Briquette (diminutive of Fr. brique, brick), a form of fuel, known also as "patent fuel," consisting of small coal compressed into solid blocks by the aid of some binding material. For making briquettes the small coal, if previously washed, is dried to reduce the moisture to at most 4%, and if necessary crushed in a disintegrator. It is then incorporated in a pug mill with from 8 to 10% of gas pitch, and softened by heating to between 70° and 90° C. to a plastic mass, which is moulded into blocks and compacted by a pressure of &FRAC12; to 2 tons per sq. in. in a machine with a rotating die-plate somewhat like that used in making semi-plastic clay bricks. When cold, the briquettes, which usually weigh from 7 to 20 lb each, although smaller sizes are made for domestic use, become quite hard, and can be handled with less breakage than the original coal. Their principal use is as fuel for marine and locomotive boilers, the evaporative value being about the same as, or somewhat greater than, that of coal. The principal seat of the manufacture in Great Britain is in South Wales, where the dust and smalls resulting from the handling of the best steam coals (which are very brittle) are obtainable in large quantities and find no other use.
Some varieties of lignite, when crushed and pressed at a steam heat, soften sufficiently to furnish compact briquettes without requiring any cementing material. Briquettes of this kind are made to a large extent from the tertiary lignites in the vicinity of Cologne; they are used mainly for house fuel on the lower Rhine and in Holland, and occasionally come to London.