Many special objects are made of ceramics or glass. These materials include porcelain, earthenware, “crystal,” pottery, and art glass to name just a few. Jewelry, dolls, sculpture, tableware, tiles, kitchenware, and many other items can be made from ceramics and glass.
Ceramics are often classified by their body type. Earthenwares are porous ceramics that have been fired at relatively low temperatures. Stonewares are fired at a high enough temperature that the stoneware body is impermeable to water. Porcelains are very fine bodied ceramics that are fired at very high temperatures to create a vitrified, or glasslike, body.
Ceramics are often decorated with colored slips and glass slurries and are then glazed for decorative purposes. In the case of earthenwares, to provide water impermeability.
Glass is a mixture of ground silica (sand) and other mineral modifying agents (usually metallic salts) that are melted together creating molten glass. The molten glass is formed by molding and blowing into a shape then allowed to slowly cool and harden. If a glass object does not cool slowly and properly (annealing), it will crack or shatter from uneven internal stresses.
The primary means by which ceramics and glass objects deteriorate is through accidental cracking and breaking. This is often a result of improper handling, shipping, storage, or display. Other sources of deterioration includes deterioration of the clay body or glass resulting poor manufacturing methods or materials. Porous ceramics can also deteriorate due to the presence of soluble salts deep within the ceramic body itself. The salts dissolve and re-crystallize as the relative humidity fluctuates. When the salts re-crystallize they expand in size and crush the surrounding ceramic structures. You may have seen this happen with a flowerpot that has become saturated with fertilizer salts over time. Freezing water within the ceramic body may also damage porous ceramics that are left outdoors during winter.
Leaving liquids inside vessels for long periods of time can damage glass. Some constituents of the glass dissolve into the liquid, making the interior of the vessel appear cloudy or appear to have residue inside. All efforts to remove this “residue” will fail. The inside of the vessel has actually been etched away and may have a very fine network of surface cracks.
One may assume earthenwares are more subject to deterioration than other ceramics due to their higher level of water permeability. Porcelains can be extremely fragile due to their highly vitrified nature. They are often made to have paper-thin, delicate walls and thus are subject to cracking and breakage.
Handling Ceramics and Glass :
A major source of damage to ceramics and glass can be improper handling and carelessness. A thoughtless tap of a glass goblet on the storage shelf can result in a chip or complete breakage. Careless handling can also lead to the formation of internal cracks that weaken the ceramic or glass structure. It is always best to overestimate the brittleness and underestimate the strength of an artifact. Anyone who has poured hot water into a cold glass and heard a delicate “chink” sound remembers the heartache of breaking a favorite piece.
When moving ceramic and glass objects, always carry one object or one part of an object at a time. Place your hands around the body of the object rather than using an existing handle, rim, or spout for support. Ensure you have an adequate level space available to place the object, and a clear path before removing the piece from its original location. Carry objects in a padded basket or box rather than in your hands. Were you to trip or fall with your hands full, you would crush the object and possibly injure yourself. Use soft padding to prevent ceramic and glass objects from clinking against each other during transport or in overcrowded conditions.
Storage and Display :
Ceramics and glass, generally, should be stored and displayed on sturdy, level surfaces that are secure from bumps and jarring. Objects should be covered or enclosed to protect them from dirt and dust. Alternatively for storage, pieces can be wrapped in acid-free, lignin-free tissue and stored in acid-free cardboard boxes. Newspaper and acidic newsprint paper can cause discoloration and stains. This should not be used for wrapping or long term storage of ceramics and glass. Any storage box should be strong enough to support the weight of the objects inside with a secure bottom. The container should also be large enough to enclose the entire object. Objects should not be allowed to bump or fall against each other.
Ceramics are often displayed vertically on walls with spring- loaded mounting brackets. These brackets may exert too much pressure on ceramic plates and often cause cracks and damage. Other vertical plate racks are made that do not exert undue pressure and are much safer for your prized objects. Separate prongs can also be used in place of either type of mounting device. It helps to pad the part of the mount with a synthetic felt to prevent any scratching onto the surface of the object.
Another common problem in the display of ceramic and glass pieces involves the gradual, incremental movement of objects on surfaces due to underground vibrations. The movement can be caused by any constant, transmitted vibration source like subways, trains, underground equipment, and normal building vibration. Objects in museums are often carefully secured to their display surfaces with very small dots of soft wax preventing them from “walking” off their display vitrines. Caution should be used, however, when using wax. One must take into consideration whether the wax may be safely removed from the object.
Cleaning Practice :
Ceramics and glass objects should be kept free of dust, debris, and oily residues. In general, it is not a good idea to routinely wash these pieces. Each time a piece is handled for cleaning there is a greater risk of breakage through accidents and mishandling. It is better to protect pieces from soiling and dust in the first place, rather than wash them too often.
Porous ceramics, like earthenware, should never be immersed in water. They will absorb the water into the body like a sponge and draw surface stains of residues deeper into the ceramic body if left to soak. Before cleaning any important ceramic or glass artifact, contact a conservator for advice and recommend safe cleaning methods.
Old Repairs and Restorations :
A very common problem with ceramic and glass objects is the presence of old repairs and restorations. Be very wary of previous repairs and restorations. They are sometimes very difficult to detect. Sometimes shining a black light on the object may help in distinguishing areas of previous repair. Older glues are weaker and more brittle than glues used today. As a result, old restorations may have aged enough that they no longer support the broken pieces of the object. They often yellow and peel and become unsightly, as well as dangerous. Objects can sometimes just fall to pieces by themselves. Be extra careful when lifting or handling repaired ceramics and glass. Also, think very carefully before you decide to take a repaired object apart yourself. If the object is important to you, consider having a professional objects conservator examine it first and provide advice.
When Disaster Strikes :
For ceramic and glass objects, the most serious threats during disaster situations are scratching and breakage. Objects that have become wet during an emergency should be rinsed with clean, distilled or deionized water and then dried with clean cotton or paper towels. Be careful not to scratch objects by wiping off grit or soil or by using towels that are dirty or gritty. If conditions are such that dry towels are not available, objects can be placed in the warm sun to dry.
Porous ceramics should not be allowed to remain wet or submerged in liquids. The permeable body will draw the dirty water and stains into the ceramic. If earthenware is already submerged or waterlogged conntact a local conservator for advice about rinsing and drying the object.
Source : American Institute for Conservation of Historic and Artistic Works : Rescource Glass and Ceramics. The recommendations in this document are intended for guidance only. AIC does not assume responsibility or liability.