A simple and accurate apparatus may be had for determining the amount of water the glue will absorb for best working results, and also whether the amount of water actually used is the proper amount for this particular glue.
It is important to know these facts, since the more water a glue will absorb under proper working conditions, the cheaper that glue is to use.
The apparatus in question is illustrated on the next page. It consists of a copper pot and a hydrometer arranged for a temperature of 75° C., or 167° F. A sample of the glue to be tested is poured into the pot and the hydrometer is slowly allowed to sink into the solution until it finds its correct position. If the glue-solution is, for instance, 1 part glue to 3 parts water, the hydrometer will drop to 25 on the hydrometer scale. This will show that you have 25% dry glue in the solution. The hydrometer is fitted with a temperature correction scale that enables the readings to be adjusted to the temperature of the glue-solution.
By noting the working qualities of glue prepared with various proportions of water, you can determine what is the correct amount of water to use, and then by using the hydrometer as each batch is prepared, you can be sure that the correct proportions are always being used. By making readings from time to time with the hydrometer, you can also determine the amount of evaporation that is going on, and in this way guard against the glue becoming too thick for proper use.
No special skill is required to use the hydrometer, and the readings are so quickly made, that tests can be made in every department in which glue is used without loss of time.
MELTING, OR DISSOLVING, GLUE
After the glue has been soaked in the manner described, the most important part of the process is undertaken—that is, the melting of the glue by application of heat.
The words “most important” are used advisedly. It is safe to say that most of the damage done to glue occurs in the melting process. There are all kinds of ways of melting glue, but many of them absolutely ruin glue for practical work. As this is a very important subject, it is well to get the rudiments thoroughly in mind—and for this purpose the reader should remember what has been said about the nature of glue—that it is made from animal matter; and that it is composed of innumerable small fibres on whose strength the holding power of the glue depends.
Whatever injures and breaks down these fibres inevitably weakens the glue; so that in melting, every care must be observed to avoid the breaking down of the glue-fibres.
The most common destructive agent is heat. Just as the application of heat breaks down the fibres of a roast of beef, rendering it “tender” as the saying is, so the prolonged application of heat, or heat of too great an intensity, will destroy the glue-fibres, and therefore radically impair their value for actual use.