The sponge is one of the most useful articles in the household and in the arts, and it is well to know both how to choose it and how to care for it. The best sponges come from the Mediterranean, and are found compressed and dried, so that when soaked and fully expanded they increase to several times the bulk which they have in the compressed state. In selecting a sponge see that it is not loaded with sand and limy matter. To cleanse sponges from these impurities they are beaten, washed in water, and sometimes soaked in acid. It is said, however, that the use of mineral acids destroys the fiber of the sponge, and this is very probably the case. It is possible that dilute acetic acid might be used without any bad effects; and the cheap acid obtained from wood by destructive distillation would answer every purpose.
Second-hand sponges are frequently offered for sale. These are picked up in various places, washed, soaked in solution of chloride of lime or soda, again washed in clean water, and dried. Such sponges do not last long: they are frequently half rotten before they reach the bleacher's hands; and if he does not do his work thoroughly they may even convey infectious matter. But being cheap and pretty they meet with a ready sale.
For all ordinary purposes the dark color of the sponge is no objection; but when a white sponge is desired the following method of bleaching has been highly recommended: -
Having made the sponges free from sand and calcareous matter by gently beating them, wash them in water, squeeze them as dry as possible, and then place a few at a time in a solution of permanganate of potassa, made by dissolving one hundred and eighty grains of the salt in five pints of water, and pouring a portion of the solution into a clean glazed vessel. Let them remain a few moments until they have acquired a dark mahogany-brown color, when they are to be squeezed by hand to free them from the solution. They are then dropped a few at a time into a bleaching solution made as follows: Hyposulphite of soda, ten ounces; water, sixty-eight fluid ounces; when dissolved, add five fluid ounces of muriatic acid. This solution should be made the day before being wanted for use, in order that the sulphur precipitated by the acid may be easily separated. This solution is poured off from the sulphur, and, if necessary, is strained through muslin into a glazed vessel. The sponges are allowed to remain in this solution a few moments, squeezing them with the hand occasionally in order that every part may be reached by the fluid; then squeeze out and wash through several waters to rid them of the sulphurous odors. They may be completely deodorized by washing them in a weak alkaline solution of bicarbonate of soda, about one hundred grains to the pint, and then washing through several waters to free from any traces of the alkali. Much caution must be used in this last operation lest the bleaching effect of the previous solutions be partly neutralized. When the sponges are nearly dry immerse them into a solution of glycerine water, one half ounce to the pint, squeeze them as dry as possible, and dry them in the shade. Be sure and not let direct sunlight on them until dry. They will be as soft and white as wool.