The plaster sticks are prepared as follows: Reduce pure crystallized gypsum to a fine powder, pass the latter through a sieve having 324 meshes per square centimeter and heat it to between 284° and 302° F. For casting the sticks moulds of soap-stone are used. The moulds are made by boring in a piece of soap-stone, at a distance of about 1 centimeter from each other, holes with a diameter of 6 millimeters on top and 7½ millimeters on the bottom.
The gypsum is weighed off in portions of 1 gramme each, mixed with 1 gramme water and cast in the moulds. The solidified sticks are first dried at a moderate heat and then over calcium chloride, and kept for use in an air-tight vessel.
Fig. 66.
The glue solution is prepared as follows: The glue dried at 212° F. is weighed, soaked over night in water, then melted in a small glass in the water-bath, and finally enough water is added so that the solution contains 10 per cent. of dry glue.
The plaster sticks are for one or two minutes immersed in the glue solution heated to 212° F., and then placed vertically upon a glass plate until superficially dry, when they are completely dried at 212° F. It is recommended to color the glue solution with indigo, the uniform saturation of the sticks being thereby more easily recognized.
The apparatus for testing the strength of the sticks consists of a brass ring a, Fig. 66, having two notches to receive the stick, and its diameter is divided into two equal parts by an indicator. The ring is supported by a pin, by means of which it is secured in a stand. The apparatus is completed by an iron or glass cup which is suspended by three cords, i, and the hook f to the plaster stick b. The hook f is placed in the position indicated by the indicator. Mercury is now poured into the cup until the plaster stick commences to break. The weight required is recorded and compared with a standard glue. During the experiment the cup is suspended by the three cords h to the ring a, and is thus caught when the plaster stick breaks. On the lower end of the cup is a clip for emptying the mercury into a vessel so that none of it is lost.
The “Artillery Werkstätte” at Spandau has adopted the severing strain of two blocks of wood glued together. The test is executed as follows: Three parts of glue (but not less than 250 grammes) are mixed with 6 parts of water and boiled in a steam bath until the weight of the boiled glue amounts to only 5/9 of the original mixture. The reason for this continued boiling is to ascertain whether the glue to be tested retains the required adhesive power even after six hours continued heating in a steam bath as frequently happens in the workshop. With the glue thus prepared the following breaking test is made:
Blocks of hard or soft wood 420 millimeters long and 40 × 40 millimeters cross section are cut in two, so that each piece thus obtained is 210 millimeters in length. These two pieces are then again glued together across the grain with the glue to be tested. The block is then placed in a dry room at a temperature of 62° to 68° F. for 72 hours, when the joint is tested as to its resisting power. At a distance of 180 millimeters from the joint a hole is bored in the block. Through this hole is pushed a bolt furnished on the lower end with a hook to which a scale is suspended. The block of wood is clamped to a table so that the joint projects 1 centimeter beyond the edge. The scale is at the start loaded with 25 kilograms, the load being every five minutes increased 5 kilograms till fracture takes place.
Two such blocks, one of hard and one of soft wood, are subjected to the test, and a serviceable glue must stand at least an average load of 70 kilograms.