_Iron pyrite_ (fig. 2) and _marcasite_ (fig. 3)--both of which are forms of iron sulphide--occur in many clays, particularly those of the Coal Measures. _Mundic_ is another form of pyrites which resembles roots or twigs, but when broken show a brassy fracture. When in pieces of observable size the pyrite may be readily distinguished by its resemblance to polished brass and the marcasite by its tin-white metallic lustre and both by their characteristic cubic, root-like and spherical forms; the latter only show a brass-like sheen when broken. Even when only a small proportion of mundic, pyrite or marcasite is present, it is highly objectionable for several reasons. In the first place, half the sulphur present is given off at a dark red heat and is liable to cause troublesome defects on the goods. Secondly, because the remaining sulphur and iron are not readily oxidized, so that there is a great tendency to form slag-spots of ferrous silicate, owing to the iron attacking the clay at the same moment as it parts with its remaining sulphur. For this reason, clays containing any iron sulphide seldom burn red, but form products of a buff colour with black spots scattered irregularly over their surface and throughout the mass--an appearance readily observable on most hard-fired firebricks. If chalcopyrite (copper-iron sulphide) is present the spots may be bright green in colour. [Illustration: Fig. 2. Pyrite.] [Illustration: Fig. 3. Marcasite.] Slightly magnified. (_From Miers'_ Mineralogy _by permission of Macmillan & Co._) _Carbon_, either free or as hydrocarbons (chiefly vegetable matter) or in other forms, is a constituent of most clays, though seldom reported in analyses. Its presence exercises an important influence in several respects. On heating the clay, with an ample supply of air, the carbonaceous matter may distil off (as shale oil), but more usually it decomposes and burns out leaving pores in the material. If the air-supply is insufficient and the heating is so rapid and intense that vitrification commences before the carbon is all burned away, the pores become filled with the fused ingredients of the clay, air can no longer reach the carbon particles and a black 'core' or heart is produced. Under peculiarly disadvantageous conditions the material may also swell greatly. This is a serious defect in many classes of clay used for brickmaking, and its causes and prevention have been exhaustively studied by Orton and Griffiths (1)[3] but, beyond the brief summary given above, these are beyond the scope of the present work. _Water_ is an essential constituent of all unburned clays, though the proportion in which it occurs varies within such wide limits that no definite standard can be stated. This water is found in two conditions: (_a_) as moisture or mechanically mixed with the clay particles and (_b_) in a state of chemical combination. [Footnote 3: References to original papers, etc. will be found in the appendix.]