Opacity, or enamel, as in lamp-shades, is produced by adding As2O3, Sb2O3, SnO2, cryolite, etc.
"An Introduction to Chemical Science"
R.P. Williams
At length he tried a stone from Greenland called "cryolite," which had already been used for making a kind of porcelain.
"Diggers in the Earth"
Eva March Tappan
cryolite is now melted by electricity.
"Diggers in the Earth"
Eva March Tappan