Masonry Cleaning

Chris Daniels


  A conservator in protective clothing applies a cleaning poultice to a trial area
  Chemical cleaning should be tested on trial areas under controlled conditions (and with the benefit of the appropriate protective clothing) as in this poultice trial.

The cleaning of masonry facades, whether of stone, brick or terrracotta, is the most visible aspect of building conservation work. When it isn’t carried out correctly, it has the capacity to cause damage either immediately, while cleaning is in progress, or over a period of time after the restoration work has been completed.

Sadly, abundant evidence of the harm caused by the inappropriate removal of dirt and stains still does not prevent those who are unqualified and inexperienced from continuing to damage our built heritage.

This article is not the place to find answers to all the dilemmas posed by masonry cleaning (and you won’t find them on the label of proprietary cleaning materials either). The aim here is to create an awareness of the fact that, as with all heritage conservation, challenges are only resolved through careful investigation, identification and trial by suitably qualified and experienced professionals.


Before any cleaning takes place crucial questions should be answered to the satisfaction of everyone involved. Obviously the major one is: why clean? That is to say: is the cleaning going to benefit the building and, more importantly, is it necessary?

  Church brickwork marked by white bloom  
  Inappropriate cleaning methods produced the insoluble white bloom visible on the surface of this brickwork.  
  Panel of clean brickwork below stained glass window  
  This steam-cleaned panel was presented as a desirable outcome during consultations on the cleaning of a church interior. Sadly, full records had not been kept so the exact method was a mystery.  

While good maintenance will keep a building free from the accumulation of dirt, it will allow natural weathering to proceed, whereas the abrasive or corrosive actions of many cleaning methods can remove or destroy the protective surface that can form on masonry with the unsurprising result that the building, once cleaned, doesn’t stay that way. This isn’t a case of it raining straight after you’ve cleaned the car: agents of decay and soiling can attack and gain a foothold on a freshly cleaned surface far more easily than on one that has its own collection of dirt, a process clearly demonstrated by the speedy loss of the pristine finish.

The answer the enterprising contractor might suggest is to apply a ‘protective’ coating. Wrong answer! Any protective finish designed to change the way the surface absorbs moisture and dirt will also change the way it releases moisture. The effects can be complex but, for this reason alone, all coatings should be avoided.(1)


In a nutshell, dirt can be defined as unwanted material on the surface of a material. It is not only necessary to understand the nature of the dirt and how to get rid of it, but also what’s it doing, and how removing it will affect the underlying material.

Deciding whether a building can be safely cleaned thus requires knowledge of both the dirt and the material it is attached to. The building’s history should be taken into consideration as should the question of whether the removal of the dirt is likely to affect the masonry and the other substrates. Even the most monolithic building will have a mortar different from the stone, whilst many historic buildings comprise a variety of materials often dating from different episodes in its history.

Common sense would dictate that due to the huge variety of stones, bricks and other materials used in buildings, one type of cleaning is not going to be universally suitable. Unfortunately, common sense does not always prevail, and cleaning methods appropriate for one substrate have often been used on all the surfaces of a building and, progressively but illogically, on all the buildings in a particular area, regardless of the material, and then on all buildings of the same material, regardless of their different locations.


The objectives of masonry cleaning vary widely. Property owners and investors may want pristine stonework to justify the money they have spent; surveyors and occasionally archaeologists may prefer clean surfaces for the ease of ‘reading’ the building; town planners may want to present well-maintained heritage townscapes; conservationists may want only harmful material removed to protect historic fabric; and contractors may well want to showcase the work they have done. If the building is to be cleaned, it is important that all parties agree what is to be expected of the process.

  A conservator in protective clothing spray cleaning ornate column capital
  Possibly the most destructive method used in the past, the grit blaster (below) has been superseded by gentler systems (above).

While the light weathering of stone to break down the fresh surface of newly worked stone can be seen as providing character to a building, the accumulation of layers of soot or surface treatments detract from the aesthetic quality of the structure and will be seen as a valid reason for cleaning. It is imperative, then, that the stone is only cleaned of harmful materials by using the least aggressive methods to achieve the desired result.

The phrase ‘desired result’ is crucial here. It refers to the level of cleanliness reached that has been approved by those responsible for the project outcome. Measuring how much cleaning is necessary and deciding what methods to use are tasks that should be completed before the project goes to tender, as it should be the contractor’s role to fulfil a requirement rather than set a level.

Pre-project trials should be compulsory for work of this type and should be undertaken by skilled and experienced conservation contractors or accredited conservators to ensure that the work is appropriate, non-destructive and practical. The company carrying out the trials should be obliged to compile a report covering all aspects of the building, material, tests carried out and recommendations. These should be discussed with the client to ensure that the likely results are understood and accepted. Only when the standard has been agreed is it possible to go to tender, as this defines the quality of the work required from the contractors, so all are tendering to the same specifications. This process also ensures that all subsequent cleaning can be measured against the standard, thus preventing hidden extras from forcing up the budget.


Cleaning projects prescribed solely on the appearance of the building and the aesthetic considerations of the client rather than investigation and fact are not going to be effective. Before the scaffold goes up certain criteria should be satisfied:

  • What is the material of construction? This may seem obvious in the masonry world but those with experience will agree that in this rocky little isle there is a huge variety of stone used in construction. Historically, it was not unusual for any handy materials to be used, be they from a field, quarry, tumbledown building or even a ship’s ballast. Mistaking one stone for another and applying the ‘right’ cleaning agent to the wrong stone can be disastrous.
  B/w photo of grit blasting in progress
  Grit blasting
  Sandstone memorial depicting Christ on the cross and alaborate floral decoration
  Wrongly identified as limestone, this sandstone memorial narrowly escaped an aggressive cleaning method before correct analysis determined the material (below: after cleaning).
  • What is the dirt in question? Remember the saying: ‘dirt is material in the wrong place’. There are many types of soiling and correct identification is crucial. It is not sufficient to rely on identification by the naked eye alone: correct identification depends on a thorough process of examination to ascertain the true nature of the dirt. If, for example, the soiling is biological, should it be treated as a protected species (which many lichens are), or as a disfiguring colonisation that is actively consuming the stone and possibly providing the foundation for further staining?

  • Where does the soiling originate? Determining whether the dirt is material leached out of the stone by chemical reaction or an accumulation on the surface produced by airborne deposition influences the approach. The building may have design faults, poor maintenance may have created environments which promote the accumulation of soiling, or the umbra of trees located close to walls may be encouraging algal staining.

  • Is the dirt harming the building or is it a symptom of some other process of decay or neglect that needs to be addressed before effective cleaning can be carried out?

  • Can the dirt be removed without harming the fabric of the building? This is probably the most important of all these questions, bringing together all the other strands within this necessarily interventive area of work. Conservation principles dictate that the method used should be the least aggressive to reach the desired level of cleaning, whereas the contractor may wish to use the strongest available method to ensure the cleaning gets rid of everything quickly and efficiently (and therefore cheaply). Masonry provides a porous surface for dirt to inhabit. Unless this dirt can be persuaded to come out (by poulticing or washing), physically removing it will inevitably involve some loss of stone.

  • How clean should the building be? The natural weathered patina of stones is probably their best loved feature, caused by the gentle polishing of the surface by the elements. Even the most mild abrasive will remove this and once the surface is dry it will be cleaner but dull. Is this what’s wanted? Another problem is that cleaning the building too aggressively can cause disharmony with a surrounding townscape of dirtier buildings, while leaving a dirty facade can have the same effect.

  • Once the criteria for cleaning have been determined, the project team should be introduced to a sample of cleaning that must be agreed on and adhered to for the whole term of the project.

  • Will the effect of the cleaning last and is it safe? Since cleaned buildings get dirtier faster, the cleaner it is the sooner it will become dirty again. However, there is a more serious problem in that the materials used can harm the stone over time causing minerals to migrate, change colour or disintegrate. The best tool to counter this is communication: look around and ask questions of other practitioners and heritage bodies. The conservation profession is dedicated to education and progress through the dissemination of experience and information so there should be no obstacle to obtaining records of previous cleaning, the materials used and the effects produced during and after the clean.

  • Water is the most widely used cleaning agent and rightly so as it is extremely effective. Conversely, it is also the force behind the majority of decay problems in buildings. If it is used unwisely the building could be clean outside but rotting inside.

It is inevitable that buildings will get dirty and if this is detrimental to the wellbeing of the structure or detracts from its aesthetic statement it may be appropriate to clean it. Before cleaning takes place, everything must be known regarding the relevant materials, techniques and effects.



(1) For a summary of the issues see Elizabeth Garrod’s article on Stone Consolidation.


The Building Conservation Directory, 2009


CHRIS DANIELS trained as a stone mason after serving in the Royal Marines. He has studied and worked in architectural heritage conservation in the UK and abroad, and has been a senior conservator at both Rattee & Kett and Herbert Read. He is currently a freelance consultant and conservator in all aspects of stone and architecture (see

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