The most serious impurities in refractory clays are lime, magnesia, soda, potash and titanium and their compounds as they lower the refractoriness of the material. Iron, in the state of ferric oxide is of less importance, but pyrites and all ferrous compounds are particularly objectionable. Pyritic and calcareous nodules may, to a large extent, be removed by picking, and by throwing away lumps in which they are seen to occur. There is, at present, no other means of removing them. Fireclays may be ground directly they come from the mine, but it is usually better to expose them to the action of the weather as this effects various chemical and physical changes within the material, which improves its quality as well as reduces the power required to crush it. To take full advantage of the refractory qualities of a clay it is necessary to select it with skill, prepare and mould it with care, to burn it slowly and steadily, to finish the heating at a sufficiently high temperature and to cool the ware slowly. Rapidly heated fireclay is seldom so resistant to heat under commercial conditions as that which has been more steadily fired. Rapid or irregular heating causes an irregular formation and distribution of the fused material during the process of vitrification (p. 37) and so produces goods which are too tender to be durable. It is, therefore, necessary to exercise great care in the firing. =Shales= are rocks which have been subjected to considerable pressure subsequent to their deposition and are, consequently, laminated and more readily split in one direction than in others. Some shales are almost entirely composed of silica or calcareous matter, but many others are rich in clay, the term referring to physical structure and not to chemical composition. The clay-shales occur chiefly in the Silurian and Carboniferous formations, the latter being more generally used by clayworkers. Clay-shales are valued according to (_a_) the proportion of oil which can be distilled from them, those rich in this respect being termed _oil shales_; (_b_) the colour when burned, as in _brickmaking and terra-cotta shales_; (_c_) the refractoriness, as in _fireclay shales_ and (_d_) the facility with which they are decomposed on exposure or on heating and form sulphuric acid as in _alum shales_. _Oil shales_ contain so much carbonaceous matter that on distillation at a low red heat they yield commercially remunerative quantities of a crude oil termed _shale tar_. In composition they are intermediate between cannel coal and a purely mineral shale. To be of value they should not yield less than 30 gallons of crude oil per ton of shale, with ammonia and illuminating gas as by-products. They are of Silurian, Carboniferous or Oolitic origin, the Kimeridge shale associated with the last-named being very valuable in this respect. The most important oil shales occur in Scotland. The _fireclay shales_ have already been described on pages 53 and 116.