Five stone cleaning mistakes that damage historic buildings

1. Pressure washing brick

The pressure washer is probably the single most destructive tool in common use on historic masonry. On London stock brick specifically, the damage is irreversible. The brick has a hard outer skin formed during the firing process. This fire skin is thin — perhaps 2-3mm — but it is the layer that gives the brick its weather resistance and its visual character. A pressure washer removes it in minutes.

Once the fire skin is gone, the softer interior of the brick is exposed. It absorbs water more readily. The freeze-thaw cycle acts on it harder. Atmospheric pollution embeds itself more deeply. A building that was cleaned by pressure washing in the 1980s often looks worse now than a building of similar age that was never cleaned at all.

The correct method for London stock brick is DOFF superheated steam. It lifts soiling without pressure. It does not touch the fire skin. The difference in the result is visible immediately, and it lasts.

2. Using acid on limestone or calcareous brick

Hydrochloric acid (often sold as brick acid or muriatic acid) has a legitimate but narrow use case in construction: removing efflorescence from new engineering brick, or cement smears from freshly laid dense clay brick. That's it. It has no place on historic masonry.

Portland stone is oolitic limestone. Its primary constituent is calcium carbonate. Acid dissolves calcium carbonate. The reaction is not subtle. Acid turns the surface of Portland stone into a faintly fizzing paste, removes the fine tooling marks, rounds the arrises, and leaves a roughened surface that is significantly more absorbent than the original. It also removes the biological patina — the lichen and algae colonies that give aged stone its characteristic colouring — which can take decades to re-establish.

London stock brick contains chalk, which is again calcium carbonate. The same chemistry applies, with the additional problem that the brick is less dense than limestone and the acid penetrates more deeply and more unpredictably.

There is no situation in which acid is appropriate for cleaning Portland stone or London stock brick. If a contractor is proposing it, find another contractor.

3. Grit blasting without trial panels

Abrasive grit blasting is a legitimate method for certain situations — stripping paint from dense engineering brick, for example, or removing very hard, well-adhered deposits from robust masonry that has already been assessed to tolerate it. The mistake is not using grit blasting as a method. The mistake is using it without first establishing, through a properly sized trial panel, that this specific substrate in this specific condition will tolerate it.

Grit blasting removes material. Every blast removes a layer of the substrate. On soft or friable masonry, the effect compounds quickly. On ornamental stonework with fine detail — capitals, cornices, string courses — detail that took a Victorian mason days to carve can be destroyed in minutes. Once it is gone, it cannot be restored to its original form.

Any contractor proposing grit blasting on historic masonry should be able to point to a trial panel and a record of the assessment. If they cannot, the method has not been properly specified.

4. Pointing with hard cement mortar

This is less a cleaning mistake than a repair mistake, but it usually follows cleaning — after a building has been cleaned, defective pointing is visible, and something gets done about it. The something that gets done is frequently wrong.

Ordinary Portland cement (OPC) mortar is harder than most historic masonry. Historic brick and stone were built with lime mortars that were deliberately weaker than the masonry units, so that movement and moisture could be accommodated in the joints. When OPC is used instead, the mortar cannot flex. The building still moves — thermally, through settlement, through moisture cycling — but now the movement is transferred to the masonry unit instead of the joint. The brick cracks. The stone spalls. The face of the unit breaks away.

The correct approach is to match the mortar to the original: usually a non-hydraulic or feebly hydraulic lime with a sand aggregate matched for colour, grading, and texture to the original mortar. This is more time-consuming and more expensive than using a bag of OPC. It is also the only approach that does not damage the building.

5. Cleaning without a test panel

The test panel is not a bureaucratic requirement. It is a diagnostic tool. Before any cleaning method is applied to a historic facade, a small representative area should be cleaned using the proposed method at the proposed parameters, left to dry completely, and assessed. The assessment should answer three questions: has the method removed the soiling? Has it affected the substrate? And does the result meet the visual standard required?

Without a test panel, you are making an assumption. Sometimes the assumption is correct. Often it isn't. DOFF at the wrong temperature can whiten Portland stone. TORC at the wrong aggregate size can abrade soft brick. A method that has worked on every previous project of the same type may fail on this one because the substrate condition is different.

A test panel costs an hour of labour and a small section of the facade. Undoing the damage from cleaning without one can cost the facade entirely. There is no reasonable argument against it.