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.
_Determination of adulterations._ White lead, sulphate of lead, zinc white, or chalk in quantities varying from 4 to 8 per cent. are frequently mixed with the melted glue to improve the appearance of the finished product. Thus, according to analyses by A. Faisst, Russian glues contained in 100 parts foreign admixtures as follows:
I. II. III. IV. Zinc white 1.66 Chalk 2.40 2.95 3.79 2.10 Sulphate of lead — 4.16 2.35 3.18 —— —— —— —— 4.06 7.08 6.14 5.28
The so-called patent glue which is opaque and of a white color is produced by adding considerable quantities of white lead to ordinary glue.
According to Barreswil, glue is frequently mixed with lead acetate solution to protect it from putrefaction. Such an addition, as well as the presence of white lead or sulphate of lead, is detected by the introduction of sulphuretted hydrogen into very dilute glue solution. In the presence of lead acetate a black precipitate of lead sulphide appears in the clear solution; if white lead or lead sulphate is present, the white powder settling on the bottom is blackened by the formation of lead sulphide.
For the detection of other earthy additions, prepare a very dilute solution of the glue in question and allow it to stand quietly for a few hours. The heavy additions subside, and after decanting the supernatant fluid is collected upon a small filter and examined by the customary analytical methods.