Pumice stones are first presoaked 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 localized washing effect with clear blue/white contrast. This technique is called acid washing. This is also termed as ‘snow washing’ or ‘ice washing’.
Acid washing effect gives indigo jeans sharp contrasts. The process was created in Italy and was patented in 1986. Interestingly, acid wash denim does not use actual acid in the wash process. Instead the process uses pumice stones soaked in bleach to alter the color in a nonuniform manner. The result is often splotchy yellowish patterns.
This finish provides jeans with sharp contrasts in tonal color. This is also known as ice or snow washing. This is achieved by dry tumbling with pumice stones soaked with bleaching agents to produce a ‘snow’ pattern effect on denim. Stones are used to deposit a chemical on garments to strip off the color. This chemical deposit removes color only from the outer surface of the garments and produces a frosted appearance. Indigo and selected sulphur dyes are the most popular candidates for this procedure.
Acid washing is also known as stone washing, which is accomplished with pumice stones. For acid wash, base color of the garment is taken out by spraying acid on the specified areas. The wash is performed in two steps: in the first step, garment is washed without water and in the second step, with water.
Process Flow Chart of Acid Washing:
The basic steps of denim acid washing are shown as follows:
Desizing (10–15 min)
Soaking stones with strong oxidising agent
Draining excess liquor
Dry tumbling (10–30 min)
Rinsing to remove stone dust
Rinsing with oxalic acid (in case of potassium permanganate)/anticolor with reducing agent (in case of sodium hypochlorite)
The normal acid washing procedure is described below:
1. Soak volcanic stones in potassium permanganate solution. Stones absorb chemicals and become saturated. The stones are then dried in normal air or sun. The stones are ready for work.
2. Denim garments are now made ready for wash. They are desized and destarched in water in a tumble washer and dried in a spin dryer.
3. The garments are put in a separate tumble washer filled with treated stones. Water is not added. Now run the tumble dryer and wash the garments without water. Tumble washer is run to wash the garments without water. Stone will abrade the garments, especially the exposed parts. Hidden parts will not be abraded.
4. Thereafter, the garments are taken out of the tumble and transferred to another tumble filled with water for washing and rinsing. After rinsing is over, the prominent acid wash effect will show up.
The treated stones carry the chemical to bleach the exposed parts and bleach them to white. But the hidden parts remain untouched. Whitening agents are often added to water during rinsing, to make the white color in the blue jeans whiter to display the acid wash.
The process of acid washing jeans involves soaking porous pumice stones and chlorine or PP bleach. Sodium bisulfate is used for neutralization. It is recommended to use two separate washing machines for acid washing, neutralization, etc.
Porous pumice stones are soaked in a chlorine or PP bleach solution for about 20–60 min. After complete the stones are completely soaked in the bleaching agent, excess water is removed from them. Then the stones and denim garments are put into a washing machine for about 20 min. In this process there should not be a single drop of water. The drain should be open during the complete process.
When the required look is achieved, the fabric is transferred into another washing machine for regular washing for neutralization. After neutralization, the garments are dried.
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