Walk through almost any street in inner London built before 1900 and you're looking at London stock brick. The yellow-grey, slightly irregular, handmade brick produced from Thames valley brick-earth defines the domestic architecture of the city — Georgian terraces in Islington and Bloomsbury, Victorian streets in Hackney and Lambeth, the side walls of warehouses in Bermondsey, the rear elevations of mansion blocks in Kensington. It's everywhere.
It's also consistently mishandled by contractors who clean it as though it were modern brick. The damage is permanent and, in the worst cases, disfiguring.
What London stock brick actually is
London stock was made from brick-earth — the alluvial clay found extensively in the Thames valley — mixed with chalk and ash. The ash, typically from domestic coal burning and industrial processes, contributed to the distinctive colour and affected the chemical composition of the clay. The mixture was hand-formed into moulds, dried, and fired in clamps: large outdoor stacks of bricks fired from within rather than in purpose-built kilns.
The clamping process produced significant variation. Bricks nearest the heat source fired harder and darker (these became 'seconds', used for rear walls and internal construction). Bricks further from the heat fired more lightly and retained the characteristic pale yellow-grey colour. Within a single brick, the density and hardness varies — the surface may be slightly harder than the body beneath.
The practical implications of this are significant. London stock is softer than modern machine-made brick. Its surface has no protective glaze. The lime mortar it was laid in is softer still. The whole system — brick, mortar, and joint — was designed to work together, to move slightly as the building settles and the seasons change, to allow moisture to move through and evaporate. Anything that disrupts this system causes damage.
Why acid cleaning is not an option
Hydrochloric acid (muriatic acid) is used routinely on modern brick. On new construction it removes mortar smears, efflorescence, and surface contamination without harming the hard-fired, dense modern brick face. Contractors who move from new build to Victorian restoration sometimes don't realise that London stock is a fundamentally different material.
Hydrochloric acid attacks the calcium carbonate in the brick's composition directly. On London stock, which contains chalk and lime in its brick-earth mix, the effect is rapid and visible: the surface fizzes, the face erodes, and the colour changes permanently. This isn't staining — it's chemical dissolution of the brick material. Neutralisation washes don't reverse it.
We have assessed buildings where contractors have used acid on London stock and the only response available was 'the damage cannot be undone.' In some cases, the brick faces had to be repointed repeatedly to manage salt release triggered by the acid treatment. In others, the loss of the original brick face was simply a permanent part of the building's appearance.
Why high-pressure washing is not an option
The softness of London stock means that high-pressure water erodes the brick face faster than it removes soiling. The surface texture is damaged, the fired skin of the brick is compromised, and the mortar joints — which are soft lime — are eroded significantly. The result is a building that looks superficially clean but has structural and weathering issues that will develop over the following years.
The increased surface porosity from pressure washing also accelerates biological recolonisation. London stock that has been pressure-washed frequently redevelops algae and lichen growth faster than it did before.
What the correct approach looks like
Survey first. Before anything is specified, we assess the brick condition, the mortar condition, and the soiling type. London stock in good condition with atmospheric soiling requires a different approach from stock with active salt problems, spalling, or damaged pointing. Treating all of these the same way produces the wrong result for most of them.
DOFF cleaning. Superheated steam at low pressure is the appropriate primary method for London stock brick with atmospheric soiling and biological growth. The steam lifts soiling and kills biological matter without introducing significant water and without any mechanical abrasion. It is the method we specify most often for Victorian brickwork.
Chemical systems for soft brick, if appropriate. There are specialist low-pH or neutral chemical systems formulated for soft historic brick that work by softening the soiling rather than reacting with the masonry. These require careful specification and neutralisation. We use them in specific cases where DOFF alone is insufficient and the brick condition rules out TORC.
Correct repointing. Brick cleaning and repointing often go together. If the mortar joints are eroded or incorrect — particularly if hard cement has been used on soft stock — this needs to be addressed alongside the cleaning. Wrong mortar does more long-term damage than wrong cleaning. Lime mortar, properly colour-matched to the original, is the correct specification for London stock.