Page:The chemistry of paints and painting.djvu/47

Rh across both strips by means of a broad brush. The pigments used may be French ultramarine, chrome yellow, and carmine. Unless they are employed in very dilute admixture, the changes produced by alum and other chemicals will not be perceptible. There should be no bleaching of the ultramarine or the carmine, or any blueing of the latter, and no dulling of the chrome, even after the lapse of a week from the date of the experiment. Washes of tincture of azolitmin from litmus, tincture of dahlia flowers, and tincture of methyl-orange may be similarly applied to paper-strips; in this case it will probably be found that the two former tests will show an acid reaction, and the methyl-orange a basic or alkaline reaction. This seemingly strange result has been found to arise from the presence of a derivative of the alum in the size, namely, an aluminium sulphate which is acid to some tests and basic to others. This point has been established by the experiments of Messrs. Cross and Bevan, Mr. C. Beadle, and Drs. P. N. Evans and Quirin Wirtz, who have proved that all the drawing-papers of well-known makers which they have examined contained no free sulphuric acid. Of course, the question remains, 'How far, if at all, is the basic aluminium sulphate in drawing-paper injurious to sensitive pigments?' This inquiry can, I think, be answered by applying the colour-tests already described, not only to the suspected papers themselves, but also to extracts from them made with cold distilled water and also with hot. Other useful tests are the following:

1. Burn 100 grains of paper to a white ash ; not more than 1.5 grains of incombustible residue should be found.

2. Extract 100 grains of paper repeatedly with boiling