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 white1.66
Chalk2.402.953.792.10
Sulphate of lead—4.162.353.18
4.067.086.145.28

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.

It is difficult to say what quantities of earthy constituents actually constitute an adulteration, since it is claimed by many practical men that a considerable content of earthy parts is of advantage as regards the cementing power of glue. Generally speaking it may, however, be said that a glue containing more than 6 to 8 per cent. of earthy constituents must be considered adulterated.

For many purposes, especially if the glue comes in contact with colored materials as, for instance, in book-binding, a content of free acid would exert an injurious effect upon the colors, destroying or changing them. It is therefore advisable to test the glue with blue litmus paper which, in the presence of free acid, is reddened.

In testing a large number of samples of glue, Kissling obtained the following results:

Skin Glue.Number
of samples.		Smallest
percentage.		Largest
percentage.		Average
percentage.
Water1513.418.115.7
Ash161.04.132.15
Fat210.010.0900.037
Volatile acids, free80.0840.2380.178
Volatile acids, fixed0.0840.3340.191
Bone Glue.
Water2511.517.713.4
Ash261.165.072.46
Fat50.0470.2170.113
Volatile acids, free70.0881.4510.655
Volatile acids, fixed0.0970.7210.460

However, those who from their practical knowledge are most competent to judge the commercial value of a glue, scarcely require such complicated tests, as by taking the sample in their hand and looking at it, they can in most cases tell its quality. Great hardness, a clear, rattling sound when struck, and resistance to breaking are signs of good quality, and if the cake is cut thick, it shows that the jelly possessed great consistency. Many kinds of glues are intentionally cut thin so that they will dry before spoiling. The derivation of a glue from a sound jelly is recognized by the fine cutting lines. When the jelly possesses but little gelatinizing power and is in the first stages of decomposition or putrefaction, or formation of sugar has already set in, it cannot be poured into moulding boxes, as it would putrefy before it has a chance to gelatinize. Such sick jelly is poured in thin layers upon glass or metal plates so that it may acquire sufficient solidity to allow of its being cut up into leaves and dried upon nets. If the edges of the glue are deeply indented and raised, the jelly, before drying, possessed but little concentration (25 to 30 per cent.), and if notwithstanding this, it acquired sufficient consistency to allow of its being cut, it must have been very sound. Glues cut thick and showing no indentations and raised edges, are derived from excessively concentrated jellies (30-35-40 per cent.). Such jellies lose in quality in evaporating.