Red Chalk





The colouring matter of which is sesquioxide of iron, is used as a

crayon. Some specimens are excessively hard, so much so that they are

difficult to crush, even in an iron mortar; while others have the

consistence of the softest iron-ochres. They vary too in tint from a

fawn colour to the softest brick-red, occasionally being almost as

bright as a mixture of equal weights of vermilion and Venetian red. The

amount of iron oxide present has been found to range from four to

thirty-seven per cent, according to the depth and hardness of the

samples. When a specimen of red chalk tolerably rich, but not too rich,

in iron oxide is finely powdered and strongly ignited, it offers a

remarkable change of colour, becoming a dull sage-green. Perhaps this,

if it were permanent, might prove useful in foliage tints.





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