To clean sponges, soak them for about half an hour in warm dilute hydrochloric acid (1 part of strong acid to 5 parts of water); remove, rinse in water, and then steep in methylated spirit for a further thirty minutes. The hydrochloric acid decomposes the lime soap which is precipitated in the cells of the sponge, and dissolves the lime, leaving the fatty acids of the soap, which are removed by the spirit.