Yolk And White Of Egg - Introduction

The materials described in the present chapter owe their peculiar properties - at least, in great measure - to the presence of chemical compounds which contain the element nitrogen. Now, this element is not a constituent of any of the artists' materials already described, nor, indeed, of any others, except a few pigments, such as aureolin, Prussian blue, and indigo. The presence of nitrogen in an organic compound is very often accompanied by a measure of instability, or proneness to change; the nitrogenous constituents of eggs, and of size, afford illustrative examples. Another source of weakness in the composition of the nitrogenous constituents, both of the white and of the yolk of eggs, lies in the presence of another element - namely, sulphur. Part of this sulphur readily leaves the original substance, yielding simpler compounds, such as sulphuretted hydrogen, and ammonium sulphide, which possess the objectionable property of discolouring many of the metallic pigments used by artists. On the other hand, all these nitrogenous bodies are susceptible of coagulation, whereby they become insoluble, and very much less prone to change. Indeed, the majority of them may be turned into a substance which is virtually leather, a material which resists decay in the most marked manner.

This tanning operation may be readily effected by treating the substances in question with a solution containing tannin, the active ingredient of oak-bark, sumach, nut-galls, etc.