Although stone artefacts may be damaged by factors as diverse as vandalism, floods, tourism pressures and even earthquakes these factors will not be examined in any detail in this chapter. Rather other significant agents of decay such as the following will be examined:

• weathering and eroding agents (sun, wind and rain);
• pollution;
• salt contamination; and
• biological activity.

Igneous stones such as granite are hard, non-porous and fairly stable. Even so, the surface of a granite monument will deteriorate when exposed to weathering agents and pollution. It may become obscured by soot or damaged by biological growth.

Sedimentary stones such as sandstones and limestones are relatively soft, porous and have friable surfaces. The matrix which cements the various particles together is readily damaged by weathering, pollution or salts. The surface of the stone is weakened and disintegration follows.

The characteristics and durability of metamorphic stones are variable. Marble for example, is of medium hardness, able to be polished and porous. It stains easily and deteriorates when exposed to conditions which promote weathering. Heat, pollutants and acidic conditions are damaging. Slate is a fine-grained, foliated rock that is susceptible to weathering and splitting into thin sheets (Figure 1).

While the chemical composition of stones may be similar, deterioration may vary depending on other factors such as the porosities of the stones and the size and distribution of their grains. Anything that causes a difference in the behaviour of the outer surface of stone structures compared to the inner regions can lead to stresses that eventually cause cracking and exfoliation of the outer surfaces.

Figure 1: Slate headstone showing exfoliation caused by exposure to the elements.

All porous or rough stone surfaces are dirtied by the deposition of airborne particles such as dust and smoke. Moisture condensing on a stone surface will increase the amount of deposition. In addition to disfiguring the stone, accumulated debris can contribute to chemical and biological attack.

Carbon dioxide and sulphur dioxide in the environment are incorporated in falling precipitation known as acid rain. Acid rain will dissolve the calcite in limestone, marble and sandstone. Acidified rainwater slowly dissolves the calcite in limestone monuments, leaving whitish patches. In areas protected from direct rain, the dirt accumulates as a black gypsum powder or crust. Studies have shown that the blackness of these crusts is largely due to anthropogenic sources, in particular the combustion of fossil fuels (Přikryl et al 2004). Diesel emissions are particularly disfiguring to stone structures.

Concern has also been expressed about the role that climate change may play in stone deterioration. In Scotland for example, higher temperatures and increased rainfall is expected to lead to increased biodeterioration of stone structures (Duthie et al 2008).

Salts may form in stone from airborne pollutants (sulphates and nitrates in particular) or be blown by wind from the sea onto stone structures. Porous stone in contact with the ground will also eventually suffer from salt damage. The base of the stone absorbs water and soluble salts which rise by capillary action. A cycle of wetting and drying is established with the pressure exerted by crystallising salts eventually disrupting the stone. Deterioration occurs in the area of stone which experiences fluctuating humidity, not where the stone is constantly wet. Visible damage includes pitting, spalling, crumbling and delaminating and is accompanied by salt efflorescence. The amount of salt in stone is correlated to the severity of the damage present in the stone. In addition to the damage caused by crystallisation, salts can also damage stone due to differential rates of thermal expansion between the incorporated salts and the stone crystals. Salts such as sodium chloride for example, have a much higher expansion rate than calcite crystals at the same temperature. Numerous examples of salt damage can be seen on sandstone, marble and slate gravestones and monuments (Figure 2).

Figure 2: Salt crystals forming on a limestone wall.

Moisture also contributes to the corrosion of metal components on stone monuments. Green, black or brown stains from corroding metal can disfigure stone and corroding iron dowels can cause fracturing.

Biological activity on stone is encouraged by a combination of factors including light, warm temperatures, high humidity levels and the presence of organic matter (from pollution or oils and waxes from previous conservation treatments). The type of organism likely to thrive on a stone substrate will be determined by the particular conditions associated with the site. Algae for example, need only light, a few inorganic compounds and a little water to develop. They may be found in relatively dry places. Once established algal growth is able to trap water and establish an environment conducive to the growth of other organisms such as bacteria, lichens, mosses and ferns. Damage to the stone is both chemical and physical. The growth process releases chemicals such as acids and chelating substances which attack the stone. Penetration of the stone surface by the organism exerts pressure which may lead to fracturing of the stone surface (Figure 3).

Figure 3: Limestone headstone showing erosion and discolouration from lichen.

Although damage to stone objects is not just an outdoors’ issue, moveable objects may be protected from extreme conditions and pollution by moving them indoors. Even then factors such as heat, humidity, acidity, salt and dust can continue to cause problems. Heat from spotlights and fireplaces for example, can cause irreversible damage to marble. The calcite crystals of marble have different directional coefficients of thermal expansion. Heating, even to 100°C, causes more expansion of the crystals in one direction than in the other. This distortion causes stresses and cracking on the marble surface, which may develop a white ‘sugary’ appearance.

Acidic vapours can build up in timber showcases or storage areas, especially at high temperatures and relative humidity and damage calcareous stones such as limestone and marble.

Careless handling can result in breakage, staining or scratching of softer stones.

Stone analyses are important so that deterioration mechanisms and rates can be determined. Analyses range from the simplest visual inspections and photographic documentation to more high powered techniques like 3-D laser scanning, fluorescence light detection and radar (LIDAR), ultrasonic scans, magnetic resonance imaging, thermography and even ground penetrating radar. While the former techniques (to LIDAR) are well suited to surface characterisations, the latter techniques are capable of providing information about the interiors of stones. For many situations however, the outer appearances provide enough information about stone deterioration to guide conservation treatments and the implementation of preventive conservation strategies.

Preventive conservation practices for stone objects should involve minimal intervention. Attempt to do something to improve the stone object’s environment and limit the use of treatment materials that may be damaging to either the stone or the environment. In addition to controlling the temperature and relative humidity levels associated with stone structures, other preventive measures could focus on keeping water away from the stone, pollution and ground water control, visitor management, construction of shelters and wind breaks and even reburial (Doehne and Price 2010).

Environment

Avoid sudden or large changes in temperature and relative humidity for stone on display or in storage. Although preferred environmental conditions will vary depending on the particular type of stone, if possible maintain temperatures of less than 20 °C and relative humidity levels in the range 45 – 55 % with maximum variations of 4 °C and 5 % respectively within any 24 hour period.

Do not expose marble or limestone to direct heating from strong lights, sunlight, radiators and the like.

Handling

Potential problems associated with handling of stone objects include:

• scratching the surface of highly polished or soft stones (talc, soapstone, marble);
• the absorption of dirt, oil and salts from hands by porous materials (marble, alabaster, limestone); and
• breakage when carelessly handling or moving objects.

To minimise the risk of damage, wear cotton gloves when handling stone. Wear clean disposable latex gloves to reduce the risk of dropping smooth, highly polished stone objects.

When moving stone objects, adopt the following practices:

• check the attachment between an object and its pedestal or base before moving;
• move large objects one at a time. Rest them on thick, soft padding such as high density polyethylene foam;
• use padding such as soft tissue paper, dacron or polyethylene bubble wrap both on the tray and between small objects being moved; and
• pack around protruding parts and distribute the weight evenly.

Storage

When storing stone objects consider the following guidelines:

• store small, lightweight objects in acid-free cardboard boxes that have a rigid base;
• place padding under and around objects. Clean cottonwool, acid-free tissue or polyethylene bubble wrap are suitable for padding. Do not compromise the stability of the object by this padding;
• to minimise dust problems, place large heavier objects in closed cupboards or on open shelves with curtains;
• store on painted metal trays or shelves rather than in or on wooden furniture;
• place the largest objects on the lowest shelves; and
• line the shelves with layers of acid-free blotting paper or cotton towels.

Seal wood or wood-based composite materials if they are used for storage (see the chapter Preventive Conservation: Agents of Decay).

Stone Monuments

Control salt contamination, biological attack and weathering of outdoor stone monuments by isolating the stone from water and salt sources. Achieve this by adopting the following strategies:

• construct a shelter for protection from rainwater. Direct the run-off away from the base;
• construct wind breaks to reduce drying impacts and the impact of airborne salt; and
• isolate the stone monument from the ground or any cement (which also contains soluble salts). Place an impermeable layer such as lead sheet under the stone.

Chemical methods for creating an impermeable layer have also been investigated. Seek the advice of a conservator so that the most appropriate isolation method is used.

Avoid applying protective coatings such as oils, waxes or acrylic resins as these will degrade over time. Although many water-repellent coatings have been developed, review their use carefully before application as these materials have the potential to trap water behind the treated surface layer. In the longer term this may cause surface spalling.

Examination

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

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.

Stain Removal

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.

Repair

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.

Introduction

Rocks and minerals may be collected for either research, education or aesthetic reasons. The colours and crystalline forms of many minerals make them particularly attractive to collectors (Figure 4).

Figure 4: (a) and (b) Examples of rocks, minerals, ores and fossils.

General rock types, their modes of formation, factors affecting deterioration and conservation were described at the beginning of this chapter. Whereas the materials described earlier in this chapter were considered stable, being used for monuments and similar structures, the many and varied samples that make up geological collections are not necessarily so.

Expressions such as ‘solid as a rock’ give the wrong impression of the stability and fragility of many materials that make up geological collections. It has been estimated for instance, that about 10% of all minerals are unstable (Howie 1992) and prone to deterioration unless adequately cared for.

One of the main purposes of this section is to alert readers of the need to care for rocks and minerals as they are often overlooked and consequently neglected because of their perceived stability.

• Store or display stone in an environment with temperatures below 20°C and a relative humidity in the range 45 - 55 % with maximum variations of 4 °C and 5 % respectively within any 24 hour period.
• Do not expose marble and limestone to direct heating.
• Use cotton gloves for handling most stone objects and disposable latex gloves when handling highly polished stone.
• Protect specimens from dust and store in enamel-coated metal cupboards.
• If wooden storage or display cabinets are used they must be properly sealed to protect calcareous stone objects from acidic vapours.
• Isolate outdoor stone monuments from water sources to avoid or control salt contamination, biological attack and weathering.
• Examine and fully document objects before any treatment.
• Use dry cleaning methods before any solvent cleaning is undertaken. Take care to avoid abrasion of surfaces.
• Test treatments on an inconspicuous part of the object before any large-scale application.
• As most stones are porous, use a minimum amount of water or solvent for cleaning.
• Use easily removable adhesives that have good ageing characteristics to repair stone. Large or heavy stone repairs may require dowels and stronger adhesives.
• Mild cleaning techniques, such as nebulisation, may be suitable for some stone monuments.
• Apply a suitable biocide to biological growths on stone monuments before removing them by mechanical cleaning.

Booth, B., 1993, Rocks and Minerals, Quintet Publishing, London.

Dajnowski, A., Jenkins, A. and Lins, A., 2009, The use of lasers for cleaning large architectural structures, APT Bulletin, vol. 40, pp. 13-23.

Doehne, E. and Price, C. A., 2010, Stone conservation: an overview of current research, 2nd edition, Getty Conservation Institute, Los Angeles.

Duthie, L., Hislop, E., Kennedy, C., Phoenix, V. and Lee, M. R., 2008, Quantitative assessment of decay mechanisms in Scottish building sandstones, in J. W. Lukaszewicz, J. W. and P. Niemcewicz (Eds), Proceedings of the 11th International Congress on the Deterioration and Conservation of Stone, 15 – 20 September 2008, Nicolaus Copernicus University, Toruń, Poland, pp. 73-80.

Howie, F.M. (Ed.), 1992, The Care and Conservation of Geological Material: Minerals, Rocks, Meteorites and Lunar Finds, Butterworth-Heinemann, Oxford.

King, B., 1992a, Appendix II: Collecting rocks and minerals, ibid, p. 127.

King, B., 1992b, Appendix III: Cleaning minerals, ibid, pp. 128-132.

King, B., 1992c, Appendix IV: Repair and consolidation of minerals and rocks, ibid, pp. 133-134.

Nassau, K., 1992, Conserving light sensitive minerals and gems, ibid, pp. 11-24.

Price, M., 1992, The stability of minerals, ibid, pp. 1-10.

Price, C., 2007, Consolidation in A. Henry (Ed.), Stone Conservation: Principles and Practice, Donhead Publishing, Shaftsbury, UK, pp 101-126.

Přikryl, R., Svobodová, J., Zák, K. and Hradil, D., 2004, Anthropogenic origin of salt crusts on sandstone sculptures of Prague’s Charles Bridge (Czech Republic): Evidence of mineralogy and stable isotope geochemistry, European Journal of Mineralogy, vol. 16, pp 609-18.

Wheeler, G. S., 1992, Stone objects, in Caring For Your Collections, National Committee to Save America’s Cultural Collections, Arthur W. Schultz (Chairman), Harry N. Abrams Inc. New York, pp. 122-127.