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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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