120 Sir Humphry Davy's experiments and observations 
nor by the action of water, could I detect the presence of any 
wax varnish, or animal or vegetable gluten. 
The pot of colours to which I have already referred, found 
at Pompeii, was blackened by smoke, as if it had been recently 
on a fire of wood. I thought that this might be owing to 
some process for dissolving gluten or varnish in the prepara- 
tion of the colour; but I could detect no substance of this 
kind mixed with the colouring matter. 
Pliny states, that gluten (our glue)* was used in paint- 
ing with blacks : and this specific mention of its application 
would induce the belief that it was not employed with other 
colours, which adhered without difficulty to, and were im- 
bibed by, a surface so polished and well prepared as the 
Roman stucco ; and the lightness of carbonaceous matter alone 
probably rendered this application necessary. 
X. Some general observations. 
It appears from the facts that have been stated, and the au- 
thorities quoted, that the Greek and Roman painters had almost 
all the same colours as those employed by the great Italian 
masters at the period of the revival of the arts in Italy. They 
had indeed the advantage over them in two colours, the Vesto- 
rian or Egyptian azure, and the Tyrian or marine purple. 
The azure, of which the excellence is proved by its dura- 
tion for seventeen hundred years, may be easily and cheaply 
made; I find that fifteen parts by weight of carbonate of soda, 
twenty parts of powdered opaque flints, and three parts of 
copper filings strongly heated together for two hours, gave a 
* Lib. xxxv. Cap. 25. " Omne atramentum soleperfiritur, librarium gummi tecto- 
rium glutino admixto.” 
