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The ash or mineral matter in paper may be derived from three sources, namely, traces of the original mineral substances taken up by the flax plant from the soil, and still remaining associated with the felted pulp; the mineral matters, such as soda and alum, introduced with the size; and, lastly, the mineral compounds used to whiten, to weight, or to finish the paper, or in bleaching the fibre and as 'antichlors.' In common and adulterated papers the ash greatly exceeds 1 per cent., twelve parts per hundred of paper being no unusual proportion. This 'filling' may contain or consist of the following substances: kaolin or china-clay, silicate of lime or 'pearl-hardening,' chalk or whitening, lead-white, baryta white or 'white dressing,' artificial gypsum or 'satin-dressing,' and a mixture of aluminium hydrate with magnesium carbonate or with calcium carbonate, known as 'satin-finish' or 'satin-white.' Other substances which increase the amount of ash left when a paper is burnt are blue colouring matters, introduced to counteract the natural yellow tint of the pulp.

These include artificial ultramarine, smalt or cobalt blue, and Prussian blue.

Last Update: 2011-01-23