Conservation and restoration of ceramic objects is a process dedicated to the preservation and protection of objects of historical and personal value made from ceramic. Typically, this activity of conservation-restoration is undertaken by a conservator-restorer, especially when dealing with an object of cultural heritage. Ceramics are created from a production of coatings of inorganic, nonmetallic materials using heating and cooling to create a glaze. These coatings are often permanent and sustainable for utilitarian and decorative purposes.Ceramic Tile and Stone Standards . Ctioa.org. Retrieved on 2012-03-28. The cleaning, handling, storage, and in general treatment of ceramics is consistent with that of glass because they are made of similar oxygen-rich components, such as silicates. In conservation ceramics are broken down into three groups: unfired clay, earthenware or terracotta, and stoneware and porcelain.
Ceramic deterioration All materials used for construction eventually degrade and deteriorate. Degradation of an object occurs as a result of the interaction with the environment or with the materials that form the object; however, in the case of ceramics, environmental factors are the major cause. There are several ways in which ceramics break down physically and chemically.
Additionally, the type of ceramic will affect how it will break down. Unfired clay, like mud and clay adobe, is clay that is fired under 1000°C or 1832°F. This type of clay is water-soluble and unstable. Earthenware is clay that has been fired between 1000–1200°C or 1832°–2192°F. The firing makes the clay water insoluble but does not allow the formation of an extensive glassy or vitreous within the body. Although water-insoluble, the porous body of earthenware allows water to penetrate. A glaze can be applied that will protect the vessel from water. Due to its porosity, earthenware is susceptible to moisture and creates problems including cracks, breaks and mold growth. Porcelain and stoneware are fired at the highest temperatures between 1200–1400°C or 2192–2552°F. Porcelain clay mixtures are fired to create a non-porous and very hard surface. However, the materials also create a very brittle surface which increases the potential for chips, cracks and breaks.
Physical degradation Due to their fragility, damage to ceramics typically comes from mishandling and packing. However, other factors, such as vandalism, frost, mold, and other similar occurrences, can also inflict harm.
Manufacturing defects Also known as inherent vice, the intrinsic instability of the fabric and components of an object can lead to its own physical degradation. This is difficult to prevent because it occurs within the fabric of the material and therefore is a natural occurrence. Deterioration of an object can happen even before the object is used. How the piece is created can instill manufacturing defects in the piece. This means that objects can be damaged even before they are used. This would include a body that contains inadequate quantities of filler materials. A second typical defect is from poor design and construction. An example of this would be a ceramic piece with a handle too thin to support the weight of the cup. A third manufacturing defect is careless firing: a ceramic piece that has been fired too rapidly or allowed to dry unevenly will crack or break.
Impact and abrasion With its delicate nature, ceramics that have been used over a period of time will sustain cracks, nicks, and blemishes. Additionally in a museum environment, damage can occur from packing, storing, and handling of objects.
Frost Damage can occur when ceramics are exposed to freezing temperatures and frost. The problem occurs when ice crystals form inside of the pores of the ceramic piece. The frost inside of the pores will exert pressure onto the fabric of the pottery and cause the material to crack and break.
Mold growth When humidity is high molds can begin to form on ceramic, particularly ones in which there is no glaze. Mold spores are found throughout the atmosphere and will attach to suitable substrates, including ceramics. Earthenware ceramics are frequently affected due to their porosity and lack of glaze.
Chemical degradation
Chemical degradation of objects occurs not in the physical structure of the object but rather at the chemical level. The degradation of the chemical constituents of an object will hinder or weaken the stability of the object when exposed to environmental factors such as water, air, pollution, heat, humidity, and the like.
Water Water can dissolve or deform ceramics that have been low fired, i.e., at temperatures around 600°C. Ceramic fired at high temperatures may also have water-soluble mineral constituents, for example gypsum or calcite. Additionally, water may carry solutes that damage ceramics. For example, dissolved carbon dioxide increases the solubility of calcite by reacting to form calcium bicarbonate which is comparatively soluble. Stagnant water is less damaging because the carbon dioxide is not exhausted.
Soluble salts A common degradation issue in ceramics involves soluble salts. Soluble salts can either enter the clay body from the environment, for example from being buried underground for decades, or they are already naturally occurring due to the components of the materials or clay used. Non-archaeological objects, such as modern dishware, can acquire salts from normal use such as storing salt. Soluble salts respond to changes in humidity both high and low. In high humidity salts become soluble and in low humidity they crystallize. The changing from soluble to crystallization and back damages the surface of the ceramic because salt crystals are larger than liquid salt and therefore will shrink and expand the ceramic body. A white haze on the surface is the first indication of soluble salts, which is the salt crystallizing. Over time, the physical component of the body will crumble until it is completely destroyed.
Preventive care of ceramics
In the realm of conservation there are two distinct practices: non-interventive and active conservation. Non-interventive types of conservation are used to control the surrounding environment such as light, humidity, and temperature. Active conservation is when a conservator practices treatments to alleviate physical problems in the object such as fading, chipping, or breaks.
Display Although ceramics are utilitarian, some pieces are made to be artwork and therefore displayed. Displaying an object improperly can cause damage either physical or chemically from the environment. One of the most common causes for damage is a ceramic piece falling over or off a shelf. To prevent this issue, many historic houses will line storage and display shelves with a thin layer of ethafoam (polyethylene foam) or bubble wrap.
Storage
Ceramics are very delicate in nature and damage can occur even when they are stored away. The most common way in which ceramics become damaged is when they are stacked one inside the other. Unless this is part of the original design, this will typically cause nicks, cracks, or breaks. Some ceramics, depending on their provenance, survive better in different temperature and humidity conditions. Pottery that has been buried, such as from an archaeological site, is better stored at a constant low humidity. This will help to keep any salts from efflorescing, a process which can mar the surface as well as remove the surface glaze.
In general ceramics are typically inert and are not sensitive to elevated light levels. However, extreme changes in temperature and humidity can cause chemical and physical damage. Typically, museums strive to store ceramics, as well as many other material types, in a stable temperature of 68 °F with ± 3°. Additionally relative humidity should be stabilized at 50% also with a ±5%. Storing objects near windows, heaters, fireplaces, and exterior walls can create an unstable environment with temperature and humidity fluctuation and increase potential for damage.
Some storing materials can be harmful to ceramic objects. Wool felt attracts and harbors insects including moths and silverfish which can be potentially very harmful to other collection material types. Polyurethane foam deteriorates over time, leaving a sticky and acidic by-product.
Handling One of the rules in object handling is to treat every object as if it is fragile and easily breakable. Museum technicians, curators, and conservators are trained to prepare a moving plan before an object is even touched. A vessel, or any object, should be held and handled by its strongest part, such as the base, and with both hands. Areas such as the handle or neck of a vessel tend to be the weakest points and may break if picked up by these components.
Removal of previous conservation actions Damage also can occur to ceramics from previous restoration. Although the intent was to repair the object for use or display, some dated practices are now known to increase damage either physically, from rivets or staples, or chemically, from formerly used adhesives that off-gas.
Removal of surface coating Overpaint is a technique that is used to cover imperfection on the surface of a ceramic piece. Differences can be seen to the naked eye due to discoloration, being matched poorly, and change in texture or gloss. Subtle difference can also be seen by restorers by using lighting and magnification. Overpaint and surface coatings can be removed either mechanically or with the use of solvents.
Mechanical removal of overpaint include physical techniques to remove the coating from the surface. On a glazed surface a sharp needle or scalpel can be used. If mechanical removal is not possible without damaging the surface, solvents can be used instead. The archetype solvents typically used are water, white spirit, industrial methylated spirits (denatured alcohol), acetone, and Dichloromethane which is usually found in the form of a commercial paint stripper. The appropriate solvent works by being applied to the ceramic surface by a cotton wool swab and is rolled on the surface rather than being wiped. Wiping the solvent on the surface will push the paint into the surface rather than lift it off.
Removal of filling materials Fill materials are used to fill in missing parts or breaks in a ceramic piece in order to stabilize the piece. A wide range of materials and techniques have been used to restore losses in ceramics. Today the most common filling materials are made from calcium-sulphate-based fillers or synthetic resins such as epoxy, acrylic, or polyester resin. These new resins are stronger and do not harm the object. Removing previous filling materials, either mechanically or chemically, and replacing them with new fillers can help keep the piece strong and stable.
Fillers can be removed physically by mechanical ways, depending on the filler material type. Cement mortar can be chiselled away with a hammer and chisel gradually. Plaster is easily removed through mechanical methods such as chiselling and chipping away with sharp implements. Saws, drills, and other mechanical methods can be used to remove the bulk of protruding materials; however, scratches, chips, and breaks can occur.
Filler material can also be removed chemically. Typically, chemical removal is used once the bulk of filler material is left and only a small portion is left.
Unlike adhesives, fills tend to be easier to remove from ceramics. Plaster of Paris is one example of a fill that comes apart easily with warm.
Removal of adhesives