In wool scouring,
cotton bleaching, and other processes requiring the use of alkaline
carbonates, ferric oxide is precipitated on the fibre. A yellowish tinge
is communicated to bleached fabrics, and to dye bright and light colours
is rendered almost out of the question. You may always suspect iron to
be present in water flowing from or obtained directly out of old coal
pits, iron mines, or from places abounding in iron and aluminous shales.
Moreover, you sometimes, or rather generally, find that surface water
draining off moorland districts, and passing over ochre beds, contains
iron, and on its way deposits on the beds of the streamlets conveying
it, and on the stones, red or brown oxide of iron. All water of this
kind ought to be avoided in dyeing and similar operations. The iron in
water from old coal pits and shale deposits is usually present as
sulphate due to the oxidation of pyrites, a sulphuret or sulphide of
iron. Water from heaths and moorlands is often acid from certain
vegetable acids termed "peaty acids.
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