ELUTRIA′TION

Cleansing by washing. The term is commonly applied to the operation of washing insoluble powders with water, to separate them from foreign matter, or the coarser portion. It is usually performed by grinding or triturating the ma with a little water until reduced to a very fine powder, and this paste is suddenly diffused through a large quantity of water, contained in a deep ve el, from which, after the subsidence of the gro er portion, the liquid is poured into another ve el, and allowed to deposit the fine powder it still holds in suspension. When this has taken place, the clear supernatant liquor is decanted, and the sediment drained and dried. The coarse sediment deposited in the first ve el is now submitted to a fresh grinding and diffusion through water, and the entire operation is repeated until the whole of the pulverisable portion is washed over. The proper length of time for the liquid to remain in the first ve el depends solely on the density of the powder and the degree of finene required in the product; heavy powders subsiding almost immediately, while light ones often take several minutes to deposit the coarser portion. Sometimes three or more ve els are employed, and the muddy liquor, after remaining a short time in the first, is poured into the next one, and this, in a short time longer, into the third, and so on, until the last ve el is filled, by which means powders of different degrees of finene are obtained, that deposited in the last ve el being in the minutest state of division. The elutriated paste or moist powder is then drained, and dried. On the small scale the trituration is performed with a stone and muller, or in a mortar; on the large scale, in a mill, driven by either horse or steam power. Antimony chalk, bistre, and other pigments, as well as various other substances insoluble in or unacted on by water, are commonly obtained in the state of an impalpable powder by elutriation, or ‘washing over,’ as it is called by amateurs and operatives.