[CHAPTER XI.]
TESTING GLUE AND GELATINE.
It is of importance that the manufacturer as well as the dealer should know how to test the quality of a glue. This may be done by chemical means and in a mechanical way.
Determination of moisture. For this purpose a weighed quantity of the sample, finely powdered, is for fourteen hours exposed to a temperature of 217° to 230° F. It is then cooled under the dessicator and reweighed. The content of moisture is then calculated from the loss in weight.
Determination of ash. The origin of a sample of glue may be traced by examining the ash for phosphates of lime and magnesia, bone-glue containing both, while skin-glue is free from phosphates. Reduce a portion of the sample to a fine powder, and weigh it in a tared constant crucible. Heat slowly over a Bunsen flame until carbonized, then remove the crucible to the muffle, and heat to bright redness for 10 hours. Cool under the dessicator and weigh. The increase in weight of the crucible is the ash of the sample. This will vary from 1 to 2 per cent. in a gelatine, 2 to 3 per cent. in a good glue, 6 to 8 per cent. in a common quality.
Determination of Acidity. Kissling elsewhere determines the acidity by suspending 30 grammes of the sample in 80 Cc. of cold water for 10 to 12 hours in a flask connected with a condenser. The volatile acids are then driven over by a current of steam into a graduated cylinder. When the distillate amounts to 200 Cc., distillation is interrupted, and the distillate treated with standard decinormal alkali. When the distillate contains sulphurous acid, a known quantity of the standard alkali is previously added to the cylinder.
An undue quantity of acid may be detected by the taste. Glues may be alkaline from the addition of excess of lime in the manufacture to correct the sourness of the jelly. For uses where colors are concerned the glue must be neutral to litmus; for adhesive uses this does not matter unless the alkalinity or acidity is due to defective preparation (Samuel Rideal).
Determination of Glutin. The percentage of glutin in a glue solution is determined by precipitating it with tannin. The dense white precipitate formed is thrown on to a tared filter, washed with hot water, dried and weighed. The calculation is made on the basis that the tannate of glutin has a percentage composition of 42.74 per cent. of glutin and 57.26 per cent. of tannin.
Bisler-Beumat while employing the same principle prepares two solutions: a. 10 grammes of pure tannin to the liter. b. 10 grammes of pure isinglass and 20 grammes of alum to the liter. The ratio in which the tannin is precipitated by the isinglass solution, which Risler considers as pure glutin, is then determined by titration. The tannin solution is then diluted so that exactly an equal volume of glue solution is precipitated by it.
In order to test a sample of glue, dissolve 10 grammes of it together with 20 grammes of alum in a liter of water, heat being applied if necessary. Next 10 cubic centimeters of the tannin solution are taken to which an equal bulk of glue solution is at once added, as one may be sure that this quantity is not sufficient for the precipitation of the measured quantity of tannin, because no glue found in commerce is as pure as isinglass. The vessel containing the mixed liquids being well shaken and the precipitate having subsided, another cubic centimeter of glue solution is added to the tannin solution which is next filtered through a moist cotton filter. If one drop of the glue solution still produces a precipitate in the clear filtrate, another cubic centimeter is added to the tannin solution, and then again filtered, these operations being repeated until the filtrate is no longer rendered turbid by the glue solution.