Sound Board Glue Used in Days Gone By

Hot hide glue is a gelatinous substance extracted from the hides, bones and connective tissues of animals. Its molecules are very fine and are able to link together in very long chains and has a great affinity for other certain types of materials. These types would include leather, cloth and wood. The stickiness of the glue is due, for the most part, to this molecular attraction to itself.
Too high a heat will destroy to a great extent this molecular attraction which destroys this stickiness so important a characteristic of this glue. What heat is too high? Any temperature higher than 145 degrees. A report by O. Linder and E.C. Frost before the 1914 meeting of the American Society for Testing of Materials showed that "overheating glue is found to be weakened by forty to fifty percent."
The Madgeburg hemisphere test, performed in 1650 before the German emperor by Otto von Guercke is illuminating. If we completely exclude the air from between a joint with glue, we are utilizing the force of atmospheric pressure to hold the parts together. It required sixteen large horses to pull apart these twenty-two inch "hemispheres" perfectly fitted together and then pumped free of internal air to create as near a vacuum as was then possible. (Practical Physics, Black and Davis, pp. 101-102.)
Hot hide glue will find its way into the pores of wood and make an absolutely air-tight joint. In the same report by Linder and Frost, it was brought out that blocks of wood glued and tested for the strength of glue required from 1,100 to 1,950 pounds of force to the square inch before the blocks would pull apart. The blocks were clamped together in the "usual manner" in what might be called a hair line joint.
In the successful gluing of a soundboard, namely the edge gluing of the boards to make the panel, the gluing of the ribs and bridges to the completed board and the gluing of the board to the frame or inner rim of the piano, every particle of glue that will squeeze out must be squeezed out so as to permit the actual touching of the molecules of the two parts to be glued together. Further, the greatest care must be taken to have the pressure uniform throughout the lengths of the parts being glued.