1 x
1¼, 1⅜, 1½, 1⅝, 1¾, 1⅞
The only way to determine which is the best glue to use, is by trying out various grades in actual practice. The best glues for ordinary uses are well understood (see pages [79] to [85] for suggestions on this subject). Between two or more glues of any one type, actual experiment is the only safe guide. Most glue men will give you good advice; but above all, keep accurate records of results of the different grades used. When you have found the right glue, keep to it.
THE TESTS
Glue is graded on physical characteristics rather than on chemical composition. Various chemical tests have been proposed, but they are unimportant so far as practical working value is concerned.
The most important physical tests are those for viscosity, and jelly strength.
VISCOSITY OR FLUIDITY TEST
The test for viscosity, or fluidity, is based on the idea that the greater the tenacity of the glue, the greater will be its cohesiveness, and the less will be its flowing power. In other words, the higher will be its viscosity.
In testing for viscosity, water is used as a standard. A solution is made of the glue, and the rate of flow of the solution at a certain temperature is compared with the rate of flow of water under the same conditions.
Several devices are on the market under the name of “viscosimeter” for measuring the viscosity of glue.