SAMPLING
In the first place you should carefully sample your glue with a view to testing.
Take several samples from various parts of the barrel. Flake glue is often made up of different varieties, and a single sample may not be at all representative. Ground glue, from its very nature, is easily adulterated. It should be examined in a good light, for evidence of foreign substances. Examine flake glue carefully also for uniformity of odor and general appearance. If glue has been adulterated while in original form it is practically impossible to determine the adulteration by external appearance. Subsequent adulteration may be detected.
BUBBLES
If you should notice white bubbles, in the shape of round blots, on the surface of the glue, you have found evidence of decay. If there is any doubt in your mind you can complete the evidence by moistening the glue. If it gives off a sour odor you have an additional indication of putrefaction. Such glue should be avoided.
Bubbles may appear within the glue—not on the surface—without being an indication of putrefaction. As a matter of fact, bubbles are practically always found in certain high-grade glues, though practically never in low-grade bone or hide glues. They are supposed to be due to the air which gets in when the glue is poured into the moulds. When glue is originally dried on nets in very cool and dry weather, such bubbles are frequently found. Always beware of glue showing surface bubbles.
SURFACE INDICATIONS
Besides being free from bubbles or blots a good glue is smooth, though not necessarily glossy. Sometimes the very best glue will be of a dull color, and many inferior glues even have a very shiny surface. The surface should be uniform in color and in appearance.
COLOR INDICATIONS
These are not important, as a rule. The color of any particular lot of glue should be nearly uniform; otherwise it is subject to the suspicion of adulteration.