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Conservation of Ceramics and Glass

By Sandra Davison,Peter Winsor and Stephen Ball

Every museum and general collection includes examples of the huge range of objects manufactured from ceramic materials or glass. These ubiquitous items - from the smallest domestic ceramics and ceremonial vessels to ornaments and large-scale statuary - encompass the entire span of recorded human history.

Ceramic

'Ceramic' is a broad term covering all types of fired clay, including terracotta, earthenware, stoneware and porcelain. Ceramic objects range from utilitarian and decorative vessels to tiles, sculpture and dolls. All ceramic objects are made from various types of clay, with or without additives; some may be painted, glazed, enamelled or gilded. Clays are natural products formed by the long-term weathering of rocks in the earth's crust. As techniques improved, potters used ever more complex mixtures of clays and minerals to produce the required properties and effects.

The firing of the shaped clay may comprise more than one stage at different temperatures - typically, a first or 'biscuit' firing to stabilise and set the object, and later firings for glazing, colour or decoration. Firing techniques and methods greatly influence the final properties of the object: low-fired pottery and soft-paste porcelain tend to be more fragile and porous, whereas high-fired stoneware and hard-paste porcelain are normally more durable and non-porous.

There are four broad categories of ceramic material. The most basic but still widely used form is low-fired pottery or earthenware. Examples of this relatively soft and porous type include Neolithic, Greek, Roman and Chinese artefacts, and a vast range of other 'common' or studio pottery; tin-glazed Hispano-Moresque pottery and Italian maiolica, French faience, Dutch delft and English delftwares; lead-glazed Islamic pottery and fine earthenwares; and slipwares, such as those from Staffordshire.

Higher firing produces the second group - the vitrified, non-porous and durable material known as stoneware. Examples include Chinese Yueh ware and celadons; salt glazed Rhenish and English stonewares; lead glazed Staffordshire stonewares and cream wares; and Wedgwood's unglazed basalt and jasper wares.

The other two groups are the porcelains - hard, white materials with a degree of translucency. Soft paste or 'imitation' porcelain was produced by Capodimonte, Rouen, St Cloud, Vincennes, Sèvres, Bow, Chelsea, Derby, Worcester and many others, and also includes bone china and unglazed Parian wares. Hard paste 'true' porcelains include Chinese and Japanese porcelains, unglazed biscuit wares, and the products of Meissen, Vienna, Sèvres, Plymouth and Bristol.

Glass

'Glass' may appear to describe a single unvarying substance, but like 'ceramic' it is a broad term that refers to a number of different but related materials. The main raw materials of inorganic glass are silica (sand), an alkali (usually sodium oxide or potassium oxide, producing respectively soda and potash glasses), and calcium (lime). Modifiers, stabilisers and colourants are added to make the glass workable or to add decorative characteristics.

Glass is an intriguing substance. It is generally regarded as a supercooled liquid, having characteristics of both solid and liquid phases. If left long enough (perhaps over centuries), a vertical sheet of glass 'runs' to become thinner at the top and thicker at the bottom. This is not the only characteristic that changes over time: although strong when first made, glass acquires minute surface defects that act as points of stress concentration. These in turn initiate deeper flaws, and the process carries on in a cumulative fashion. Hence old glass is more fragile than new glass.

Apart from glass itself, other glass-like substances or finishes include glazes (vitreous coatings applied to other materials, such as a ceramic), enamels (glazes applied to a metallic base), and Egyptian faience (a glaze of fused, fritted - powdered - silica, but unconnected with the earthenware known as faience).

Glass may be clear, or modified or coloured by the addition of metallic oxides, metal particles, and opalising agents. Lead glass or lead crystal is a 'soft', sparkling glass that is easily cut and incised. Often known simply as 'crystal', lead crystal is produced by adding lead oxide to glass and should not be confused with rock crystal, a naturally occurring form of quartz. Glass can blown, floated or moulded, and may be decorated in both molten and hardened states. The range of glass or glazed objects in even the smallest museum is potentially very large: as well as the familiar utilitarian and decorative glassware and glazed ceramics, there may be sculptures, enamelled objects, chandeliers, mirrors, stained glass windows and fragments, reverse paintings on glass, and so on.

Cleaning and Handling

Routine cleaning presents two main risks: from the cleaning itself, and from the associated handling or moving operations. In general, clean ceramic and glass objects infrequently using a soft brush held in one hand while the other hand holds or steadies the object. Keep hands and brushes away from gilding, which could be rubbed off in time, and do not apply friction or force to repairs and other fragile areas.

Before handling the object, examine it carefully for cracks or weaknesses. Lift and set down the object very carefully, and always remove lids and other loose items that could fall before moving the main object. Never lift an object by its handles, knobs, rims or other protuberances, and instead use both hands around the base or similar substantial area of the piece. Check the weight and balance before you lift. Take cleaning equipment to the object, rather than the object to a cleaning station - indeed, always find ways to avoid moving ceramic and glass objects from place to place.

If you must move an object to another place, wrap it in tissue then place it on its own in a box or basket before moving it. Choose a time of day when there are few people about, and ask a colleague to open doors and clear the way for you. Concentrate on what you're doing - don't talk to others and ignore distractions en route.

Wet cleaning should not normally be necessary unless obvious soiling builds up in spite of the dusting programme, and even then not more often than every 2-3 years. Perhaps needless to say, a dishwasher is not suitable for cleaning museum objects; it may cause irreversible damage. Wash small areas at a time, without immersing the object, using small swabs of cotton wool moistened with water. Use only very small quantities of safe, museum quality (non-ionic type) detergents, and make sure that all traces are thoroughly rinsed off. Small objects can be washed over a washing-up bowl padded with polyethylene foam in case the object falls. Gently pat larger items dry with paper towels. Intricate items such as figurines should be left to dry for 24 hours before being returned to their display position.

Never wet unglazed ceramics, damaged glazed earthenware or soft paste, repaired or restored areas, gilded decoration or painted glass, or early (pre-1700) glass.

Display and Storage

Display cabinets provide the best protection for ceramic and glass objects. Allow plenty of room for objects, avoiding overcrowding on the shelves. Make sure that plate stands are big enough for the chosen object; they must support the plate well above the centre of the back and also be wide and deep enough to be stable both laterally and from front to back. Acrylic stands deteriorate over time, so inspect them for cracks.

If you must use plate hangers, use the plastic covered wire variety, but remember that sprung hooks apply a constant load to the edges of the piece. This applied stress may chip fragile edges or open up cracks. Never display a piece with open cracks; the holder or stand, or the object's own weight, may be enough to cause failure. Instead, consolidate it and refer it to a conservator (see below).

Smaller objects are frequently displayed on furniture of the same period, so do not forget the conservation needs of the latter. Many ceramic and glass objects have unfinished bases that could scratch polished surfaces, so use a suitably sized felt mat between the object and the surface.

Objects in store should be arranged on shelves, again without crowding. Cover each row of objects with a sheet of tissue to protect against dust - remembering not to pull the paper sideways to gain access but to lift it vertically upwards. The arrangement of the objects largely follows common sense principles aimed at preventing damage. For example, large piles of plates and bowls are unstable and heavily load the bottom items: limit piles to 10 items at the most, preferably fewer, and place acid-free tissue between the layers. Bowls and other rounded objects must not be stacked in such a way that the weight of the objects above applies outward stresses on the sides of the items beneath, as where the rounded body of a bowl squeezes against the rim of the next bowl down. Do not use handles for hanging cups and jugs, however traditional this may appear.

Repair and Restoration

Ceramic and glass are generally more stable than many other museum objects, and sound pieces are not particularly sensitive to variations in the environment. However, their brittleness is an all too familiar characteristic, and an unforgiving one. Catastrophic breakage or significant damage often result from careless handling, dusting and carrying operations, or from poor siting that permits an object to be knocked over by passers-by, curtains or during dusting. For this reason, ceramic and glass objects are common candidates for repair or restoration.

If the worst happens, carefully collect every fragment. The pieces of a dropped object may be widely scattered. Wrap the larger pieces individually in layers of tissue, and place the fragments in envelopes. Do not attempt to fit pieces together yourself, or tape fragments together or on to paper - though you may place tape across running cracks to prevent them from lengthening. Put the packages and envelopes for the item into a strong box with plenty of packing material, together with identifying details and a description of the contents.

Ceramic and glass do not normally deteriorate while awaiting repair. Provided you have collected, wrapped, protected and labelled the pieces, the box can wait in store indefinitely until the time - or the budget - is right for repair.

Always inspect new or previously unexamined objects for signs of damage or repair. Small chips and blemishes are the marks of an object's history, but larger damage may render a piece unsuitable for many types of display unless it can be restored in some way. Remember that restoration and conservation approach the problem from different vantage points, and that you may have to decide (perhaps with advice) which approach to follow.

Existing repairs or restorations sometimes cause problems as early materials lose integrity and old methods show their shortcomings - adhesives fail or fillers and paints discolour, for example. Traditional repairs often used very obvious rivets and staples to keep separate parts together. Although these are ugly, they are part of the object's history and should normally be retained; their main purpose was to keep an item in continued use, so it is unlikely that the rest of the piece will be in pristine condition. Refer rivets, staples and other previous restorations to a conservator when the repair shows signs of failing.

Restoration is not for the amateur. Modern adhesives and repair materials may appear temptingly easy to use, but this is deceptive. The repair and restoration of valuable objects demand a knowledge of technology, historical styles and the chemistry of materials. Never try to repair an object yourself; even if you partially succeed, it is likely that a restorer will have to undo your handiwork in the future, and at high cost.

Information and Sources of Advice

Buys, S. and Oakley, V., The Conservation and Restoration of Ceramics, Butterworth-Heinemann, London, 1993.
Newman, H., An Illustrated Dictionary of Glass, Thames and Hudson, London, 1977.

Newton, R. and Davison, S., The Conservation of Glass, Butterworth-Heinemann, London, 1989.

Plowden, A. and Halahan, F., Looking after Antiques, Pan Books, London, 1987.

Sandwith, H. and Stainton, S., The National Trust Manual of Housekeeping, Penguin with The National Trust, London, 1993.

Savage, G. and Newman, H., An Illustrated Dictionary of Ceramics, Thames and Hudson, London, 1974.

Simpson, M. T. and Huntley, M., Caring for Antiques: A Guide to Handling, Cleaning, Display and Restoration, Sotheby's, London, 1992.

For more information about private conservation work please contact:

Conservation Register
Tel: 020 7721 8246

Conservation Register (Scotland)
Tel: 0131 668 8668

Copies of this fact sheet can be provided in alternative formats. Please contact Viola Lewis, Information Officer at MLA for further information.

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