(1) Saturate in a quart of buttermilk for 24 hours, and rub between the hands. (2) Soak in dilute muriatic acid (1 acid to 1} water) for 12 hours, wash well with water, to remove lime, then immerse it in a solution of 2 lb. hyposulphite of soda in 12 lb. water, to which 2 lb. muriatic acid has been added a moment before. After it is sufficiently bleached, remove, wash again, and dry. (3) Soak for several days in cold water, renewing the water and squeezing the sponges occasionally. Then wash in warm water, and put into cold water acidulated with hydrochloric acid. Next dry, take out, and wash thoroughly in soft water; then immerse in an aqueous sulphurous acid (sp. gr. 1. 034) for a week. Afterwards wash in plenty of water, squeeze, and allow to dry in the air. (4) Soak in dilute hydrochloric acid to remove the lime, then wash in water, and place for 10 minutes in a 2 per cent, solution of potassium permanganate. Their brown appearance on removal from this is due to deposition of manganous oxide, which may be removed by steeping for about 2 minutes in a 3 per cent, solution of oxalic acid, to which a little sulphuric acid has been added. As soon as the sponges appear white, they are washed out in water to remove the acid.
Very dilute sulphuric acid may replace the oxalic acid. (5) First wash in tepid water, and then in a solution of hydrochloric acid (5 cc. per litre = 5 fl. dr. per 7 pints), which frees the pores from carbonate of lime; next immerse for 24 hours in a solution composed of 5 pints hydrochloric acid in 100 of water, with addition of 6 pints hyposulphite of soda. (Blondeau.)