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. See also Brick Face brick where no plastering is specified London Brick Company References www.archaeologyinmarlow.org.uk Retrieved February 2012 Alan Cox 1997. A vital component: Stock bricks in Georgian London. Construction History 13, 57-66. Ian Smalley 2021. London Stock bricks: from Great Fire to Great Exhibition. British Brick Society Information 147, 26-34. Notes Architecture of London Bricks