Conservation Cleaning Decorative Stone Objects
The methods and materials identified below form part of the armoury used in the conservation cleaning of historic objects. This could be, but is not exclusive to, polished stonework including fireplaces, sculpture and monuments.
Materials and techniques employed for the removal of soiling should always be determined by the properties of the different types of stone and the nature of the dirt they are exhibiting (be that soot, grease or airborne particulates). Open-textured, porous limestones are normally only cleaned using dry cleaning methods: to mitigate the potential mobilisation and migration of stains which may occur as the result of wet cleaning. A good rule of thumb is always to begin with dry methods. Dry methods are least interventional and are usually least likely to cause further damage to the object.
Dry Cleaning Materials can include;
- Mars Eraser – A plastic eraser.
Chemical Composition: PVC, calcium carbonate, and phthalate plasticizer, (dioctyle phthalate).
- Wishab Sponge – A two-part pH neutral eraser: one side is an orange sponge-like pad and the other is a blue, stiff foam abrasive. The orange side is a soft rubbery sponge, which lifts light staining on contact. The blue side is a harder plastic used for heavier scrubbing. Wishab’s are also available in 3 different compositions: soft, hard and extra hard.
Chemical Composition: Synthetic rubber – a styrene-butadiene copolymer.
- Magic Eraser – A mass produced rubber widely available in supermarkets produced by P&G.
Chemical composition: polymer based on melamine resin
- Groom Stick – A sticky, beige kneadable eraser available from specialist conservation suppliers.
Chemical Composition: Vulcanized cis – l, 4- polyisoprene rubber and titanium dioxide.
- Latex Cleaning Poultice – These are available in a range of concentrations, depending on the level of cleaning required. The product contains ammonia and comes in a creamy consistency that cures on exposure to air, to form a rubbery film. Once cured, the latex is peeled away from the substrate, with the soiling adhered to the film. Latex poultices are not suitable to be used on polished stone, because the latex contains the non-selective chelating agent EDTA (ethylene-diamine-tetra-acetic acid); which can etch polished surfaces. Technically latex poultice can be considered a dry application, because it doesn’t require any real quantity of water and therefore can be used in sensitive environments (as it was to such great effect at St Pauls cathedral during the major conservation and restoration works completed in 2011).
A solvent by definition, is one chemical which can dissolve another. Water is often referred to as the “universal solvent” because it dissolves more substances than any other liquid.
Wet cleaning techniques act by dissolving or mobilising soiling. It is important that once mobilised, these are lifted from the cleaned surface and so the use of cotton wool swabs or absorbent paper is often combined with the ‘solvent’ to prevent redistribution.
Wet Cleaning Materials include;
- Saliva – Considered to be an effective cleaning solvent. Saliva contains enzymes which help break down and lift some surface dirt. It is also widely available, cheap and more importantly eliminates the need for chemical cleaning agents
- Deionised water – a highly purified water that has gone through reverse osmosis and had all the ionic contaminates removed
- Water – Clean, fresh H20 drawn from the tap. Polished marble can be cleaned using water, an effective, polar solvent.
V&A Mix – V&A is a tried and tested V&A Mix or Kill Quick is an emulsion of 50:50 White Spirit, water and non-ionic surfactant. This is effective for removing both greasy and water-soluble dirt and is applied via cotton wool swabs to lift dirt.
- White Spirit – A useful solvent widely available for degreasing and removing dirt and dust. White spirit is the most widely used solvent in the paint industry and easily accessible
- Acetone – Commonly recognised as nail varnish remover. Acetone is an organic compound with the formula (CH₃)₂CO. It is a colourless, volatile, flammable liquid, and is the simplest and smallest ketone. Acetone is miscible with water.
- IMS (Industrial Methylated Spirits) – A pure form of methylated spirit for use in industry and the laboratory. It is an ethyl alcohol denatured by the addition of 5% methyl alcohol
- Solvol Autosol – A diatomaceous earth putty containing ammonia. It can be applied in small areas with cotton wool swabs or a toothbrush, or to larger areas with a polishing cloth or a cotton wool pad. Once cleaning is complete, all residues should be removed with the application of white spirits.
In combination with the solvents listed above, poultices are sometimes advocated. Poultices are a medium for holding a solvent against a surface, for longer, more intimate contact. They are particularly suitable for application to vertical surfaces. A poultice moves the front of evaporation outwards, so that ingrained dirt is softened or solubilised by the solvent being drawn to the surface.
In most circumstances, following wet cleaning, a microcrystalline wax will be applied. This is not a heavy wax coating, but a light sacrificial application specified by museums, art galleries and conservators worldwide. Microcrystaline wax is not visible, does not discolour and provides a level of protection; such as finger prints, humidity, heat, dust, environmental destruction and most ordinary wear. It is matt when applied and not buffed, but can be also polished to achieve a high sheen.
Other than the obvious exceptions, all the chemicals listed above are subject to COSHH (Control of Substances Hazardous to Health) Regulations 2002. As always, trials should always be completed in advance in an inconspicuous location to confirm that the methods proposed are appropriate. Suitable to task PPE should obviously always be employed.
In conclusion, control is the most important attribute of a conservator’s approach to cleaning and the ability to define the correct method to clean (but, not over-clean) an object is critical. It is important to try and achieve a consistent finish that meets expectation, but does not strip an object fully. As such, I cannot emphasise enough that trials really are essential!