A little oxide of cobalt or zaffre is frequently added to alter the shade.
zaffre is the roasted mineral ground with sand, while smalt, a term used more frequently, is the fused mixture with sand.
The first practical use of cobalt was in the form of zaffre or cobalt blue.
zaffre, digested in aqua regia, and diluted with four times its weight of water, is sometimes employed; a green tint results.
zaffre is now made from the cobalt ores found in these hills.