With the known relation of the tannin solution to pure glue (isinglass), a conclusion may be formed from the number of cubic centimeters of glue solution used as to the content of glutin in the sample examined.
The percentage of glutin varies, of course, according to the quality and origin of the glue. Bone glue of good quality contains from 50 to 52 per cent. and skin glue from 65 to 75 per cent.
S. Dana Hayes analyzed two samples of American glue of best quality and obtained the following results:
| a. | b. | |
|---|---|---|
| Water (escaping at 212° F.) | 16.70 | 16.28 |
| Glue substance | 79.85 | 80.42 |
| Calcium carbonate | 1.42 | 1.33 |
| Calcium sulphate | 0.41 | 0.34 |
| Magnesium phosphate | 0.35 | 0.31 |
| Alkaline salts | 0.17 | 0.12 |
| Silica, ferric oxide, etc. | 0.09 | 0.08 |
| Zinc oxide | 1.01 | 1.12 |
The chemical modes of testing glue give only the quantity of glutin contained in it, but do not prove that the substance combined with tannin corresponds to the actual adhesive power of the glue, for it is possible that a glue containing a large quantity of glutin may possess but little adhesive power, and a jelly from which the glue is formed may contain an equal amount of glutin with the latter, but not possess an equal power of adhesion.
It is certain that the determination of the glutin alone is not a criterion of the quality of glue. In the absence of a reliable method of direct analysis, attempts have been made to deduce the quality of glue from indirect properties.
One of these methods consists in immersing the sample of glue in a large quantity of water at 59° F. for a considerable time. The glue swells up, absorbing 5 to 16 times its own weight of water. The more consistent and elastic the glue in this state is found to be, the greater its adhesive power, and the larger the quantity of water absorbed the more economical the glue will be in use. This method does not give thoroughly reliable results, and should only be employed with bone-glue, as skin-glue does not behave in a similar manner.
Fig. 65.
A more reliable method is to test the strength of a glue by the method first proposed by Lipowitz, which is based on the weight-sustaining power of a jelly of known strength and temperature. The test is executed as follows: Soak 5 parts of the sample in water, then dissolve it in sufficient hot water to make the weight of the solution equal to 50 parts, and allow to stand for 12 hours at 64.4° F. in a glass cylinder of uniform width to gelatinize. Cover the glass cylinder with a tin cap perforated in the centre, Fig. 65. Through the perforation is passed freely a stout iron wire, at the lower end of which is soldered a piece of tin shaped like a saucer, the convex side of it resting on the surface of the jelly. To the upper end of the wire, which together with the tin saucer should weigh 5 grammes, is soldered a funnel weighing 5 grammes, and capable of holding up to 50 grammes of fine shot. The apparatus is gradually loaded with shot until the saucer-shaped piece of tin is forced into the jelly. The greater the strength of the jelly, the heavier will be the weight required. From the determined weight of the shot used, the relative value of the glue as to its adhesive power is determined.