We should not forget that remarkable substance of which the well-known Dinas bricks are made. The proportion of silica present ranges from about 96 to 99 per cent., the remainder consisting principally of alumina, though traces of iron, lime, and magnesia frequently occur. There is not, of course, sufficient natural flux for this “clay,” so a small proportion (2.5 to 3 per cent.) of lime is added, which produces the desired effect. In other words, if we can obtain a pure siliceous sand, with hardly any lime, iron or magnesia in it, we have the material of which the better kinds of fire-bricks are made. Such sandy earths are not uncommon in the South of England, but strange to relate, they are not used for the purpose indicated. The earths from which the superior Stourbridge bricks are made, are approximately of the following chemical composition:--Silica, 64.10; alumina, 23.15; iron oxide, 1.85; magnesia, .95; water and loss, 10.00 per cent. It will be observed that the proportion of iron and magnesia here is very small, whilst lime is altogether absent. It is a most excellent earth for the purposes for which it is used, and the chemical results may be taken as a standard for that class of material. Another Stourbridge earth yields as much as 4.14 per cent. of iron, however, whilst its proportion of silica is lower, 51.80, and alumina higher, 30.40, which serves to remind us of the variability of even good earths used in the manufacture of fire-clay goods. Let us now turn to the consideration of pottery clays, of which the following results may be taken as typical:-- _Chemical Composition of Pottery Clays._ ------------+-------+-------+-------- | 1 | 2 | 3 +-------+-------+-------- Silica | 46.38 | 49.44 | 58.07 Alumina | 38.04 | 34.26 | 27.38 Iron oxide | 1.04 | 7.74 | 3.30 Lime | 1.20 | 1.48 | .50 Magnesia | trace | 1.94 | trace Water | 13.57 | 5.14 | 10.30 ------------+-------+-------+--------