Caring for ceramic and glass objects

Composition of ceramics

Ceramics are as variable as the cultures and individuals who created them. In general, ceramics are made of fired clay with inclusions of other non-plastic materials, called “temper,” that reduce shrinkage and allow gases to escape when the ceramic is fired. The most common inclusions are crushed, previously fired ceramic (grog), chert, volcanic ash and quartz sand. The type of clay will determine the firing temperature of the ceramic, which affects its physical properties, such as porosity and hardness. The term “fabric” is used to describe the clay body of a ceramic and any inclusions. The fabric is sometimes sealed or decorated with glazes or slips. A combination of surface finishes may also be used.

The more highly fired a ceramic is (i.e. the higher the temperature used), the more impervious it is to its environment. It will still be brittle but more resistant to water. The porosity of the fabric, the type of surface finish and, to some extent, its colour are indications of a ceramic's firing temperature. Ceramics with porous, coloured fabrics—typically buff, red or red-grey—are fired at a lower temperature than ceramics with a white fabric.

Ceramic finishes

Glazes

Not all ceramics are glazed; glazes are added as decoration and to seal the surface of a ceramic. A glaze is a layer of glass formed over a ceramic fabric. It is formulated to match the thermal expansion and contraction rates of the fabric to which it is applied. Glaze is composed of silica with fluxes that lower its melting point, allowing it to fuse to the clay body during firing. Typical fluxes are oxides of lead, sodium and potassium. The temperature at which glazes form must be the same as, or below, the optimum firing temperature of the clay.

Glazes are the most common form of finish on European and Asian ceramics. The highest fired ceramics are the true porcelains, which have a white, non-porous fabric and a glaze that is very similar in composition to the fabric (Figures 1a and 1b). The glaze on Chinese porcelain, termed "feldspathic," appears to be an integral part of the porcelain body. The two fuse together so completely in the firing process that it is very difficult to see a boundary between the glaze and fabric.

Almost as highly fired as true porcelain is stoneware. Stoneware fabric is non-porous and is often greyish in colour. Stoneware glazes are often formed from the clay itself. Such is the case for salt-glazed and ash-glazed stonewares. Both salt and wood ash contain fluxes that melt the silica in the clay minerals on exposed clay surfaces, thus creating a very thin yet resistant layer. Salt glazes are described as having an "orange peel" or dimpled texture. Stoneware vessels have been used as storage containers due to their resistance to damage from water and chemicals.

The most common type of European ceramics in Canadian museum collections will have a glaze based on a mixture of lead oxides and silica. These glazes are clear or coloured and will be found on coarse earthenware as well as on more refined wares such as creamware, pearlware and transfer-printed earthenware (Figures 2a and 2b). Lead glazes are low to moderately high fired and their stability will depend on the ratio of lead to silica. Lead glazes can be damaged by acidic food and beverages and, because these glazes are softer than those on porcelain or stoneware, they are usually scratched if the object was used or handled a lot. Lead glazes may also have fine crazing (a pattern of small cracks) over their surface (Figure 3).

At the other end of the firing spectrum from porcelain is tin-glazed earthenware, which is composed of a soft, porous fabric and a soft, opaque white glaze that is sometimes poorly fused to the underlying ceramic. The opacity of the glaze is caused by the addition of tin oxide to a clear lead glaze. Objects made with this type of glaze are referred to as "faience," "Delftware," "majolica" or "tin-glazed." The white glaze is often painted with bright, fired-on patterns (Figure 4).

Glazed wares can be further decorated by applying designs under or over the glaze. Any design applied over a glaze is more susceptible to damage than a design under the glaze. A good example is gilding on dinnerware, which can be abraded away by repeated use.

Burnishing

Burnishing, or smoothing a damp clay surface, aligns the clay particles and results in a thin, semi-lustrous finish when the object is fired. Burnished surfaces are usually red or black, with the colour coming from the iron oxides in the clay (Figure 5).

Forming a ceramic

There are many methods of forming a vessel or object from clay: casting liquid clay into a porous mold, throwing on a wheel, coiling and paddling, pinching, building in slabs and sections, pressing slabs into molds and sculpting. When handling ceramic objects, it is important to keep in mind that not all such objects were made in one piece. Handles, spouts and molded decorations are usually added to a previously formed vessel. These are held in place using liquid clay and are then fired on (Figure 6).

Firing ceramics

When clay is fired, the minerals in it undergo changes in their crystal structure. Each type of clay has an optimum firing temperature, which is the temperature at which it will become hard, or sinter, without sagging. In porcelains and stoneware, the altered clay minerals actually fuse together into a glassy matrix that gives them their characteristic vitreous quality. All clays shrink when fired.

The reds, greys and buff tones of earthenware are caused by iron oxides that change colours depending on the amount of oxygen in the air around the clay. Oxygen content will also influence the colour of glazes, since metallic oxides are the colouring agents in glazes.

Most glazed ceramics are high-fired in kilns in which the temperature and amount of oxygen can be controlled. Low-fired wares might have been fired in poorly regulated kilns, or in firepits, with little control over temperature and oxygen. It is possible to find a piece of ceramic with some sections underfired and some overfired. Low-fired pottery can be damaged by water.

Physical forces

The chief agent of deterioration for ceramics is physical forces—the brittleness and fragility of ceramics is well known (Figure 9). Handles, spouts and molded decorations are particularly prone to damage. These pieces are held in place using liquid clay before firing. Often, the joints are not strong, and sometimes an improperly secured handle may be attached only by a layer of glaze.

Ceramics can have traces of use, such as scratches or cracks in the glaze (Figure 10), worn overglaze decoration and possibly spalls in the glaze. Pieces created as artworks will not generally carry wear marks of this kind. Some decorative works of ceramic art may contain non-ceramic elements, such as paper, paints and lacquers. These are susceptible to damage through incorrect handling and cleaning.

Designs applied over a glaze are more susceptible to damage than designs underneath. Gilding on dinnerware, for example, can be abraded easily by repeated use and incautious handling or cleaning. Decorations painted onto fired ceramics are easily damaged.

Ceramic objects might have been broken and repaired and will be weaker at these points. Signs of repair are adhesive residues, glue lines, metallic staples or rivets (Figure 11) and infills or overpainting. Repairs are an important part of an object's history and should be documented.

Water

Usually, water entering a museum accidentally is dirty and contains dissolved residues that can be absorbed by porous ceramics and can stain glazes by seeping into cracks. This also happens to archaeological ceramics buried in humid or wet soil (Figure 12). Low-fired ceramics, those fired below 600°C, can deform or disintegrate if soaked in water. Water-soluble tempers, such as gypsum in porous ceramics, can dissolve. Water can also damage materials that were used to repair ceramics, such as plaster or adhesives. Old repairs could discolour, swell or fall apart.

Pollutants

In a dusty or polluted environment, ceramics will become dirty if not protected from particulates. If dirt is left on the surfaces of objects, particularly porous, low-fired or unglazed ceramics, staining can develop. Dirt can also lodge in cracks and areas of repair, resulting in unsightly discolouration. Preventing the deposition of dust or dirt is important because dusting or washing ceramics increases the risk of damaging the object during handling. Staining may occur in both the fabric and glaze from materials that have been absorbed during use or during burial for archaeological material. Salt, iron and oil are common contaminants. Iron will form unsightly rust-coloured or blue-black stains, which are difficult to remove, while oil will form dark grey patches. Salty ceramics, if kept in fluctuating RH, can be physically damaged by repeated crystallization of the salt (for more information, consult Agent of deterioration: incorrect relative humidity). In the case of archaeological and ethnographic ceramics, remnants of food or black fire marks may be indications of historical use and should be preserved.

Incorrect relative humidity

RH is not a concern for most ceramic collections, as the constituent materials of their fabric and glazes are not affected by ambient RH. However, at high RH (above 65%), mould can occur on ceramic surfaces if there are even small amounts of soiling, oils and food residues. Earthenware objects are more likely to be affected due to their porosity. Consult Agent of deterioration: incorrect relative humidity, which provides a chart indicating the rate of mould growth by humidity level.

High RH can also cause water-sensitive adhesives used in repairs to become tacky and, above 75% RH, weak enough to risk releasing fragments.

RH is, however, a greater concern for ceramics contaminated with salts. When the RH rises above a certain critical RH, the salts deliquesce (they absorb enough moisture to form a solution) and later will crystallize when the RH falls below this critical RH. During the drying and crystallizing stage, the salt solution moves towards the zone of evaporation and crystals grow to the point of exploding the pore structure. The resultant flaking of the ceramic is called spalling (Figures 13a, 13b, 13c and 14).

The most common soluble salts are chlorides found in seawater, urine, blood, acids, bleaches, leather tanning products and salt used in food preparation and preservation; nitrates found in decaying organic materials, urine and gunpowder; and phosphates found in ash from kiln sites. Pure salts of chlorides and nitrates deliquesce (absorb moisture from the air to form a liquid compound) at around 70–76% RH; however, mixtures of soluble salts deliquesce at an RH significantly lower than that of pure salts (consult Rörig-Dalgaard 2014), so RH should be maintained well below this level. Since, experimentally, salt mixtures have been shown to deliquesce at an RH as low as 53%, it would be best for these items, to be safe, to remain below 50% RH. Fluctuations in RH above and below a critical RH which cause salts to deliquesce and recrystallize are a major cause of damage and should be avoided.

Spalling is well known in archaeological ceramics from salty environments, but this can also happen to non-archaeological objects that once contained salty solutions, particularly those with unglazed, poorly glazed or crazed surfaces. Chamber pots are a good example: spalling of the glaze might be observed in the lower portion.

Manufacturing defects

As part of the manufacturing process, some pieces may be damaged by mistakes made during forming, glazing and firing. If these defects are severe, the piece may be discarded by the manufacturer. Ceramics are graded by their quality: the best being the most expensive, and the seconds, with minor imperfections, being cheaper. Canada, as an English colony, was a market for wares produced by British potteries, and most of the ceramic tableware exported to colonial markets was not of top quality. Mistakes or sloppiness during manufacture can result in poorly attached handles and spouts, microfissures within the fabric, poorly fused overglaze decoration, unstable glazes, kiln marks in the glaze, unglazed areas and misshapen pottery. This should be kept in mind when handling pieces in museum collections, as these objects may be more susceptible to physical forces and damage from water or pollutants.

Physical forces

Like ceramics, glass is brittle and the most common form of damage is breakage. Since glass is amorphous, once a crack has been propagated, it tends to continue unimpeded due to the lack of crystal boundaries. Glass undergoes a certain amount of elastic deformation when it is broken; for this reason, it is difficult to align broken shards of glass.

Breakage is particularly noticeable in glass due to its transparency. Even after a professional repair, breakage will be visible (Figure 15). Stable adhesives that minimally discolour with time should be used. Adhesives that match the glass's refractive index can be used to create a less noticeable glue line under normal lighting. Consult Koob (2001, p. 48) and Down (2015) for more information on adhesives used for glass objects and on matching the refractive index between an adhesive and glass.

Glass objects might have been repaired, and if so, they will be weaker at these points. Signs of adhesive residues, glue lines and infills are all indications of repair. Repairs are an important part of an object's history and should be documented.

Water

Historic glass, being a non-porous material, is not greatly affected by water over the short term. However, in the case of archaeological glass excavated from wet burial sites (pH 9 or lower), the alkali component of the glass will have been leached from the surface, forming iridescence. Multiple layers of remaining silica interfere with the transmission of light to create the iridescence. Extremely degraded archaeological glass will have many layers of fragile iridescent glass, referred to as "onion skin" degradation, which will flake or crumble when touched. This glass is too fragile to wash (Figure 16). Glass from burial sites where the soil's pH is very alkaline (greater than pH 9) will lose its silica component and the surface will appear etched.

Very deteriorated glass, such as archaeological specimens, is porous and can absorb not only water (from leaks, floods, etc.) but also any dirt, minerals or organic extracts dissolved in the water, resulting in staining or physical damage (Figure 15). Further information on the effects of water on glass can be found under Incorrect relative humidity.

Lead crystal containers, such as decanters, that were used to store liquids often become cloudy on the inside. This is usually a sign of deterioration due to use. The lead has been leached from the glass and the surface will appear cloudy. Fluids left in lead crystal containers for long periods may become contaminated with lead and become toxic.

Pollutants

Like ceramics, in a dusty or polluted environment, glass will become dirty if not protected from particulates. Even slight amounts of dust will be visible on the transparent surfaces of glass, resulting in an unaesthetic appearance. Preventing dust deposition is far safer than having to deal later with dusting and cleaning—which may not always be safe or recommended, and which, at the very least, involves handling and therefore unnecessarily increases the risk of physical damage.

Because of the potential for deteriorated glass to attract moisture, dust and dirt can stick to the fragile surface and can be extremely difficult to remove without damaging it. Conversely, dust and dirt are hygroscopic and aid in attracting atmospheric moisture to the surface, where it can contribute to deterioration (consult Incorrect relative humidity for more details).

Light and ultraviolet

Some types of glass were tinted with manganese oxide to make them appear colourless. If exposed to ultraviolet (UV) light, this type of glass can turn purple. If glass contains a small amount of arsenic and selenium, it will develop a brown tint. This shift in colour is called "solarization." In some cases, this is considered aesthetically pleasing, but it is not the object's original colour. Solarization only occurs after long periods of high-intensity UV exposure. There is a low risk of this occurring under the low to moderate light levels and filtered UV typically found in a museum environment. However, adhesives used to repair glass may become discoloured from exposure to light and UV.

Incorrect relative humidity

Most glass in collections is stable and can be safely stored at an RH between 40–50% (Koob 2006, p. 133). Fluctuations in RH should be avoided. Unstable glass, however, requires extremely tight control of the atmospheric environment to slow the deterioration from glass disease. The proportion of stabilizers within the glass determines its chemical stability. Glass that has less than 4% lime (calcium oxide) will be extremely sensitive to moisture. In soda-lime glass, the excess sodium in the glass absorbs moisture directly from the air to form sodium hydroxide, a strong base, which then dissolves the silica in the glass. It is a cyclic reaction, as the removal of silica frees up more sodium for reaction. The glass appears foggy and has a soapy feel (Figures 17a and b). At a higher RH, the result is weeping, a condition in which the glass surface becomes damp and forms droplets. Initially, washing will remove the hazy layer and return the transparency of the glass; however, over time, as more damage occurs, the result is permanent and the cloudiness cannot be removed. Leaching of the silica produces very fine cracks in the glass. Strict control of the RH is essential for unstable glass. Sodium deliquesces at 42% RH; therefore, to prevent severely deteriorated glass from deteriorating even more, the RH must be lower than 42%. However, at an RH below 30%, severely deteriorated glass will desiccate and turn opaque as a network of fine cracks is exposed. This condition is called "crizzling" and is irreversible.

Potash-based glass is more sensitive to leaching. Placing an unstable glass object in direct sunlight will cause the RH to drop drastically due to localized heating, which will desiccate the glass and cause crizzling. As well, heat from lights in display cases should be avoided as this may cause local desiccation and crizzling. If not kept in stable RH, crizzled glass will eventually disintegrate.

Even stable glass, if stored in extremely high RH, can develop glass disease. Microclimates, created by enclosed spaces such as the inside of decanters, clock faces, daguerreotype covers or framed artwork, often produce moist environments that can lead to crizzling. Dust and other pollutants attract moisture and can contribute to the problem. The alkaline deposits of weeping glass beads, with a pH of 10 or more, can damage the textile or leather substrates to which they are sewn, as well as the thread holding the beads onto the object. Beads may become detached or lost. Consult Deteriorating glass beads for a case study of this type of deterioration.

Handling and storage

Ceramics and glass should be handled carefully. Damage commonly occurs when the objects are dropped or knocked against a hard surface, or when they fall off a shelf. Handling, moving and cleaning are times of highest risk.

Recommendations

Handling ceramics and glass requires care and common sense:

• Do not handle unless necessary.
• Handle extremely carefully and plan all moves to prevent breaking the object.
• Support the object with one hand directly under its centre of gravity, and use the other hand to stabilize it and prevent any movement.
• Handle with clean hands to prevent transferring oils or salts to the surface. Do not use hand lotion.
• Do not wear gloves when handling as gloves can be slippery, especially cotton ones. For unstable glass where wearing gloves is necessary, use nitrile gloves as the glass should not be exposed to moisture or salts from the hands.
• Never pick up objects by handles or spouts since these could break off (Figure 18).
• Beware of loose pieces, such as lids, as these may fall and break when handled.

Consult Handling heritage objects for more guidance.

Padded storage containers and enclosures, including closed storage and display cabinets, will protect these objects from physical damage as well as dirt and dust. Safe storage measures should include the following:

• Protect all objects from dust, thus reducing the need to clean them.
• Enclose water-sensitive objects in waterproof containers.
• Do not stack objects, as the excess weight can cause abrasions and stress.
• Pad out items to prevent rolling or touching of other objects, which may cause abrasion (Figures 19, 20 and 21).
• Provide tall or unstable items with form-fitting mounts to prevent them from tipping and breaking.
• When moving fragile ceramic or glass objects, place them in padded boxes to prevent them from touching each other or rolling.

The environment

Ceramics and glass in good condition are usually robust and do well in general museum environmental conditions. Unstable ceramics and glass, such as salt-contaminated ceramics or glass showing glass disease, require special environments.