Souring:

The fabric after bleaching contains metallic salts such as calcium carbonate, magnesium carbonate etc., and other mineral matter. These salts, if left on the fabric, will not only give a harsh feel but also affect the uniformity of the shade during dyeing. In order to make these salts soluble in water, the fabric is treated with either dilute hydrochloric acid or sulphuric acid, the former is used for fabrics meant for dyeing whereas latter, being cheap, is used for fabrics meant for white finish. Hydrochloric acid which forms soluble calcium salts is better then sulphuric acid. The concentration used is about 0.5 to 1.0% on the weight of fabric.