The Todd Conservation Report
Submitted by: Andrew Todd 17 June 1993
The following report addresses the problems faced by the Society in its efforts to maintain the condition of gravestones in the City of Victoria, BC. The Pioneer Park Cemetery next to the Cathedral was examined in March 1993 and the recommendations included here are based on the observations of that visit.
Conservation of stone monuments and sculpture is a highly specialized field in the preservation of historic and artistic works. Research is constantly being carried out in order to understand the processes of deterioration and the steps required to arrest the decline of the material.
Sedimentary Rocks: Mudrocks
The finest-grained sedimentary rocks are really clays and clay minerals. Some of these, like smectites, respond to moisture and temperature conditions. For example, smectitic clays are referred to as swelling clays, based on their ability to absorb water and swell in size. As a result, smectitic clays are not practical for outdoor sculpture. Even indoors, sculpture of this material should be kept in low humidity-controlled environments or specially designed display cases.
Sandstones are mainly composed of quartz, feldspar and some rock fragments deposited together and cemented in a sedimentary bed. The three main categories of sandstones are quartz sandstone, arkose and lithic sandstone. They are distinguished from each other by the ratio of quartz/feldspar content in addition to the other rock fragments which can affect colour and density of the rock.
Sandstones are cemented together with carbonate cements or silica cements, most commonly. Clay-based, or argillaceous cements and iron cements also are responsible for bonding the grains of sandstone together. The calcareous carbonate cements that are most common in sandstone are relatively soft, of medium density, and soluble in acid. Silaceous cements are contrasting in qualities of hardness: they are dense, durable, and not readily soluble in acids. Iron cement has varying durability and is prone to rusting with subsequent staining of the material and any other materials in a runÂ¬off position. Argillaceous, clay-based cements are not very durable, soft, and subject to swelling in moisture (Bell and Wright 1985.)
These sedimentary rocks are of major importance in architecture and sculpture. They are divided into groups also, the principal ones being calcite and dolomite: however, other rare minerals can be present in limestone. In addition, fossil remains are found in limestone formations, at times in such abundance that they become the principal constituent of the rock. Fossil-bearing rocks have found decorative placement in architecture, an example being the Alberta Provincial Museum building in Edmonton, which is entirely clad in a fossil-rock facing.
Calcite is a fine, colourless, glassy mineral, but is often coloured by associated minerals. Micrite is a fine-grained, compact mineral in limestone which has the dense qualities valued for lithographic stones when finished smooth and flat.
Dolomite, colourless as a mineral, can be stained or it can appear as a white, coarse to medium-grained crystalline rock.
These rocks have been structurally transformed into thin crystalline structures by heat. Igneous, sedimentary and existing metamorphic rocks can all be transformed by the action of heat and pressure from an intrusion of molten magma. When heat is introduced into igneous rocks they are changed into fine-grained, structureless, heavy rocks called hornfels. Depending on the proximity to the heat, different stages of metamorphism take place.
The transformation of sedimentary rocks into metamorphic rocks by heat creates some new rock structures called spotted shales, slate and hornfels. Slate is a metamorphosed mudrock and is found as a material in outdoor sculpture. Although mostly carved in relief rather than in the round, slate was a common material for tombstone carvers for many years. The early slate markers, placed as early as 1720 in Nova Scotia were probably imported stones from New-England (Trask 1978).]
Deterioration of Stone
The factors that cause deterioration of stone sculpture in the outdoor environment are normal precipitation, acidic precipitation, particulate and gaseous air pollution, air-borne particles, water-borne salts, ground water, and plant growth. Increased public use of sites can have adverse physical effects on stone sculpture (Riederer 1986). Unfortunately, weathered stone is a brittle and porous material which is susceptible to damage from vandalism.
Acidic damage to stone sculpture usually takes place in the presence of moisture, whether or not the acid is deposited in rain or by airborne pollutants. Acidic deterioration occurs as a chemical reaction first on the surface and then proceeds into the interior of the stone by capillary action. The dissolving of the mineral composition of the stone by these acids is described differently according to the kind of stone. For example “sugar decay” is the term that has been mentioned for marble. Limestone and sandstone are affected by feldspar breakdown due to weathering in the pores by acidic water. Clays are abundant in the stone once weathering has taken place, and their tendency to swell causes the surface of the sculpture to be pushed off in a process which is referred to as exfoliation.
Soluble salts which occur naturally in the soil or are introduced for ice control can cause major damage to outdoor stone sculpture. Salts become dissolved in ground water, which can wick or be absorbed into the stone if it is directly in contact with the ground. Once the water begins to evaporate from the stone, the salts crystallize out from solution forming crusty solids; this usually happens at the surface, although it can occur at a break, an internal natural fault line or interface; in the rock structure. The forming of salt crystals results in their acting as tiny wedges and levers; this causes physical breakage and spalling of the surface or exfoliation. Once this type of damage has begun, the treatment required becomes quite complicated in the relationship and order of the steps needed to arrest the damage and repair the stone. Careful analysis has to be conducted to determine if consolidation should take place before removing the salts (desalinization). Desalinization can involve prolonged and repeated exposure to distilled water either by poultices or a series of immersions. Prior to treatment the conservator may arrange for an x-ray examination of the structure to determine if any metal pins had been inserted in the sculpture to reinforce previous repairs. There can be great danger to the stone if a prolonged water treatment is begun without a thorough analysis of all the materials potentially present in the sculpture.
Ground water in contact with a stone sculpture is an additional danger when it is a source of nutrients for mould or plant growth. Excessive or pooling ground water may render the location physically unstable, undermine the sculpture and require provisions for drainage.
Biological growth in association with stone sculpture can take place on the surface: common examples are micro-organisms (such as bacteria and moulds), algae, lichen and mosses. These may cause direct physical damage by digesting minerals or weakening it with growing roots. Also, these organisms produce and trap particulate matter, such as dust and grit, which in turn traps moisture. Acidic moisture trapped in dirt on the surface increases the rate of chemical breakdown of minerals in the stone which in turn allows more moisture to enter the pores, perpetuating the cycle of deterioration.
Plant growth overhead slows down evaporation of moisture from the environment around the sculpture. Moisture from overhanging trees, dripping regularly, can damage materials over time. Dripping sap and resting birds can cause chemically active deposits and stains. Decaying leaves and seeds are additional unwanted materials on the surface of the sculpture.
As a mechanical threat, trees growing in close proximity to a sculpture pose a hazard to stability through upheaval due to active root growth. Inspections in the vicinity of the sculpture, if carried out on a regular basis, will indicate a time-frame for insuring site security. Long-range planning in relation to the anticipated rate of plant growth is required to provide the best environment for preservation. Landscape architecture shares this concern for the relationship between the object and the environment. The aesthetic design of a landscape with sculpture is certainly enhanced by sculpture in good condition.
Vibration and Stone Sculpture
Stone is probably the sculptural material most susceptible to physical damage through vibration. Sources of repeated vibration include transportation routes, such as roadways, railways and airports. Industrial activity can also cause vibration which can be transferred to the sculpture.
Inspection of stone sculpture should include close visual examination for any interruptions in the structure, such as flaking or hairline cracking. If cracks are observed, their progress can be monitored. Cracks are measured and carefully recorded on a diagram which locates their positions on the sculpture. Investigation of the cause can be earned out by observing related activities in the area until the source of vibration becomes apparent. In areas where earthquakes are known to occur, it may be possible to engage consultants experienced in vibration measurement. The results of earthquake or seismographic studies can be obtained from environmental monitoring agencies. Although earthquakes cause dramatic vibration and shock, clearly the more likely and insidious sources are traffic on streets, highways, and railroads or industrial production.
If damage is occurring due to vibration and the source is a roadway or railway, then a way to prevent the damage from continuing must be found. Obvious physical solutions are not necessarily the most appropriate; for instance, moving the sculpture might remove it from its historic or artistic context. Moving a roadway is not likely to be acceptable either. One possible way to remove the threat of vibration damage is to provide a stone sculpture with a shock-absorbing pad. Modern synthetic gasketting materials are available which are inert and will withstand long exposure to the outdoor environment. A product called Panacea (TM) sheet, a stabilized poryvinyl chloride, is available in different thicknesses and colours to provide a cushioned pad. It is important that the cushioning material is chemically inert under local environmental conditions and stable over a long period of exposure. Alternative inert cushioning materials may be recommended combined with engineering advice. Engineering expertise is often valuable when a move is required or a cushioning pad is needed.
If a sculpture can appropriately be moved to a new location, then the guidelines should be obtained for installation and handling.
Vandalism of Stone Sculpture
Graffiti or writing applied to stone sculpture to deface it can be very complicated to remove; one is advised to contact a conservator immediately in order to describe the nature of the attack. If any evidence, such as spray cans or markers, have been left at the site they should be retained for testing. Several solvent tests and removal methods for liquid-based graffiti can be used, but they are not without risks when carried out by an inexperienced worker. The removal of “dry” marks as opposed to liquid stains also requires special treatment for removal.
Acts of vandalism which cause mechanical loss, breakage or physical scarring of the surface are equally difficult to repair; substitutes or infilling materials may discolour, come loose, or become imbedded in the stone in a way which makes it difficult to remove them if unsuitable later. If a number of complicated breaks are found together in addition to surface abrasions, then conservation treatment should be arranged. In the past, repairs to broken stone sculpture have been performed by masons using traditional mortar as an adhesive. Conservators now prefer to choose from a range of more durable and predictable adhesives; particular grades and brands may be selected from such categories as epoxy and polyester resins and acrylic plaster, for repair of broken stone.
The urgent repair of such damage can often coincide with a period of neglect. Overall cleaning and care of outdoor sculpture is often required at the same time as a repair is necessary. Cleaning of the broken edges, as well as the surrounding areas is almost certain to be needed in order to remove dirt which could otherwise become incorporated into a repair.
Coatings are presently being tested for conservation treatment of stone sculpture; as yet, the most appropriate and practical coating has not been established conclusively. It is therefore recommended for stone sculptures to remove any foreign matter on the surface which may trap and hold acidic moisture in place.
A description of cleaning tests and methods is found in the following section, “Cleaning and Washing.” As with any operation for treatment of works of art, the record keeping and documentation are extremely important.
Cleaning and Washing
Cleaning is not to be underestimated as an important operation which requires training and knowledge of materials; it is a step in conservation treatment. Conservators consider this step carefully with almost all objects which they examine for treatment. As a result they have techniques which have been specially developed for different circumstances which are tested and appropriate for many types of cleaning requirements. Conservators with sculpture experience reiterate that cleaning of sculpture be undertaken only with training and informed supervision. Sometimes the condition or materials of the sculpture make it necessary for conservators to limit cleaning themselves. Therefore it is highly recommended that training be acquired for all cleaning, including routine maintenance cleaning of outdoor sculpture. While some methods may resemble those applied to familiar domestic situations like car cleaning, or polishing silver or furniture, they vary in important details. Again, as a reminder, it is important to have conservation input for the planning and supervision of the cleaning, and to require a written proposal for what the intended procedure is, so that it can be reviewed before the operation begins.
Importance of Cleaning
Cleaning, if done carefully, contributes significantly to preventing deterioration of outdoor sculpture; if not done carefully it can be the cause of even more serious deterioration. Many works of art have been seriously harmed by badly informed attempts at cleaning and repair, even when begun with the best of intentions. For example, sandblasting is now considered to overclean works of art and architecture. Yet, until the treatment had been examined for its harmful effects to metal. At one time, many statues and buildings were stripped of important protective coatings and, in some cases, of historical details by this aggressive technique. Over aggressive cleaning by any method is not acceptable because it can remove original material or cause permanent changes which will accelerate the response of the material to the outdoor environment (Chase & Veloz 1985). There is a history of treatment of works of art that is almost as old as the earliest art; not all have been successful.
Many techniques that are used for maintaining domestic and industrial equipment are not appropriate for the survival of works of art; for example, wooden sculpture which is already painted cannot be pressure-treated with fungicides like unpainted fence posts are. Preconceived notions and old stand-by recipes need to be carefully analyzed before they are considered applicable to an art work. Anniversaries and centennials of historical events often generate new enthusiasm for statuary and monuments. In the past, eager teams of citizens have been known to descend with wire brushes in hand to begin a “brightening-up” of historical bronze statuary. This is a terrible technique to ever bring against an outdoor sculpture. Inappropriate techniques can be avoided by getting professional advice and by following an established maintenance procedure for removing loose surface dust and grime. Embedded dirt or discolouring effects such as corrosion or altered paint layers will require specialized attention.
Total, easy access to all areas of the sculpture is essential to perform the following steps of a maintenance program. In order to reach high points and extended sections, it will be necessary to have either a cherry-picker and operator, erect scaffolding or a ladder. Safety for the object must be considered with the use of any equipment. The operator of a cherry-picker must be experienced and well informed of the importance of avoiding contact with the sculpture. Scaffolding must stand clear of the surface at all points. Ladders should not be leaned against the sculpture; padded bumpers are required on these and on any equipment that is in close proximity to the sculpture.
Personal safety equipment should be worn by all workers on a site. Hard-hats, work boots and gloves as basic safety equipment will need to be supplemented with goggles and organic vapour filter masks for some operations. A complete first-aid kit is necessary on every site, even for the simplest of maintenance operations. The location of the nearest telephone to the site should be known by the workers in case of an accident or emergency.
Finally, if equipment is left overnight, barriers or a security patrol are recommended to prevent unauthorized access to the site.
Maintenance Cleaning. Dry Methods
The first step is the removal by dry methods of accumulations of loosely attached or unbonded debris and foreign matter from the surfaces, in crevices and in details of carving and decoration of the outdoor sculpture. The removal of such material as leaves, seeds, pods, twigs, bits of branches, and dirt is a good measure to prevent conditions which provide a micro-environment which can encourage deterioration. Organic material (i.e., derived from plant and animal sources), will retain moisture and therefore promote a suitable climate for deterioration activities Staining of the surface may be the first obvious sign of damage as organic material begins to decompose in moist surroundings.
Removal of organic and inorganic material:
- Working from the top down, loose debris is removed by hand, with brushes, wearing gloves for protection.
- Abrasive dirt, such as sand, gravel or stones, even twigs and branches, should not be dragged over soft or deteriorated surfaces as scratching of the surface with the material being removed will be harmful.
- Recommended tools for direct use on sculpture must be softer than the material of the sculpture; soft bristle brushes should be used to sweep up any loose dirt; small wooden picks can also be used.
- A damaged or chipped surface needs care around the edges of the chip to prevent any further loss.
- Dirt or moisture should be prevented from getting under chipped paint.
- Blotting paper or filter paper can be used to remove dirt from cracks; they can be cut into triangular shapes. One shouldn’t let the paper get jammed or broken off in a crack, or try to force open a crack to clean it.
- A firmly bonded accretion should not be chiseled or forced with a tool that could scratch the material of the sculpture, e.g., mud wasp nest; the deposit may be more firmly attached to the material than the material is to itself.
Removal of Bird Droppings
Loose debris can be removed as above by cleaning with dry methods. By the use of brushes and soft wood, nylon or similar synthetic spatulas, bird droppings can be loosened and removed. IMPORTANT CAUTION: All work to remove bird droppings must be done with masks and protective gloves. Spores in old bird waste can be inhaled when disturbed by the operator. These spores can be the cause of diseases, such as histoplasmosis and cryptocococcus, which are acute chronic lung diseases with very serious and debilitating effects on humans. (Byrne 1983). On the surface of an outdoor sculpture, it is still very important to use a good particle mask, protective goggles and gloves. Collect and dispose of all debris at waste disposal site.
Low Pressure Washing
Water spray has been used for the removal of surface dirt in some cases and can be recommended, depending on the sculpture. With a sound structure and surface, low pressure water-spray washing can be effective, but must be employed with great caution and only after careful analysis of the condition of the sculpture. In addition to the previously indicated effects of water, the material and the structure of the sculpture should be considered from a pressure point of view. For example: will even low pressure threaten the stability of the sculpture? Are there any structurally weak points? Are there any free-standing details that could be exposed to uneven pressure from the washing? Finally, the surface must be free of any cracks or fissures into which water could be forced. If any doubt exists as to the condition of the sculpture, then low pressure washing should not be considered. Hand-washing techniques are more conservative and therefore are recommended for the control that the operator has over the project.
Hand Washing: Equipment List
- plastic tubs, pails
- hose, to a source of clear tap water
- non-ionic detergent (e,g. W.A. paste), or a mild baby shampoo (2% in water)
- wooden-handled scrub brushes, plastic brushes
- stencil brushes, toothbrushes, artist’s bristle brushes, paint brushes
- distilled water, spray plunger or natural sponges for final rinse
- wooden dowel, whittled to wedge shape for removal of mud wasp nests (unoccupied)
- towels and rags, clean cotton diapers are recommended.
- step-ladder, scaffolding or cherry-picker.
The sculpture is washed from the top down, with several wooden-handled scrub brushes at hand to gently work over the high surfaces. Interior details can be cleaned with smaller stencil brushes. Wooden dowels cut to 8″ or 10″ lengths and whittled to a wedge shape, but with rounded edges, can be used to remove accretions such as mud wasp nests and compacted debris.
Stone sculpture should have attention to cleaning and the removal of organic debris which has accumulated on the surface. Freeze/thaw cycles during winter months can lead to structural damage if water in a crevice expands during low temperature periods. Frost damage is just the start of a potentially serious chain of events that can cause masonry objects to develop fissures and fractures.
Lichens, moss and seedlings can be removed from stone by careful mechanical cleaning, as mentioned earlier.
Cleaning of solid-surfaced, intact stone to remove remaining traces of loose debris can be followed by washing with water and non-ionic detergent. Again, a soft bristle brush can be used to assist in the washing of stone surfaces and removal of accumulations of dirt and biological material. After washing, a rinse with clean, clear water is recommended. Distilled water, if available, is ideal.
With regard to cleaning and washing, some sculptures can be cleaned and washed, others cannot. Those that cannot be cleaned and washed may be sculptures in need of conservation treatment for any one of several reasons.
Note: (See below) It is helpful to keep a record of any “dirt traps,” such as areas on the sculpture which are deeply hollowed-out, and which, as a result, hold puddles of water or large pockets of muddy, organic “compost.” These sites become incubators where serious corrosion or rot begins. Later, the weakness in the area may promote serious structural hazards. Plugged drain holes need regular cleaning to keep them free of debris.
Condition Reports and Maintenance Records
For works of art like outdoor sculpture, record-keeping is extremely important. In order to slow the rate of deterioration, documentation of the condition of a sculpture is necessary to begin a record of its physical state and to note any changes which may require attention in the future. Records of condition are the only way to monitor the effectiveness of a maintenance program; ideally, such records are initiated on acquisition of the work, but they may be developed at any point in its history. Hand-in-hand with condition reports goes the maintenance records. With a record of maintenance initiated, ongoing comparisons can be made with condition reports in order to determine how much the rate of deterioration has either been arrested or has advance. Evaluations can also be made of products and methods employed in the care and maintenance of a sculpture.
The most effective effort that any Society interested in the preservation of stone monuments can do at this time, aside from the above maintenance steps, is further research. In Victoria, systems are available to access world information. At the same time, local research can be combined with state of the art developments in conservation to institute a pilot research project. I stated above that documentation is a critical part of the conservation process, even local research into sources of stone, carefully recorded, and then combined with local environmental statistics, could be an invaluable source of information. Such information could in turn be offered back to the world conservation community, while benefit to the Society’s collection would be greatly enhanced. It would be a significant contribution to conservation history if some of the above steps in outdoor sculpture maintenance could be established.