134 ON THE DISCOLOURATION OF WHITE BRICKS. 
Since then other bricks from various localities near Sydney have 
been examined. All show, more or less, the same yellow and 
green colourations, and these have been proved to be due to 
vanadic acid. 
The presence of vanadium in several clays has been pointed out 
by Roscoe, and its effect on bricks has been noticed in Germany. 
Seger has found (Thonindustrie Zeitung, 1877-78) that the golden- 
yellow and grass green colours which appear in some white ricks 
if Kiinste wnd Gewerbe (3rd edition), it is stated the production 
of these coloured compounds depends to a large extent, as might 
ve expected, upon the temperature and nature of the furnace used 
in burning, and that at a very high temperature an insoluble sili- 
cate of vanadium is formed. 
_ Probably the only method of prevention in this case is to use & 
higher temperature in burning ; but then it is no longer possible 
with ordinary clay to produce such light-coloured bricks. 
It will be seen, therefore, that discolourations such as those 
described above may be due to one or two essentially distinet 
causes, either 
(1 
ie . 
dium, titanium, cerium, chromium, molybdenum, copper, and even 
of 
sents of molybdic and phosphoric acids. The former has been 
ound in certain clays, and the latter is said to invariably accom 
pany vanadic acid. 
SL Sag ee ee Oe Te en eT! 
