As with all objects, thoroughly examine and document the condition of stone objects before beginning any treatment. Record this information through a combination of notes, diagrams and photographs. Describe any deterioration, usually indicated by changes to the surface or structure of the stone. Such visible effects may include:
- staining and other discolouration;
- salt efflorescence;
- encrustation; and
- pitting, crumbling, spalling, erosion, delamination and breakage.
Take samples of any salt efflorescence as future analysis may be required. Note any previous repairs and marks which provide information about the history of the object.
Cleaning methods aim to remove unwanted substances without leaving behind any soluble salts or chemical residues and without abrading the stone surface. While laser cleaning is now routine in many countries and in large institutions, the need for specialist equipment and expertise precludes a full discussion of this technique here. It is worthwhile noting however, the advantages of laser cleaning – there is no direct physical contact with the stone (hence delicate objects and surfaces can be cleaned), no solvents or water are used and there is a high degree of control of the cleaning process (Dajnowski et al 2009).
Both dry and wet cleaning methods may be applied to stone objects. As dry methods involve the least intervention and pose the least risk to stone objects, try these methods first. Follow the guidelines below when dry cleaning stone objects:
- use a low suction vacuum cleaner with the end covered with clean, soft muslin and a soft brush;
- do not dust with a cloth as this tends to force dirt particles into the stone;
- use a soft brush or soft wooden splint to help to dislodge accretions; and
- take care not to scratch the surfaces of soft stones such as alabaster, soapstone or marble.
If required, wet cleaning may be attempted after dry cleaning. The following guidelines should be noted:
- avoid solvent cleaning with damaged or friable stone;
- always test solvents on an inconspicuous part of the stone before cleaning;
- avoid using large quantities of water as most stones are fairly porous;
- use cotton swabs, dampened with distilled water or a mixture of 50 % water and 50 % methylated spirits to remove dirt and grime from marble and other smooth stone surfaces;
- roll the swabs over the stone surface and discard them as soon as they become dirty; and
- do not scrub the swabs back and forth as this forces dirt into the stone pores.
If dirt persists, add a small quantity of soap solution to the water or water and methylated spirits mixture. Prepare the soap solution in the following way:
- dissolve one gram or about one teaspoon of pure soap flakes in a little hot water;
- add enough cold water to make one litre of solution. Then add 100 ml of this soap solution to 100 ml of water or water and methylated spirits;
- apply the soap solution with cotton swabs in the manner described above;
- use a cotton swab soaked with distilled water to rinse the soap from the stone surface; and
- do not use brushes on soft or porous stone as this will scratch or force dirt into the surface.
For cleaning more porous or rough stone surfaces, use a poultice. The steps in preparing and applying a poultice are outlined elsewhere (see the chapter Ceramics).
Poultices are also useful for removing soluble salts from objects when immersion in water is not practical or advisable. If salt contamination is suspected, for stone recovered from a marine or burial site or on the basis of a previous treatment, consult a conservator for advice on identification and removal techniques.
Removing stains usually requires some form of chemical treatment. Seek advice from a conservator before attempting this type of treatment. Also consider the following points:
- some stones such as marble acquire a patina as they age. This is an intrinsic part of the stone and should not be removed;
- exercise care if strong acids or alkalis (caustic soda, ammonia) are used as they will destroy patinas (and some stones);
- indiscriminate use of chemicals on stone may result in salts forming, disintegration of the stone or changes in colour;
- it is often difficult to remove chemicals completely after treatment;
- iron and copper stains require specialist conservation treatments;
- do not treat iron-stained stones composed of calcium and magnesium minerals with chelating agents such as disodium ethylenediaminetetra-acetic acid. In addition to iron these chelating agents tend to also remove calcium and magnesium;
- hydrochloric acid reacts with marble and limestone;
- acids cause brown stains on sandstone with a high iron content;
- alkali cleaners used on porous stone lead to salt efflorescence; and
- stains often penetrate deeply into stone and may become insoluble with age.
For these reasons, consult a conservator before any stain removal is attempted.
Cleaning oil and grease stains poses less risk to the stone matrix. Hydrocarbon solvents such as white spirits or X-4 solvent are more successful at dissolving oils and greases than water-soluble solvents like acetone and methylated spirits. Steps in the removal of oil and grease stains include:
- initial testing to determine the most effective solvent;
- testing to ensure that the stain does not spread further;
- using a cotton swab, cottonwool strip or paper poultice to apply the chosen solvent; and
- if applying a poultice, slow the evaporation of the solvent by either placing the object in a sealed container or covering the poultice with polyethylene film.
Cleaning Stone Monuments
In some cases it is appropriate to clean outdoor stone monuments by nebulisation. In this process small nozzles, such as those used in greenhouses, spray a very fine mist of water near the stone surface. Only the mist contacts the monument, not the direct water spray. Water particles slowly deposit onto the stone, giving a mild cleaning action. The small size of the water droplets results in a greater wetting and cleaning effect for the small quantity of water used. Collect the run-off water from the base of the monument.
Consult a conservator regarding the use of neutral chemical poultices to treat stains and encrustations.
Before cleaning biological growths such as lichen, algae and moss from stone, kill the growth by applying an appropriate biocide. Choose a biocide that will have no chemical action on the stone, is non-polluting and is safe to the user. The best time for treatment of outdoor stone is either spring or autumn, at the beginning of vegetative development. The dead biological crust can be removed by the use of a poultice, low suction vacuum cleaning or gentle brushing.
Seek a conservator’s advice on the most suitable cleaning products, biocides and procedures.
There are many adhesives and consolidants that may be used on stone objects and research is continuing into the best materials and techniques for repair of these objects (Price 2007). It would be prudent therefore, to consult a conservator before attempting any repairs to stone objects.
Small and medium sized stone objects can be repaired with Paraloid B-72 and UHU All Purpose Adhesive (see the chapter Ceramics). These adhesives, which can be removed or diluted with acetone, meet the required conservation standards.
Steps involved in the repair of stone objects are outlined below:
- prime porous stone fragments before joining. Dilute the adhesive with an appropriate solvent by mixing one part adhesive to four parts solvent;
- brush the dilute solution (primer) over all surfaces to be joined and allow them to dry completely;
- for a multiple repair and to avoid ‘locking out’, practise the order of joining the pieces before applying any adhesive;
- use a shallow container filled with clean, dry sand to support fragments;
- begin joining at the base of the object where appropriate;
- select two fragments to be joined and apply adhesive along the centre of one broken surface. Fit the two fragments together, pull slightly apart to allow some evaporation of solvent and to check distribution of adhesive and then push firmly together again;
- place the joined pieces in the sand tray for support while the adhesive sets. Balance the top fragment carefully and ensure the weight of the stone pushes down onto the join to help tighten it; and
- remove excess adhesive with a cotton bud dampened in solvent or by careful trimming with a scalpel after the adhesive has set.
If a large or very heavy stone needs to be repaired then dowels may be needed to strengthen the join. Although most epoxy resins yellow with age and are difficult to reverse, it may be necessary to use such an adhesive if high cohesive strength is required.
Seek help from a conservator if there is any doubt about the materials or techniques required for stone repairs.