FOAM
A simple test for foam is to beat a solution of glue with an ordinary eggbeater. Glue which shows foam, or in which foam does not quickly subside, probably contains impurities. Foam is especially frequent in alum-dried glues and in cheap bone glues.
Some authorities believe that foam is caused by overheating, due to scalding by contact with steam jacket, or by steam coming into direct contact with the glue, or by heating for too long a time, or it may be due to the fact that all the grease has not been eradicated.
Glue that foams at ordinary temperature should be avoided for good work.