CARING FOR YOUR CERAMICS AND GLASS OBJECTS

Many special objects are made of ceramics or glass. These materials cover a tremendous variety, including porcelain, earthenware, “crystal,” Depression glass, 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 made from different types of clays that are modified and colored by additives. The clay mixture, also called the body, is formed into shapes using a variety of techniques such as coiling, turning, and molding. The shaped objects are heated to drive off water and realign the crystalline structure of the clay. The clay is compacted, and sometimes melted, during firing, which results in shrinkage of the object.

Ceramics are often classified by their body type. Earthenwares are porous and often coarse-bodied ceramics that have been fired at relatively low temperatures. Stonewares can also be coarse-bodied, but 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 or, in the case of earthenwares, in order to provide water impermeability. Glazes are usually slurries of ground glassy materials, often mixed in water, that are coated over ceramic bodies and fired into a glass layer on the ceramic body. Colorants and other minerals modify the glaze to produce different colors and effects. By controlling the amount of oxygen in the kiln and the firing temperature, the potter can produce a variety of wares even from the same clay. Raku is a form of Japanese ceramic that has been fired in an oxygen-low reducing fire in the kiln.

Glass objects are made from a mixture of ground silica (sand) and other mineral modifying agents (usually metallic salts) that are melted together to create a molten glass. The molten glass is formed by a variety of methods, including molding and blowing. The shape is then allowed to cool and harden. If a glass object is not allowed to cool slowly and properly by a process called annealing, it will crack or shatter from uneven internal stresses.

Reasons for Deterioration

The primary means by which ceramics and glass objects deteriorate is through accidental cracking and breaking due to improper handling, shipping, storage, or display. Other sources of deterioration for ceramics and glass can include deterioration of the clay body or the glass as result of 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. For example, a flowerpot that has become saturated with fertilizer salts over time may exhibit this effect. 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 to have residue inside. All efforts to remove this “residue” will fail because the inside of the vessel has actually been etched away and may have a very fine network of surface cracks.

Earthenwares might seem more susceptible to deterioration than other ceramics due to their higher level of water permeability. However, porcelains can be extremely fragile due to their highly vitrified nature. They are often made with paper-thin, delicate walls and are subject to cracking and breakage. The goal in the preservation and conservation of all ceramics and glass is to prevent the cracking or breakage of the objects through extremely careful handling, display, and storage.

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, worse, 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 ceramic and glass artifacts. Extra caution in handling can prevent serious damage that can be expensive to repair. Anyone who has poured hot water into a cold glass or mug and heard a delicate “chink” will recall 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. It is best to place your hands around the body of the object rather than using an existing handle, rim, or spout for support. Be sure you have a level space of adequate size on which to place the object and a clear path through which to move before removing the piece from its original location. Carry objects from room to room or up and down stairs in a padded basket or box rather than in your hands. 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, in general, 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. Pieces can also be wrapped in acid-free, lignin-free tissue and stored in acid-free cardboard boxes. Newspaper and acidic newsprint paper can deposit inky stains on ceramics and can also cause a diffuse, creamy brown discoloration in earthenware bodied ceramics. They should not be used for wrapping or long-term storage of ceramics and glass. Any box used for storage should be strong enough to support the weight of the objects and should have 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. The edges of the object, where the bracket prongs rest, are often chipped or cracked. Other vertical plate racks are made that do not exert undue pressure. They have two separate parts that slide against each other to allow expansion of the mount. These 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 or other inert material to prevent any scratching from the mount onto the surface of the object.

Another common display challenge involves the gradual, incremental movement of objects on surfaces due 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, almost pin-sized, dots of soft wax to prevent them from “walking” off their display vitrines. Caution should be used, however, when using wax. Consider whether the wax can be safely removed from the object and the surface on which it sits without harming either surface. Expert conservators have researched ways to reduce earthquake-related damages to displayed objects. If you live in an area prone to earthquakes, you can get information about these experts and their methods by contacting a local objects conservator.

Cleaning Practices

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. For cleaning any important ceramic or glass artifact, a conservator should first be consulted to ensure the objects stability and recommend safe cleaning methods.

Old Repairs and Restorations

A common problem found with ceramic and glass objects is the presence of old repairs and restorations. Be 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. Be sure to wear UV protective glasses whenever using a black light. Older glues are not as good at repairing damaged pieces as the glues we use today. Old restorations and repairs 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, you might consider having a professional objects conservator examine it first and provide advice. A conservator can also carefully remove the old repairs and replace them with more stable and visually acceptable adhesives and paints.

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 as soon as possible. The rinsed objects should then be dried. Clean cotton or paper towels can be used. Be very 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. Using a blow-dryer with heat would not be a good idea; however, a fan can help circulate the air and, in some cases, help the drying process. Ceramics from an archaeological source may have salt contamination, and drying the object could cause crystal growth within the clay body, causing the surface to spall off or powder. If you are not sure about the condition of an object, you might want to contact a conservator before salvaging an object if you are able to get in contact during or right after a disaster. Having a pre-established rapport with one or two conservators is helpful should a disaster strike.

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, you should contact a local conservator for advice about rinsing and drying the object. Other questions about preserving your ceramics and glass during and after a disaster can be answered by your local conservator.

When to Call a Conservator

If you have questions about the care of your objects, or if your objects require special intervention like cleaning, repair, restoration, or replacement of missing parts, you should contact an objects conservator. They will give you advice about the safest means by which to preserve and restore your special items. AIC provides a free brochure containing information to help you select a conservator and also offers a free guide to conservators in your area.

Additional Resources

Schultz, Arthur W., ed. Caring for Your Collections. New York: Harry N. Abrahms, Inc. 1992.

Schultz, Arthur W., ed. Caring for Family Treasures. New York: Harry N. Abrahms, Inc. 2000.

Prepared by Julie A. Reilly with assistance from David Harvey and Julie Lauffenburger, 2001.

For more information, contact:

American Institute for Conservation of
Historic & Artistic Works (AIC)
1156 15th St., NW, Ste. 320
Washington, DC 20005
Telephone: (202) 452-9545
Facsimile: (202) 452-9328
E-mail: info@aic-faic.org
Website: http://aic.stanford.edu

The American Institute for Conservation of Historic & Artistic Works (AIC) is the national membership organization of conservation professionals dedicated to preserving the art and historic artifacts of our cultural heritage for future generations. Among other services of AIC is the Guide to Conservation Services, which provides a free list of conservators in your geographic region. The AIC brochure, Guidelines for Selecting a Conservator, will help you make an informed choice.

The recommendations in this brochure are intended as guidance only, and AIC does not assume responsibility or liability.