402 Professor T. E. Thorpe [May 4, 



action of ordiuary acids and alkalis. For this reason the analytical 

 chemist prefers to use vessels of hard porcelain rather than of glass 

 in operations in which it is desirable to exclude minute portions of 

 foreign substances. At the same time, experience has shown that the 

 susceptibility of glass to the action of water, or solutions of alkalis, 

 acids and salts, may be considerably modified by due regard to its 

 composition. 



By far the greater portion of the domestic and sanitary ware and 

 china made in this country, together with a great variety of articles, 

 such as glazed bricks, wall- and hearth-tiles, so-called " china furni- 

 ture" — door-knobs, finger-plates, escutcheons, bell-handle fittings, 

 discs for water-taps, &c, — electrical sundries, as insulators and fittings 

 for electric-light installations — and countless other articles employed 

 partly for use and partly for ornament, are glazed with materials 

 containing compounds of lead. 



The lead may be present in the dipping tub as white lead or 

 red lead, together known technically as "raw" lead, or these sub- 

 stances or litharge may be previously heated with some form of 

 sufficiently pure silica — usually in this country calcined and pow- 

 dered flint — whereby a fusible lead silicate is formed, which is then 

 powdered and mixed with the other materials of the glaze. Or 

 the red lead or litharge may be added to a mixture of powdered flint 

 and china-stone, or china-clay and felspar, and the whole fused 

 together to form what is substantially a double silicate of lead and 

 alumina, with, it may be, small quantities of admixed lime and 

 alkalis, this fritt being added as before, in the requisite proportion 

 to the rest of the glazing materials, such as borax, chalk, flint, 

 china-stone, also, for the most part, previously fritted together ; 

 or, lastly, the whole materials of the glaze may be melted together, 

 so as to form a vitreous, homogeneous mass. 



The practice diifers in different works, and is, for the most part, 

 purely empirical ; each manufacturer adhering as closely as he can 

 to the composition which he finds by experience gives what he is 

 content to regard as satisfactory when used in conjunction with the 

 particular mixture of ball-clay, china-stone, china-clay and flint, &c, 

 he employs for the body of the ware, and when fired or otherwise 

 manipulated as he deems best. Hence it follows that no two manu- 

 facturers use a glaze of precisely the same proximate or ultimate 

 chemical composition, unless, indeed, they have been supplied, as not 

 infrequently happens, with their glazing material from a professional 

 glaze-maker. 



The amount of lead, calculated as lead oxide, and on the dry 

 material of the glaze, in the case of earthenware or ordinary English 

 china, may vary from 12 or 13 per cent, to as much as 21 or 22 per 

 cent. That used for covering tiles, especially the decorated and 

 coloured varieties, may contain upwards of half its weight of lead 

 oxide. This lead oxide may either be as " raw " lead — that is, white 

 lead or occasionally red lead; or it may be "fritted" lead — that is, 



