Page:Proceedings of the Royal Society of London Vol 2.djvu/45

Rh prepared from shell-fish is in a broken vase in the baths of Titus, containing a substance which at the surface has become of a cream colour, but in the interior has a lustre approaching to that of carmine. The colouring matter of this substance was found to be combustible, constituting about one thirtieth part of its weight, the remainder being a compound of siliceous, aluminous, and calcareous earths. It may, therefore, be regarded as a lake; but it would be very difficult, if not impossible, at this distance of time, to determine whether it be of animal or vegetable origin. In either case its durability, even in the interior of the mass, is a very curious circumstance, although the part exposed to the air has suffered the changes to which such colours have been too often proved to be liable, and accordingly no traces of it remain in any of the ancient fresco paintings.

All the blacks observable in the baths of Titus or elsewhere accord with the descriptions given by ancient authors, who speak of them as carbonaceous substances, obtained either as common charcoal or as soots of woods or resins.

The browns are sometimes mere oxides of iron or ochres, and sometimes mixtures of the oxides of iron and manganese; and it appears that the Romans had some knowledge of the properties peculiar to the latter substance, as Sir Humphry Davy has analysed two specimens of ancient Roman purple glass, both of which were tinged with manganese.

Among the whites of the ancient paintings, the author was unable to discover any ceruse, although it is known to have been in common use on the authority of Theophrastus, Vitruvius, and Pliny. The whites found are in general carbonate of lime, or fine white clays.

The ground to which the colours are applied in the ancient fresco paintings, is precisely such as is described by Vitruvius, powdered marble cemented by lime, highly polished and beautifully white. With regard to the mode in which their colours were applied, Vitruvius and Pliny agree as to the employment of wax in encaustic painting, which was subsequently liquefied by heat so as to give a varnish to the painting. But the author has not in any instance been able to detect the presence of wax, nor yet of any animal or vegetable gluten, in any of the fresco paintings, or even in the pot of colours found at Pompeii.

From the facts above stated, it appears that the Greek and Roman painters had the advantage over the great Italian masters, since the revival of civilization, in two of their colours, the Tyrian purple and the Egyptian azure, although the latter may easily and cheaply be imitated; for if a mixture of about fifteen parts of soda, twenty parts of powdered flint, and three parts of copper filings, be strongly heated together for about two hours, a frit is produced extremely similar in appearance and degree of fusibility to the ancient blue frit. 