Stone Cleaning in the UK
Careful maintenance and cleaning of stone and masonry will reduce the need for repairs. Keep the rainwater disposal system weathertight and in good working order; ensure that joints are properly pointed; control or remove organic growths and vegetation attached to or in close proximity; and maintain protective coatings on ferrous metals.
Decide if stone cleaning is appropriate:
• Is the dirt adhering to the surface causing or contributing to its deterioration of the stone?
• Is cleaning to improve the appearance of the stone, slow deterioration or provide a clean surface for carrying out repairs?
• Is the soiled appearance of the stone worse than what the clean appearance will be?
There are many methods for cleaning stone on English historic buildings and structures but only those that will cause no detriment to the significant fabric of a building or structure are approved.
A thorough understanding of the physical and chemical properties of stone will help prevent inadvertent use of damaging cleaning methods and substances.
It is important that the building’s materials are correctly identified before developing a stone cleaning program in the UK. Distinguishing types of stone is not always easy some sandstones can be confused with limestones, and cast stone and concrete can be mistaken for natural stone and are often used in combination with natural stone as trim or sills.
An understanding of the weathering of the surface of various types of stone is required. Cleaning a uniformly soiled surface may reveal an untidy and patchy surface and cleaning alone may be inadequate.
The nature and source of dirt or soiling must be identified to establish the gentlest possible means of removal. Soot, smoke, exhaust fumes, oil stains and metallic stains can all discolour historic stone but require different cleaning agents. Previous treatments may also be the cause of discolouration and what appears to be dirt may be the remains of a water repellent coating which may be difficult to completely remove.
There are three types of methods for cleaning stone:
• water-based cleaners;
• chemical-based cleaners; and
• mechanical cleaners.
Water-based cleaning methods soften and rinse the dirt or soiled substance from the face of the stone.
Chemicals remove the dirt, soiled substance or paint by reacting with it to effect its release from the stone surface which is then removed by scraping, brushing or rinsing with water.
Mechanical methods are abrasive and include blasting with grit or the use of sanders or grinders to remove the dirt, soiled substance or paint (and usually some of the masonry surface). Not a system of stone cleaning that we use.
Water cleaning is generally the gentlest cleaning method and can be safely used to remove dirt from most historic stone and masonry. However, water washing of stone can be hazardous. The building must be watertight and mortar joints must be sound before using water-based cleaning methods. It is not appropriate for some badly deteriorated masonry, some sandstones or on gypsum or alabaster.
Successful washing involves the minimum amount of water for a minimal amount of time. Most problems associated with washing are to do with saturation and include staining and efflorescence cause by the migration of salts to the surface; flaking of small pieces of stone caused by salt dissolution; washing out of weak jointing material; water penetration through cracks or joints to ferrous metal fixings, filling unseen water traps; and the development of algae on flat or inclined catchment areas.
Cleaning limestone and marble
The dirt that forms on limestone and marble, unlike sandstone, is water soluble. Washing, by bucket and brush, sprays or lances is a well-established method of cleaning these surfaces. Only put enough water in contact with the deposits to remove them or to soften them sufficiently to remove by brush.
Spraying and misting
A continual wet mist over the soiled face of the stone avoids the impact of large water droplets when delivered by coarse sprays. The mist can be achieved with a fine nozzle atomised at least 300mm from the face of the masonry.
Use of intermittent or pulse washing is another suitable method of atomised spraying.
Spraying and misting is particularly effective on limestone and marble and for removing heavy accumulations of soot or crusts that form in protected areas that are not regularly washed by rain. On large structures this can be done using lengths of punctured hose or pipe with fittings that deliver continuous or timed fine sprays of mist to the surface. This is a very gentle method that does not drench the masonry.
Low pressure water washing
Water washing, done with low or medium pressure water (even a garden hose) and a natural or synthetic bristle brush, is the most commonly used method for removing pollutants from historic masonry. Water blasting is not considered to be water washing as this is very abrasive and can easily damage soft stones. Water blasting is not permitted under General Exemption (G1).
Light soiling not tightly adhered to the masonry face may be more effectively removed by low volume, high pressure (100-115KPA) water lances. Beware that there is a potential for damage from careless lancing to soft stones.
Water washing with detergents
Detergents that are non-ionic, synthetic organic compounds, e.g. Tergitol, Igepal, Triton, can be effective when used with water washing to remove oily substances.
Steam/ hot water pressurised water washing
Steam/ hot water pressurised water washing is a gentle, effective method for cleaning acid sensitive stone and may be useful for removing built up deposits and dried up plant materials. It can be effective for cleaning carved stone details and may be suitable for cleaning interior stonework.