1 22 On the Arrangement of the Tessera; in a Roman Pavement. 



ruby-glass which at present is manufactured in great perfection 

 in Bohemia, where a preparation of gold, generally chloride of 

 gold, is used for that purpose by the glass manufacturer. The 

 application of gold preparations in the preparation of red glass, 

 comparatively speaking, is of recent origin, for it appears that 

 before the 17th century the use of gold preparation for this par- 

 ticular purpose was unknown. In the 17th century we find 

 the first reference made to the use of gold for colouring glass 

 red by Cassius, who discovered and recommended a new com- 

 bination of gold, which, to the present day, is known under the 

 name of Cassius gold purple. 



" Copper thus appears to have been the material with which 

 the ancients were in the habit of colouring glass red. Various 

 methods of applying copper were in use, and though metallic 

 copper is capable of imbuing glass with a red colour, no doubt 

 on account of the protoxide of copper which is found in almost 

 every sample of copper, in most cases it was first subjected to 

 operations which tended to generate protoxide of copper. Fre- 

 quently also peroxide of copper (black oxide) was used for the 

 same purpose, but in this case the glass mass received an addi- 

 tion of substances, as tartar, charcoal, soot, iron, protoxide of 

 iron, which substances at a red heat combine with part of the 

 oxygen of the black oxide, and thus become the means of redu- 

 cing the latter to red oxide of copper. 



" This important action of iron seems to have been known to the 

 ancients, for both Cooper's and Klaproth's analyses of antique red 

 glass referred to above, as well as my own of the Roman glass 

 found in Cirencester, exhibits, besides oxide of copper, oxide of 

 iron. Later the art of colouring glass red by means of copper 

 was lost entirely, and many persons of our days even denied 

 altogether the possibility of producing a red glass with copper. 

 Very generally all red glass was supposed to contain gold. 



" The importance of the subject induced the Society of Arts 

 of Berlin to offer a prize for a method of manufacturing red glass 

 by means of copper. The prize was gained by D. Engelhardt of 

 Zinswider, who gave several directions of manufacturing red 

 glass, and who succeeded in making a beautiful red glass with 

 protoxide of copper and without using gold at all. - " ( Vide Ver- 

 handl. des Gewerhevereins, Berlin, 1828, S. 15.) 



From this analysis it will be seen that the Romans imparted 

 this red tint to glass by a very ingenious method, and it was the 

 substance used for this purpose, namely copper, which covered 

 over the tesserae as the surface of the glass had become decom- 

 posed in the form of a carbonate of that metal. 



This fact is curious in its bearing upon the pavement as a work 

 of art; as so harmoniously are the colours arranged in all the 



