Hard as a Rock
The raw materials of sculpture are resistant to erosion, but not immune. Future condition is profoundly affected by atmosphere. Exterior display will expose statuary to the most severe and damaging atmospheric extremes. Various sculpting compounds react differently to external factors. Understanding the vulnerabilities of different sculpting materials is essential to proper care and maintenance.
Stone can be defined as a solid compound found in a wide variety of mineral compositions with various properties. Environmental pressure will degrade stone regardless of its mineral composition. Atmospheric sources of damage and erosion include wind, precipitation, heat, airborne pollutants, and salts in maritime regions. Stone sculpture can also suffer from mishandling. Comprehensive strength in statuary is excellent; tensile strength is poor, creating an increased vulnerability to breakage at weak points.
Some sculpting methods were unsound, resulting in unavoidable maintenance problems. Iron dowels incorporated into sculpture to add strength have been known to cause stains and chemical reactions that can accelerate disintegration. Rusting dowels, clamps and armatures should be replaced with conservation grade stainless steel.
The most common types of stone used in sculpture are marble, limestone, sandstone, and granite. Granite is highly durable but lacks versatility. It is hard, impermeable, crystalline rock with a close-grained structure. Granite can be found in a variety of shades from gray-white to nearly black and green. Each shade usually maintains the telltale-speckled appearance. The properties that contribute to the durability of granite also create a difficult product to shape necessitating simple, less ornate forms.
Limestone statuary became popular in the19th century for use as garden ornaments. The popularity of limestone can be attributed to the similarities in appearance to marble, but comparatively low cost. Limestone was more malleable, and abundant; thus less expensive than marble statuary. Limestone is the un-crystallized form of marble and is highly porous. Composed of calcium carbonate, which is soluble in water. Limestone is particularly susceptible to the elements.
Sandstone is a porous sedimentary rock composed of grains of quartz bound by silica. Golden brown sandstone gained popularity as garden sculpture in the 18th and 19th centuries. Sandstone, as well as Limestone, can develop a crust as sulfates leach out. This reaction or case hardening appears to be and feels firm at the surface but conceals a decaying interior. To identify this condition, tap on the surface of the sculpture, if the report is hollow, the condition may be present. Correcting this condition is difficult, expensive and may not be possible.
Repairs can be made to limestone and sandstone. Limestone is commonly repaired with hydraulic lime fills that react with water to produce constituted limestone or lime putty, a bonding agent that reacts with carbon dioxide in the atmosphere to produce reconstituted limestone. The above noted methods of repair are less durable than the original limestone. Although each compounds has the same chemical structure as limestone, they were not created under the pressures over long periods of time in the earth and are not as hard.
Removing the piece from the elements best supports the preservation of stonework. If sculpture is to remain outside, it should be placed on a plinth or base that has been made impermeable to water, and surrounded by drainage channel that is to be kept clean of debris. Keep outdoor sculpture as dry as possible by placing it out of sprinkler range, away from overhanging foliage (foliage can cause etching, dampness, and biological staining), and in an area of Southern exposure, as Northern exposure can be accompanied by damp conditions in which lichens and algae flourish).
During winter months in frost prone locations, vulnerable statutes should be brought indoors or covered with well-ventilated tarpaulin. A canvas, burlap, and tarpaulin are fine but never use plastic tarp because it will cause condensation.
Stains that are likely to cause continual deterioration or disfiguration should be removed. Experts test sculpture to identify stains and the best method of removal. Stain removal is generally done chemically, but poultices may also be used to draw out the stains.
Fielden, James; Gardner, Richard; Davidson, Paul; Luckhurst, Bruce 1998 "The Antique Clinic" Barrons Educational Series, Inc., Hauppauge, New YorkBack to Tips & Techniques