Acid Washing

Pumice stones are first pre-soaked in a solution of a strong oxidising agent (either sodium hypochlorite (5–10%) or potassium permanganate (3–6%)) and are then applied to the garments by means of dry tumbling. This results in a localised washing effect with clear blue/white contrast. This technique is also termed ‘acid washing’, ‘snow washing’ or ‘ice washing’.

After the stones are soaked in a strong oxidising agent such as sodium hypochlorite or potassium permanganate, the excess liquor is removed. This can be accomplished by placing stones in net or mesh fabric before soaking. Then the stones can be removed, and the excess liquor drained off. Another alternative is to place the stones in a rotary tumble machine along with ‘waste’ fabric and tumbling for several minutes to remove the excess solution. Another alternative is to use pre-soaked stones. There are many shapes, varying levels of chemicals and other additives which produce different effects. After soaking the stones, stones and denim garments are put into the machine for tumbling for 10–30min or until the desired effects are achieved. Results are dependent upon dyestuff, fabric, concentration of chemicals, stones and equipment. In some cases, the stones can be reused for another load before resoaking, depending upon their porosity. It is advantageous to remove the denim garments from the machine and transfer them to another machine for washing to minimise the number of machines used for acid washing.

After getting the desired effect, the machine is filled with water for rinsing to remove the stone dust. If potassium permanganate is used, manganese dioxide gets formed (a brown/orange colour) and must be removed by rinsing with oxalic acid or sodium bisulphite. In the case of sodium hypochlorite, a reduction process, antichlor with a reducing agent (e.g. sodium bisulphite) is used for removing the residual chlorine. In acid washing, the addition of water is not required. The colour contrast after tumbling can be further enhanced by a subsequent optical brightening process. Soft stones that dust off easily are more suitable for this process.