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Stone cleaning: some considerations

Torc

Cleaning using Stonehealth's gentle abrasive vortex Torc system. This should only be carried out by approved operatives such as those employed by Ideal Cleaning.

Cleaning stonework is not as straightforward as it might appear. Is it even desirable to remove the signs of age from an old building? It would often be inappropriate to clean back to a pristine appearance, even if it is possible. Stone cleaning specialist Ideal Cleaning here addresses issues involved in natural stone cleaning, talks about the risks of a clean, the types of damage and the methods that Ideal Cleaning uses to ensure that the fabric of buildings is not damaged by cleaning.

Ideal Cleaning is a member of The Society for the Protection of Ancient Buildings (SPAB) and has helped with the restoration of some of the UK’s most precious architecture. It is a Stonehealth-approved contractor, using the Stonehealth DOFF superheated water and TORC gentle abrasive vortex cleaning methods (other similar systems are available from companies such as Restorative Techniques, Tensid and Hodge Clemco). Such systems use a minimal amount of water to complete the clean, which avoids saturation of the stone and the associated risks of damage. The stone will usually dry within a matter of minutes.

As no chemicals are used, the risk of a chemical reaction and decay are nulled. And as both DOFF and TORC are completely adjustable, it means they can be controlled, as long as they are used by experienced technicians like those from Ideal Cleaning, mitigating the chance of accidental damage. The technicians are able to start the clean using the lowest pressures and temperatures and gently increase either as needed.

Whether it is part of a regeneration project, the removal of damaging deposits such as lichen and moss, or as part of something more serious, such as structural repairs, there are many reasons for a professional stone clean.

The cleaning of a building’s facade is often the most obvious aspect of building conservation, but is it always the best choice?

If done incorrectly, or for the wrong reasons, it can cause irreversible and costly damage both immediately during the cleaning and over a period of time after the cleaning has been carried out.

Before cleaning commences, key considerations must be made. Firstly, you need to identify the type of stone involved, what type of dirt you are hoping to remove, how that dirt is currently affecting the stone, and how it will potentially affect the stone once removed.

Different stones vary in resistance to cleaning. Some stones are very soft and porous – the softer the stone, the greater the risk of damage during the cleaning process. Some stones (limestone and marble) are particularly vulnerable to etching and therefore any acidic cleaning products should be avoided when working with them.

Some stones are more prone to suffering from staining than others, meaning that great care should be taken when deciding how much water to use. If a material is more porous, the water can saturate the stone, which in turn can cause rotting to internal joists. Additionally, it can freeze during the winter months, creating a whole new plethora of issues.

Prior to a clean, you need to consider the building’s history. Is it a heritage building that has had extensive repairs over different decades? Has it had previous restoration or cleaning work carried out? Does it have an extension to the original structure?

An old heritage building, for example, may have had various repairs or extensions undertaken in the past and might, therefore, have different types of stone and/or mortar used. At the very least, the stones may come from different quarries. This could result in the cleaning method needing to be altered for the different areas of the facade.

The decision to clean and the cleaning method will ultimately depend on the stone used and the level of soiling present. Although unsightly, lighter soiling doesn’t usually require any attention as it doesn’t generally cause any harm to the surface and is generally best left alone. Heavily soiled stonework, however, can retain water and hold agents which are harmful to the stone.

Lichen, algae and fungi can all live on natural stone surfaces and all secrete acid, which is particularly damaging to limestone and marble, as mentioned above. Moss not only secretes acid, but it retains water that risks freezing in the winter months leading to cracking in the stone or mortar. The presence of moss and algae can also be indicative of a damp issue, as they both prefer damp environments.

Birds can also cause damage to the facade of a building by pecking at the salts in the masonry. Their droppings can cause damage to some stones as well as being unsightly. This can be significantly worse in urban environments, with the worst culprits being starlings and pigeons.

Acid rain, which includes mists and fogs, can directly dissolve stonework. The reaction between acid rain and the calcium carbonate in limestone and marble causes the stones to dissolve and crumble. A wet or dry deposition of sulfur dioxide significantly increases the rate of corrosion many stones.

PROFESSIONAL STONE CLEANING

Before carrying out any stone cleaning, consider whether the dirt is actually harming the building. And ask yourself (or an expert, if you don't know) if the soiling is a symptom of another issue such as some form of decay or neglect? Once you are certain that you want to proceed with a clean, ensure you enlist the services of a professional stone cleaning company that will carry out a site survey to ascertain the best method for cleaning your stonework.