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 and soft appearance come from the yellow local clay from which the bricks were made
During the 19th century the fields around London were built up with new housing. Commonly, a field would be excavated to expose the brickearth or 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.
In the 19th century, London stock bricks were available in a variety of grades priced according to their consistency and their regularity of shape and colour. High grade bricks were used for face work and lower grades were bought for use as internal bricks. Unfortunately it seems to have been common practice for a high grade brick to be broken in half so that it could be used twice, each end appearing as a header in the wall. The result of this parsimony was that the wall was deficient in bonding bricks, i.e. bricks tying the outer skin of brickwork back to the inner part of the wall, often resulting in the outer skin peeling away from the inner and bulging out. This issue, known as snapped or snap headers, leads to walls which need to be repaired either by rebuilding or by fitting various types of proprietary tie.
Most London stock bricks are more or less porous, as is the lime mortar in which they have traditionally been laid. The pointing should be flush pointing so that rain water can run down off the surface and not be encouraged to soak in to the wall as is the case with recessed or struck pointing. When used in this way the brickwork does not get wet all the way through and is thus effectively waterproof.
Lime mortar tends to weaken in London’s acidic rainwater and needs repointing several times a century. It has been common since the widespread availability of Portland cement to see London stock brickwork repointed using much stronger cement mortar. As repointing consists of replacing the outer 20 - 40mm of mortar, the effect of this is to make the outer 20 - 40mm of the brickwork harder and stronger than the interior of the wall. This can lead to spalling of the brick surface, and can also encourage the bulging associated with snapped headers.