Chloride of zinc
½
„
It will be noticed here that chloride of magnesium and chloride of zinc are introduced along with the china clay. Chloride of magnesium is a very powerful softener as well as a weighting material, and one of its uses is to prevent the gritty feel which the addition of clay alone would give to the cloth. It has a great affinity for water, and has thus the power of attracting moisture to the cloth in which it is used. It is this which really constitutes its softening effect.
Chloride of zinc is used to prevent mildew, which is a species of vegetable growth which often occurs in sized cloth which has been left damp, or which attracts moisture.
As chloride of magnesium attracts moisture, it is necessary to use an antiseptic which will counteract the tendency of the cloth to mildew. Chloride of zinc possesses valuable properties as an antiseptic, and therefore it is often used where chloride of magnesium is used in the size as a softening and weighting material.
If china clay is used for medium sizing without using chloride of magnesium, it is necessary to greatly increase the proportion of tallow or other softeners in the mixture. Thus, for every 100 lbs. of flour, 40 lbs. clay, and perhaps 25 lbs. tallow would be used.
Chloride of calcium has a similar effect to chloride of magnesium, but is scarcely as powerful. It is used by many in light-sizing mixtures to prevent the yarn becoming too brittle.
For heavy sizing the proportions of clay and mineral ingredients are increased. In some classes of low shirtings, over 100 per cent. is added to the weight of the yarn. The adhesive material mostly used is wheat flour, as it carries the added materials better than farina or sago; but farina is sometimes used for sizing up to 100 per cent. Sometimes two parts clay to one of flour is used for very heavy sizing. For 100 per cent. sizing about the following proportions may be used:—