The limestone of London’s grand public buildings comes from far away, but Middlesex provided the humbler building materials: oak, which grows wild as one of the main forest trees, the stiff Thames valley clay that makes London bricks, flints that form the walls of many old churches and builders’ sand. The Roman built in bricks; these were reintroduced to England from northern continental Europe in the 15th century and became generally used in London in the 17th century, especially with the building regulations following the Great Fire. However terraced London houses are still substantially timber buildings with a brick skin and a tile or slate roof.
Until the 20th century when new housing was planned for a green field site, the first step was to remove the topsoil to get to the ‘brickearth’ or London clay. This would yield bricks that would be hand made in moulds and fired in an oven in situ. Then this brickfield would be levelled and built on with the bricks provided by a neighbouring field, and so on .. This does not mean that all London’s clay subsoil is equally suitable as a building material. An ideal site for brickmaking is a stratum known as ‘Claygate Beds’ or ‘Claygate Member’ which is a clay and sand mix. The sandy silt reduces the clay’s tendency, exacerbated by its iron sulphide content, to crack during drying and burst during firing.
The authors of ‘Hampstead Heath: the Walker’s Guide‘ explain that it is because part of the heath lay on this type of subsoil that it was, damagingly, exploited for brickmaking in the late 19th century. The iron sulphide, present in varying proportions, makes London clay blue, or red ochre coloured on exposure to the air. London bricks have been yellow since the 18th century, aiming to look more like Classical stone. This is achieved by leaving the clay in the rain to flush out the sulphates, mixing it with surface silt from the Claygate Beds and then with ash and up to 15% of chalk, brought in from mines in Kent and Sussex.
Hampstead Heath was also a prime source for builders’ sand. London’s Northern Heights are, according to the guide, a sandbank laid down 40 million years ago, the ‘deposit of a vast freshwater river quite as large as the Ganges flowing from the western part of Britain.’ Sand extraction on the heath created Hampstead’s famous ponds — the water staying trapped because iron oxide has helped create a crust of sandstone.
Most of Middlesex was built up, after World War 1, at a time when for cost reasons builders had stopped using handmade local bricks and gone over to ‘flettons’, a smooth salmon-pink brick taking its name from the town of Fletton in Huntingdonshire that was the base of operations of the giant London Brick Company. Fletton clay is unusual because it contains small nodes of carbon and when fired the bricks burn themselves. Some architects consider fletton an unattractive facing material, although Fletton-type bricks appear to be the material with which the architect Charles Holden built his celebrated Piccadilly-line stations in the 1930s which he described as ‘brick boxes with concrete lids’, inspired by contemporary public buildings in Sweden and Holland.
If you wanted to demolish something in a hurry the forests provided another resource. Coppiced or pollarded hornbeam, willow or alder were the raw material for charcoal, one of the main ingredients of gunpowder, which was produced at Hounslow at least from the time of Henry VIII. This counter-intuitively saved many buildings from destruction: the Great Fire of 1666 was finally tamed by the Trained Bands carrying out controlled explosions to create fire-breaks.