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Questions to a ceramics conservator

roman cast moulded bowl

Find out more about the ceramics and glass conservation department| on this website.

How should I mend my broken plate, vase or cup?
Mending ceramic and glass objects so that the joins are disguised as much as possible is difficult. In addition, the types of glues and adhesives one should use are important. Using the wrong adhesive could damage the object. As every object is different book an opinion service appointment with a ceramics and glass conservator to find out how best to mend your object.

bowl with a picture of a Liver Bird and a decorative pattern around the rim, broken into 5 large pieces

What is 'ceramic'?
Ceramics are materials made of non-metallic minerals that have been permanently hardened by firing them at high temperatures. Objects made of such materials are also called ceramics. Traditionally, ceramics were made of clay| and other naturally occurring materials. In contrast, modern ceramics are made of silicon carbide, alumina and other specially purified materials. The word ceramic comes from the Greek word 'keramikos' meaning 'of pottering', from 'keramos', which means 'potter’s clay'.

What is 'earthenware'?
Earthenware is a type of ceramic made from potash|, sand, feldspar| and clay|. Earthenware is typically 'biscuit fired' (first kiln firing) at a temperature of around 1000ºC. It is 'glaze fired' (the final/second firing) at around 1100ºC. Sometimes earthenware can be as thin as bone china and other porcelains, though it is more easily chipped. It is not as translucent as porcelain. Earthenware is less strong and less tough and more porous than stoneware – but is cheaper to produce and easier to work with. Due to its high porosity, earthenware must usually be glazed in order to be watertight.

high sided earthenware bowl decorated with a simple geometric pattern

Earthenware Peruvian vessel

What is 'stoneware'?
Stoneware is a high-fired ceramic that is vitreous| or semi vitreous, not translucent, and is often made up of clays that are not highly refined. Stoneware commonly has some specks and some particulate material such as sand in it. Stoneware is harder than earthenware and is fired at kiln temperatures of about 1200-1300ºC. Its colour is usually grey, buff, white or brown. Stoneware is strong and can hold water, though it is not completely waterproof unless glazed.

What is 'porcelain'?
There are two groups of porcelain - soft-paste porcelain and hard past porcelain.

  • Hard paste porcelain is a white, vitrified, high temperature ware, which is translucent and rings when struck. A unique aspect of hard paste porcelain is that it can be worked like clay, but when fired properly it reaches a state similar to glass. It is a special type of clay that is either white or grey, to which kaolin (a white firing stiff clay) and white China stone (finely decayed granite, washed and prepared in small white blocks) is added. When fired at temperatures of 1280ºC and over, the body vitrifies ie it becomes completely impermeable. Glazes can be applied for the first firing, or a vessel can be decorated with a low firing glaze and can be put back into the kiln a second time. True porcelain was being made in China and Korea around 960AD. Hard paste porcelain was first made in Europe in the 18th century. Its recipe was discovered by Meissen Chemists in Germany.
  • Soft paste porcelain was first developed in Europe in 1738 to imitate Chinese porcelain. It is produced by mixing white clay with a frit (a glassy substance that was a mixture of white sand, gypsum, soda, salt, alum and nitre). Lime and chalk were used to fuse the white clay and the frit. The mixture was then fired at lower temperature than hard paste porcelain. It produced a softer body than hard past porcelain or true porcelain being fired at 1200ºC. Soft paste porcelain is soft and the body is granular since the ingredients do not melt together.

plate covered in highly detailed colourful decoration, with a coat of arms in the centre

18th century Chinese hard paste porcelain charger

What is 'bone china'?
In the 18th century, English potters invented bone china to compete with the Chinese porcelain being imported into Europe. It combines bone ash (animal bone) with the 'hard paste' porcelain (see above) ingredients of kaolin and china stone in a formula of 50% bone ash, 25% kaolin and 25% china stone. It is fired at 1200-1300ºC.

What is 'pottery'?
Pottery is a form of ceramics technology where wet clays are shaped and then dried or fired to harden them. The term is generally used only for relatively easily constructed utensils such as pots, cups, bowls etc. Pottery is an ancient technology.

What causes ceramics to deteriorate?
Environmental factors play the biggest role in the deterioration of ceramics. Although mechanical degradation| (such as a ceramic breaking after being dropped) can be rapid, chemical degradations| tend to be incredibly slow processes. Ceramics are generally chemically very stable. It is because of this that so many ceramics have survived periods covering 27 000 years and are found to be in good condition when uncovered during excavations. The range of different ceramics is wide and variations in firing temperature, composition and construction will all affect the degree to which any particular ceramic object is susceptible to individual environmental factors. Generally, it is the lower fired ceramics that deteriorate more easily than the higher fired wares. In a museum, historic building or normal social dwelling ceramics are most at risk from human intervention. They are very susceptible to mechanical shock and breaks, travelling cracks and chips will occur. General domestic use and exposure to polluted environments can cause superficial dirt or staining to the ceramic body. When ceramics are exposed to other environmental disasters such as fire| or flood|, it is usually thermal shock that causes extreme damage. Staining and chemical damage can also occur to the ceramics. It is in a burial situation that chemical changes can often occur; when there has been exposure to hostile environments. These may be so severe that the ceramic structure breaks down and crumbles. Chemical breakdown may lead to damage that can neither be reversed or nor truly disguised eg iridescent glazed surfaces.

What object that you have worked on is your favourite?
Some favourite items I have worked on are: several Panathenaic Amphorae; beautiful Roman Glass; stunning Chinese Han and Kangxi ceramics, and Buckley Pottery. Before training in conservation, I trained as a ceramicist and so I love exploring and understanding how something was made. Often one comes across the potter’s handprint in the clay or fingerprint in the glaze, which to me adds a sense of wonderment and appreciation.

What is the weirdest object you have worked on?
The ‘quirkiest’ item I have worked on is Fritz Spiegl’s Loophonium|.

What is the oldest thing you have worked on?
The oldest object/s I have worked on would probably be some of the Egyptian material| and a Bronze Age urn – all now on display in the Weston Discovery Centre| at World Museum.

What is the largest thing you have worked on?
The largest singular object I have worked on is a fabulous life size Majolica peacock produced by Minton, which is on display at the Walker Art Gallery. Architecturally, there are also several tile panels and a wonderful Birkenhead Della Robbia tile fountain that fall into this category. Of course, there are also several stained glass windows, some of which are on display in the craft and design gallery at the Walker Art Gallery.

Glossary

Potash: the common name for potassium hydroxide, a substance that has been used since antiquity.

Sand: the most common constituent of sand in inland continental settings and non-tropical coastal settings is silica (silicon dioxide) often in the form of quartz.

Feldspar: an important group of rock-forming minerals. The group of minerals consists of three silicates a potassium-aluminium silicate (the orthoclase feldspars), a sodium aluminium silicate, and a calcium aluminium silicate (the plagioclase feldspars) and their nisomorphous mixtures.

Clay: a generic term for an aggregate of hydrous silicate particles less those 4 micrometers in diameter. It consists of the small crystals of the minerals silica (SiO2) and alumina (Al2O3). There are three main groups of clays: Kaolinite-Serpentine, Illite and Smectite. Although there are about 30 different types of “pure” clays in these categories, most natural clays are mixtures of these different types as well as other weathered minerals.

Vitreous: material that is vitreous resembles glass. Either in colour, composition, brittleness, or lustre.

Vitrified: a material that is vitrified has been converted into glass or a glassy substance by heat and fusion.