London stock is one of the most misunderstood materials in Victorian building stock. Contractors treat it like modern brick. It is not. It is softer, more absorbent, chemically sensitive, and irreplaceable once damaged. Cleaning it correctly requires understanding what it is.
London stock is a handmade brick produced from Thames valley brick-earth, a distinctive alluvial clay deposited across what is now Greater London and the surrounding counties. It is not a single material but a family of related clays, all sharing the characteristic yellow-grey colour that defines Victorian London's domestic architecture. The colour comes from the composition of the brick-earth: lower iron content than typical red brick clays, with calcium carbonate from chalk mixed in during manufacture.
The bricks were formed by hand in wooden moulds, which means they vary slightly in dimension, texture, and density even within a single course. They were fired in clamp kilns rather than continuous kilns: a clamp being a large pile of green (unfired) bricks stacked with fuel between them and fired as a single mass. Clamp firing produces lower, less even temperatures than kiln firing, which means the bricks are softer and more porous than modern machine-made bricks. The fire skin, the hard outer crust formed where the brick surface is in direct contact with combustion, is the most durable part of the brick and the hardest to replace once damaged.
This history matters for cleaning because every characteristic of London stock, its softness, its porosity, its calcium carbonate content, creates a vulnerability to the cleaning methods that contractors default to on harder modern brick.
Hydrochloric acid cleaning, still widely used by general building contractors for mortar residue removal and general cleaning on modern brick, is destructive on London stock. The calcium carbonate in the brick-earth and in the chalk that was mixed into the clay reacts directly with hydrochloric acid. The reaction is visible as fizzing and bubbling at the surface. What is being dissolved is not just the surface deposit but the material that gives the brick its structure and colour.
The result after acid cleaning is a bleached, powdery surface with permanent loss of colour and texture. In cases where acid has been applied multiple times, the brick face can be so deeply etched that the surface begins to erode away. We see this damage regularly on buildings where well-intentioned previous work has used the wrong chemistry. It cannot be reversed.
High-pressure water impacts the surface with force. On a modern dense brick, this force is absorbed without significant damage. On London stock, the lower firing temperature means there is less vitrification of the surface. High-pressure water erodes the relatively soft surface material, removes the fire skin, opens the surface to deeper water penetration, and drives existing soluble salts further into the brick body. When those salts migrate back to the surface as the brick dries, they crystallise within the pore structure and cause further disruption from within.
The damage from pressure washing is often not obvious immediately after the clean, which is why contractors continue to use it. Over the following winter, as water penetrates the opened surface and freezes, the consequences become apparent. Spalling, surface crumbling, and mortar joint failure all accelerate after pressure washing on soft historic brick.
DOFF superheated steam at 150°C, delivered at low pressure (typically below 100 psi at the nozzle), is the appropriate method for London stock brick. The temperature does the work. At 150°C, steam softens biological growth, general soiling, and atmospheric deposits and allows them to be wiped or rinsed away without mechanical abrasion. The pressure is low enough not to erode the brick surface or damage the mortar joints.
DOFF does not introduce excess water into the wall. The steam volume is small relative to pressure washing, and the heat accelerates the drying process. The brick surface is not saturated.
The method is consistent with Historic England's guidance on soft historic brick, and it's the specification we use across all London stock brick cleaning contracts. Test cleans are carried out before full application to confirm the settings are appropriate for the specific brick and soiling in question.
London stock brick walls were built with lime mortar. The two materials form a system: the mortar is sacrificial, softer than the brick, allowing moisture to escape through the joint rather than through the brick face. Where hard cement repointing has been applied, the system is broken. Moisture that enters the wall cannot escape through the rigid cement joint, and it escapes through the brick face instead, carrying dissolved salts and causing spalling.
Where existing mortar is in poor condition, lime mortar repointing follows cleaning. The specification, joint profile, and colour are matched to the original material. On listed buildings, this is submitted to the conservation officer for approval.
A correct DOFF clean removes soiling and reveals the original brick colour, which is often significantly lighter than the accumulated patina suggests. The brick will not look new, because patina is a material property of old brick, not a surface deposit. But it will look clean, even, and correct. We do not aim for an artificial appearance and we would advise against any specification that does.
Acid is appropriate on modern dense calcium silicate or engineering brick where there is no reactive calcium carbonate in the material. On London stock, which contains chalk as a deliberate component of the original mix, acid reacts with the brick body itself. If a contractor is proposing acid on Victorian London stock, they are applying a modern brick specification to a material that cannot receive it. We have seen the results often enough to be direct on this point.
Cost depends on the area to be cleaned, the access required, the degree of soiling, and whether repointing is required at the same time. A typical Victorian terrace rear elevation cleaned with DOFF might start from £1,200–£2,500. Full front elevations with scaffolding vary considerably depending on the building and the soiling type. We provide written quotes within 48 hours of a free site visit.
Cleaning unlisted buildings in standard residential streets generally does not require planning permission. Listed buildings require listed building consent for any cleaning. In conservation areas, external works may require prior approval from the local authority, though minor cleaning is often permitted development. We advise on the specific position for each building at the initial site visit.