Glue that foams at ordinary temperature should be avoided for good work.
GREASE
A moderate amount of grease may be a good thing when using with clay or with colors, but a large proportion of grease should be avoided when making glazed or coated papers, or in general use.
A glue at ordinary temperature which is not overheated on which a scum rises has an excessive amount of grease. It shows that the glue has not been properly skimmed in manufacture.
KEEPING PROPERTIES
The keeping property of glue may be determined by letting the glue-jelly stand for several days exposed to the air, and noting any deterioration. It is customary to let the jelly stand at room temperature—but if the glue is to be kept under any special conditions the test should be made as nearly under these conditions as possible.
ODOR
Deterioration is always accompanied by a sour odor. Avoid using any glue that does not smell clean and sweet.
LABORATORY TESTS
Up-to-date practice in all the larger concerns using glue demands a laboratory for making tests. An expert is put in charge and the glue analyzed chemically as well as for its physical properties. This method cuts the guess-work down to a minimum. Manufacturers whose output would not permit the employment of an expert all the year round can have glue analyzed in laboratories maintained for such purposes. It means often a great saving of money in the end to learn the exact properties of the glue you propose to use, or that you may be actually using.