Calcium carbonate, or ground white chalk. Used to neutralise any remaining aluminium acetate or iron mordant. Can also be used as an assist to 'harden' water, to aid in the deepening of shades in certain natural dyes.

Add 50g of chalk (calcium carbonate) to 5 litres of warm water. Fully wet the fibre in this solution and wring out, before rinsing thoroughly. This solution can be reused for up to 10kg of fibre.