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.
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.