Colour
:
ON THE PRIMARY, BLUE.
Other things being equal, those artificial ultramarines are most durable
which possess the most colour; and all are, perhaps, most permanent in
water. If used in that vehicle, care should be taken to employ a gum
free from acid; also, whether in water or oil, not to compound the blue
with a pigment which may possibly contain acid, such as constant white.
Acid, as we have said, is the great test for ultramarine; whence if a
sample be sophisticated with cobalt, its blue colour will not be
entirely destroyed. With high-class artistic pigments, however,
adulteration is the exception and not the rule. It is as a powder-blue
for the washtub that ultramarine gets disguised, when it is ground up
with soda-ash, chalk, gypsum, &c., and sold sometimes under its own
name, but more frequently as superfine Saxon smalts.
TTITLE BRILLIANT ULTRAMARINE,