discharge

Discharge is the deliberate removal of dye from fabric through chemical treatment, most commonly using sodium hydrosulfite (sodium bisulfite), bleach, or other reducing agents. The process breaks down the molecular bonds holding dye to fiber, leaving white or near-white areas on otherwise colored cloth. It is used to create patterns, designs, and decorative effects on pre-dyed textiles without the cost and complexity of printing from scratch.

The technique works best on natural fibers, particularly cotton and wool, and on vat dyes and direct dyes that are bound tightly enough to the fiber to survive the manufacturing process. Synthetic fibers resist discharge because most synthetic dyes are engineered to withstand chemical assault. Discharge paste, a thickened solution of the reducing agent mixed with water and often gum thickener, is applied to fabric using screens, blocks, or rollers much like printing ink. Heat (usually 150, 180 degrees Celsius) accelerates the chemical reaction, causing the dye to lose color within minutes.

The most common form is bleach discharge, which uses sodium hypochlorite or chlorite and works quickly but can damage fiber if over-applied or left too long. Reducing discharge (using sodium hydrosulfite) is gentler and preferred for delicate fabrics; it typically requires steam or hot air curing. Color discharge deliberately leaves some dye behind by using lower-strength chemicals or shorter processing times, creating patterned effects in lighter shades on the original ground color.

Quality control is critical. Uneven paste application causes patchy or streaked discharge. Over-exposure weakens the fabric, causing brittleness or holes. Under-exposure leaves faint, incomplete patterns. Residual discharge chemicals must be thoroughly rinsed out; sulfites left in fabric can cause yellowing or brittleness over weeks or months in storage. Water hardness, temperature, and pH all affect the speed and uniformity of the reaction.

Discharge competes with resist printing (wax, paste, or tie-off methods) and with direct printing of white dye onto colored ground, each offering different economics and aesthetic results. The method remains common in fashion textiles, especially for creating white or light-colored motifs on dark indigo denim and on patterned cotton shirting. Environmental concern focuses on chemical disposal and wastewater treatment, particularly when sodium hypochlorite is used.