Conservation problems common to art and antiquities made of metal usually result from interactions between metal and it's recent or past environment. Since metals are reactive with their environments, the best conservation practice is to maintain safe, controlled storage and exhibition conditions. In an event that an objects environment cannot be controlled-for example, when a bronze sculpture is displayed outdoors, it can be protected by a suitable surface coating. Metal objects may also suffer mechanical damage, such as cracks and dents, or breaks that require repair. The preservation of metal objects relies upon their protection against environmental agents that cause undesirable corrosion or degradation. Moisture is the most serious environmental hazard; in the presence of atmospheric humidity iron rusts silver tarnishes more quickly, and copper and its alloys (brass and bronze) corrode. Most stable polished or patinated materials can tolerate a constant humidity level of upto 55% RH and not be damaged, but lower levels are preferable. Corroded, unstable iron and bronze require an even drier environment, and the relative humidity in an area housing such objects should not be above 45%.
Instability of metals and metal artifacts occurs when slats that have migrated into objects during burial or exposure react with the metal and with water from the atmosphere. Eventually, these reactions may produce extreme disfigurement and in time can lead to complete disintegration of an object. Sophisticated treatments for unstable bronze and iron have been developed; they consist of either chemical removal of harmful salts or the application of chemical moisture barriers to the metal surface. The simplest and most reliable way to stabilize these fragile materials against further degradation is to reduce humidity as much as possible.
It would be unwarranted on aesthetic, ethical and practical grounds to remove the artist's patination from a bronze sculpture. These same considerations apply as well to some naturally acquired patinas that not only enhance an objects beauty but may preserve form and detail when much of the original metal has been altered by corrosion reactions. On the other hand, some restrained cleaning and other surface treatments are often desirable in order to display metal objects to their best advantage.
The surfaces of archaeological bronze, iron and silver artifacts are often heavily encrusted with soil, calcareous accretions, and lumpy corrosion products that should and can be removed to reveal underlying form and detail. When possible conservators prefer to do this mechanically - that is by carefully chipping the accretion with specialized tools because this method is more easily controlled, although slow and laborious. Tarnished silver and brass do not need necessarily need the attention of a conservator, but once again, cleaning should be undertaken cautiously. Repeated polishing will wear away the metal surfaces. Commercial polishes should be used with caution; they are often abrasive, may contain ammonia, and can leave unsightly residues in low relief areas.
The application of organic surface coatings to metals has a long history. Tinted resins were applied to bronze sculptures during the Renaissance to enhance their luster, and waxes have long been used to protect outdoor bronzes. Outdoor bronzes, unless their patina is exceptionally intact and stable, will benefit from the protection of lacquers or waxes. Today, practical, functional and aesthetic criteria govern the decision whether to use surface coatings as well as the choice of coating to be used.
Metal objects are subject to mechanical damage as well as to corrosion. Frequently they are repaired using methods similar to those employed in their manufacture. However, the conventional methods of working and shaping metal, such as hammering and soldering, are not always suitable for treating historical objects. They must not be used with largely or wholly mineralized archaeological objects and should be applied sparingly even to modern, structurally sound objects. The need for caution arises from the nature of these metalworking techniques. Proper storage and display conditions can minimize the chances of mechanical damage just as a controlled environment can minimize corrosion. Objects should be placed on shelves when required and supported so that they cannot shift, roll, fall or come into contact with sharp edges or corners of neighbouring objects or structures.
The guiding principles of conservation of all metal objects remain the maintenance of a dry and inert environment, and a conservative sensitive attitude toward repair and restoration.
Carved stone can usually be thought of as any piece of stone which is cut in three dimensions to form an artwork. The extent and variety of carving techniques is matched by the range of stones from which stone sculptures can be made. Marble, granite, limestone and sandstone are the most common stones that artists work with, but each of these varies substantially in appearance and behavior.
Early recognition of decay through careful observation helps to prevent serious or even irreparable damage. Even though stone is considered one of the most durable materials, stone sculptures may decay or be damaged due to several factors.
Frequently found on the exterior surfaces of stone sculptures, environmental decay is the result of inclement weather, erosion, pollutants, the crystallization of salts on the surface, and repeated wetting and drying. As this decay is not easily controlled and tough to reverse, prevention through regular checks is important.
This is caused by lower grades of stone and changes in devices used along with the stone or in its installation.
Poor transportation, installation, repair and vandalism can also lead to damage.
Dirt on the surface of a stone sculpture, especially one stored outdoors, is not usually a problem. Many carved stones suffer from the accumulation of a black crust on their sheltered undersides. Though this may lead to decay, it is usually stable and even functions as a protective layer. The surfaces of carved stones should not be cleaned regularly, as the action of cleaning can cause accelerated deterioration (especially to sandstones) or re-soiling by opening the pores of the stone. The repeated handling of stone sculptures can also lead to the formation of stains from the oils present in skin. If it is absolutely necessary, cleaning should be carried out by a professional conservator using gentle means such as bristle brushes and water. Detergents and other chemical agents should be avoided at all costs.
Ceramic and Glass Sculptures
Ceramic is a broad term covering all types of fired clay, including terracotta, earthenware, stoneware and porcelain. Some ceramic sculptures may be painted, glazed and enamelled or gilded.
Glass also covers a number of varieties. Glass sculptures may be clear, colourless, or coloured by the addition of metallic oxides. The addition of lead oxide to glass produces a soft sparkling glass which can easily be cut, and is often referred to as crystal. Glass can be decorated in the molten or hard state.
Causes of damage
Ceramic and glass sculptures are generally stable and are not readily subject to variations in the environment, like paper and canvas. However, unstable glazes may react to environmental fluctuations and 'crizzle' or develop a fine network of cracks in their surface. Being brittle, they are easily broken. The most usual damage is breakage caused by the objects being washed, transported in inadequate packing, dropped during cleaning, or even blown over by the wind. Fortunately, there are a lot of preventive measures to avoid such damage.
Ceramic and glass objects can be dusted using a soft brush. Dry cotton wool or cloth will catch on rough surfaces, leaving their fibers behind, and possibly causing damage.
What to do when something is broken
Wrap each piece separately in acid-free tissue paper or glassine, and place them in a clearly labelled box. Include even the tiniest chips. Do not try to fit the pieces together yourself, as more damage can be done by grating the edges against each other. Do not tape fragments together as tape is difficult to remove and may damage the glazing, gilding or fragile surfaces.