London stock brick is the type of handmade brick which was used for the majority of building work in London and South East England until the growth in the use of Flettons and other machine-made bricks in the early 20th century. Its distinctive yellow colour is due to the addition of chalk. Another important admixture is 'spanish', which consists of ashes and cinders from rubbish. The spanish ignites on firing and reduces fuel costs at the firing stage. London Stocks are still made in comparatively small quantities in traditional brickworks, mainly in Kent and Sussex, for heritage work, and machine-made versions are available for use where a cheaper approximation to the traditional product is acceptable. Red stock bricks are also fairly common, but only the yellow or brown bricks are usually known as London stocks. History During the 19th century, the fields around London were built up with new housing. Commonly, a field would be excavated to expose the brickearth (which was found overlying the London clay subsoil), which was then turned into bricks on the site by moulding and firing them. The bricks would then be used to build houses adjacent to the brick field – transport was expensive. Once the building work was nearing completion the brick field would be levelled and built upon while a new brick field further out would supply the bricks. Bricks were also made in clay areas surrounding London. By the 1890s it is estimated 100 million bricks per year were being produced in West Middlesex, supplying Victorian London's demand for building materials. Bricks were also made in Kent, Essex and other areas where they could be imported to London by rail. In Stock, Essex, there is a common belief that 'stock bricks' originated there; bricks were certainly made there, but the name is a coincidence, stock being a common English word with many meanings and also a common place-name element. Air pollution in London during the 19th century and early 20th century commonly caused the bricks to receive a sooty deposition over time, turning the bricks greyish or even black, but the removal of contaminants from the air following the passing of the Clean Air Act in 1956, has enabled older buildings to be cleaned and new buildings to retain their natural colour. In the 19th century, London stock bricks were available in a variety of grades priced according to their consistency