Musical Instrument Making
For general adhesive purposes, hide glues are prepared in the range of 25 to 50 percent dry glue concentrations, depending on the test grade of the glue selected, the mechanical requirements employed for applying the adhesive, and the nature of the gluing operation. With a specific dry glue, the glue-water ratio is so adjusted that the fluidity of the glue solution is adequate for processing requirements. In most cases a paint-like flow is desirable. The higher-testing (meaning higher gel strength, gram values) grades of glue will take more water for a given fluidity than the lower-testing grades, and in many applications this feature permits lower unit adhesive costs. In other applications the lower-testing grades are more suited, where a film of low water content is desired. In most applications the cost of the glue is insignificant to the value of the substrates being glued.
The unique property of hide glue in jelling when a solution is cooled makes hide glue admirably suited for many industrial uses as an adhesive. If a process requires a quick-setting glue, a correct gluing technique may be developed by establishing the proper relationship between a specific grade of glue (high test), the concentration (heavy), and working temperature (low) of the solution. Other outside factors with a tendency to speed-up the set are: low temperature of the object to be glued, low room temperature and humidity, a thin spread of glue, an absorbent surface, and the presence of air currents around the glue film. By proper control of these factors, the conversion from the liquid to the jelly state may be sped up so as to result in a fast initial set.
On the other hand, if a slow setting property or a long "tack" period is desired, it can be had by making changes in the reverse direction in any or all of the above factors.
The time of "set" may be as short as one minute or less or as long as five or even ten minutes depending on the conditions. In other words, as an adhesive, hide glue has a wide working range which is adjustable to practically any set of conditions. The time of "set" can easily be extended with the addition of a small amount of urea or even common table salt.
Once a specific glue best adapted to the desired results has been selected, its flexibility of use permits it to remain a standard. Simple adjustments in concentration or temperature or adding a gel depressant will easily offset changes in ambient room temperature and humidity.
The guiding principle in adhesive work using hide glues is that of depositing a very thin, uniform, continuous adhesive film at the surfaces to be bonded. Warm hide glue solutions as prepared and applied for adhesive purposes deposit such desired films. The cooling effect at the time of application promotes a rapid jelling action of the glue film, thereby holding the adhesive at these surfaces, eliminating undue bleeding of the adhesive into the stock being glued, providing ample tackiness, and insuring maximum overall economy and efficiency.