Citric Acid is used for fixing animal (protein) fibres with Procion MX dyes (hot water dyeing method) and other acid dyes. Use 25g of citric acid for each 100g of yarn or fabric.
With natural dyes citric acid is principally used to change the PH of the dye bath, making it more acidic. This will change the colour achieved with certain dyes. For example, cochineal produces orange shades in acidified water rather than pinks. Citric acid is also added when using copper sulphate as a mordant.
Please note: this is not food state citric acid.