrefaction being destroyed by the acid.
[PART II.]
CEMENTS, PASTES, MUCILAGES.
[CHAPTER XII.]
CLASSIFICATION OF CEMENTS.
The great variety of substances entering into the manufacture of cements and pastes makes a division of them extremely difficult. Stohmann divides them into the following groups:
1. Oil cements.
2. Resinous cements.
3. Cements containing rubber or gutta percha.
4. Cements containing glue, or starch paste.
5. Lime cements.
Generally speaking, this division is correct; the only change we would suggest is to apply the term agglutinant or paste to bodies containing glue and starch paste.
When we attempt a division of the cements according to the bodies to be cemented, we find that the result will be a larger number of groups; as we must take into consideration whether the articles to be cemented have to be heated or not, whether they are to come in contact with water or other liquids, and other circumstances which would necessitate modifications in the composition of the cements themselves.