[A] Raw lead comprises red lead, white lead, and litharge. If introduced in this form as a constituent of glaze it is soluble in dilute acids. If, however, the raw lead is fluxed by heating with a part or the whole of the silica, it is converted into “fritted lead.” The solubility of the frit depends upon the relative proportions of material taken. Thorpe[23], as a result of numerous analyses of lead silicates (after determining their solubility as regards lead), both simple and complex, in use in the potteries and on the Continent, found that the quantity of lead dissolved had no necessary relation to the quantity of lead in the silicate. “Primarily and in the main the insolubility of the lead depends not upon any one oxide or group of oxides, but upon the maintenance of a certain proportion between the whole of the basic oxides on the one hand and the whole of the acidic oxides on the other. If the value of ratio bases/acids is higher than, or approximately equal to, two, the amount of the lead extracted is small, but if it fall much below two, the quantity of lead dissolved begins rapidly to increase.” On the subject of the use of leadless glazes, the Committee conclude that in all classes of pottery ware a great many articles can be manufactured in a very high state of perfection, with reduction in the cost of production of certain classes of common ware, such as jampots and Persian painted ware; but that in certain other classes, owing to the excessive number of “seconds,” their use would entail increased cost or sacrifice of quality, so much so as to involve loss of important markets; and, finally, that certain kinds of ware, in consequence of difficulties relative to accuracy in reproducing old patterns, colours, or methods of decoration, cannot at present be made at all without use of lead. In the case of manufacturers who are able to conform to the Thorpe test of low solubility--_i.e._, glaze which yields to a dilute solution of hydrochloric acid not more than 5 per cent. of its dry weight of a soluble lead compound, calculated as lead monoxide (PbO)--important relaxation of certain special rules are allowed, such as limitation placed on the employment of females and young persons, and periodical medical examination of the workers. H. R. Rogers[24], one of H.M. Inspectors of Factories, Stoke-on-Trent, has worked out a simple test to show approximately how much lead has been used in the glaze of a piece of pottery. Thus, by treating glazes with hydrofluoric acid for forty seconds, absorbing the liquid with filter paper, precipitating the lead on the paper as the sulphate, dissolving out the sulphate soluble in water, and then precipitating the lead on the paper as sulphide, stains are produced varying, in depth of colour, according to the proportion of lead in the glazes concerned (see Plate IV.). Briefly summarized, the recommendations of the Potteries Committee in regard to the processes are--