The most important oil shales occur in Scotland.
The fireclay shales have already been described on pages 53 and 116.
The brickmaking shales are those which are sufficiently rich in clay to form a plastic paste when ground and mixed with water. They can be made into bricks of excellent colour and great strength, but for this purpose require the use of powerful crushing and mixing machinery. They are usually converted into a stiff paste of only moderate plasticity and are then moulded by machinery in specially designed presses, though some firebricks are made from crushed shale mixed into a soft paste with water and afterwards moulded by hand. Some shales, such as the knotts at Fletton near Peterborough are not made into a paste, the moist powdered shale being pressed into bricks by very powerful machinery.
Brickmaking shales may be found in any of the older geological formations, though they occur chiefly in the Silurian, Permian, Carboniferous and Jurassic systems. The purer shales of the Coal Measures burn to an agreeable cream or buff colour, the less pure ones and those of the other formations mentioned produce articles of a brick-red or blue-grey colour.
Where the shales are of exceptionally fine grain and their colour when burned is very uniform and of a pleasing tint they are known as terra-cotta shales, the red terra-cottas being chiefly made from those occurring in Wales and the buff ones from the lower grade fireclays of the Coal Measures.
Alum shales are characterised by a high proportion of pyrites, which, on roasting, form ferrous sulphate and sulphuric acid. The latter combines with the alumina in the shale and when the roasted ore is extracted with water a solution of iron sulphate and aluminium sulphate is obtained. From this solution (after partial evaporation) alum crystals are obtained by the addition of potassium or ammonium sulphate.
The chief alum shales are those of the Silurian formation in Scotland and Scandinavia. The Liassic shales of Whitby were at one time an equally important source of alum.
During recent years a large amount of alum has been obtained from other sources or has been made from the lower grade Dorset and Devonshire ball clays by calcining them and then treating them with sulphuric acid. These clays being almost free from iron compounds yield a much purer alum at a lower cost.
Brick clays are those which are not suitable—either from nature or situation—for the manufacture of pottery or porcelain and yet possess sufficient plasticity to enable them to be made into bricks. The term is used somewhat loosely, and geologists not infrequently apply it to clays which are quite unsuitable for brickmaking on account of excessive shrinkage and the absence of any suitable non-plastic medium. Large portions of the 'London clay' are of this nature and can only be regarded as of use to brick- and roofing-tile-manufacturers when the associated Bagshot sands are readily accessible. Similarly, some of the very tough surface clays of the Northern and Midland counties are equally valueless, though designated 'brick clays' in numerous geological and other reports. It is, therefore, necessary to remember that, as ordinarily used, the term 'brick clay' merely indicates a material which appears at first sight to be suitable for brickmaking, but that more detailed investigations are necessary before it can be ascertained whether a material so designated is actually suitable for the purpose.
It is also important to observe that local industrial conditions may be such that a valuable clay may be used for brickmaking because there is a demand for bricks, but not for the other articles for which the clay is equally suitable. For instance, a considerable number of houses in Northumberland and Durham were built of firebricks at a time when it was more profitable to sell these articles for domestic buildings than for furnaces.