For the best varieties of glass, viz., optical glass, flint glass and the whitest sheet-glass, as well as for the best Bohemian glass, a very pure variety of sand is required, preferably containing less than 0·05 per cent. of iron, and not more than 0·05 per cent. of other impurities such as alumina, lime or alkali. As a matter of fact, sands containing so little iron rarely contain any other impurity except alumina in measurable quantities. The best-known deposit of such sand in Europe is that at Fontainebleau near Paris, but equally good sand is found at Lippe in Germany, whence sand is delivered commercially with a guaranteed silica content of 99·98 per cent. Sand of excellent quality, although not quite so good as the above, is obtained at Hohenbocka in Germany (Saxony) and at a few other places in Europe. In England no deposit of sand of such purity is at present being exploited. Next in order of value to these exceedingly pure sands, come the glass-making sands of Belgium, notably of Epinal. These usually contain from 0·2 to 0·3 per cent. of iron and rather more alumina, but they are used very largely for the manufacture of sheet and plate-glass. When the standard of quality is further relaxed, a large number of sand deposits become available, and the manufacturers of each district avail themselves of more or less local supplies; thus in England the sands of Leighton in Bedfordshire and of Lynn on the East Coast, are largely used. Finally, for the manufacture of the cheapest class of bottles, sands containing up to 2 per cent. of iron and a considerable proportion of other substances are employed.