By Eman Abdallah Kamel
Eman is a writer and engineer. She likes writing about crafts, as well as many other topics.
What is Clay?
Clay is a fine-grained, naturally occurring earthy material composed of crystalline, hydrous silicate minerals. Clay minerals are composed primarily of silica, alumina, and water but may contain significant amounts of iron, alkali, and alkaline earth minerals. The mechanical and chemical decomposition of rocks forms clay.
Pottery is the art of shaping pots and other objects with clay, which is fired at high temperatures to give it a hard, permanent shape.
With a total trade of $2.52 billion in 2022, clay was ranked 726th among the most traded products worldwide. The clay trade makes up 0.011% of all trade worldwide.
The United States, China, Turkey, India, and Germany were the largest clay exporters in 2022, while the top importers were Italy, Germany, the Netherlands, Spain, and Canada.
China, Portugal, New Zealand, Turkey, Japan, and Poland were among the largest pottery-exporting countries in 2022.
History
Since ancient times, clay has been used to make pottery, including:
- Prehistoric,
- Ancient Egyptians,
- Greek,
- Medieval,
- China,
- Japan,
- Iran.
1. Prehistoric
Pottery is one of the oldest crafts known to man. People carried water in woven baskets lined with clay. The clay layer dried after the water was poured out of the container. The shape shrank and separated from the basket’s sides because of the moisture loss. When the clay was taken off and allowed to dry in the sun on hot sand, the basket pattern was still present. They then learned how to make sturdy containers by hardening pottery cast in hot ash.
2. Ancient Egyptians
The Egyptians built kilns to place their pottery in for firing. The kiln was lined with insulating brick made from straw and sun-dried clay. Later, they used finer clays with a high quartz content to make delicate pottery. They rubbed the pieces with a fine stone to give them a dull shine or covered them with a thin layer of clay of another color. The ancient Egyptians used three types of kilns: updraft kilns, faience kilns, and lime kilns.
Did you know?
The Egyptians invented the art of covering pottery with enamel. Many rare colors of ancient Chinese porcelain have also been found in ancient Egyptian enamel. The same enamel was sometimes applied to fine pottery. The Egyptians were the first to use the potter’s wheel (hand-turned).
The Egyptians painted clay objects with a bluish-green substance to make them non-porous. This substance was a coating composed of quartz, soda, and copper-containing minerals, and when fired, it covered clay vessels and vases with a glass-like surface.
3. Greek
Ancient Greek vases were highly prized for their aesthetic value and finesse. The pottery was decorated with images of their lives and stories. Between the beginning of the sixth century and the end of the fourth century BC, black and red colors were used in Greek art to decorate fine pottery, and simple, undecorated pottery was used for everyday household purposes.
The firing process of red and black figure vessels consisted of three stages. During the first oxidation stage, air was allowed into the kiln, causing the entire vase to turn the color of clay. In the following step, the oxygen supply was cut off, and green wood was added to the chamber, turning the body black in the smoky atmosphere. In the third stage, the air was reintroduced into the kiln; the retained parts turned orange while the shiny areas remained black.
4. Medieval
In the Middle Ages, sand was mixed with clay to make sturdy cooking pots over an open fire. In early medieval England, crafts such as pottery were not highly regarded. By the 13th and 14th centuries, techniques and technology had advanced, and urban potters were able to mass-produce a variety of pottery shapes.
Around the middle of the 13th century, German potters began producing stoneware. It was made from softer clay and fired at a higher temperature than earthenware. Stoneware was brown or grey in color, strong, and non-porous. In the 15th and 16th centuries, “Tudor greenware” developed and became very popular. Green colors were made using powdered copper in the paint mix, and sometimes lead was added to make the green more vivid.
5. China
The history of Chinese porcelain dates back more than ten thousand years. In the Neolithic, colored pottery was made as well as red or white pottery, and later, black pottery flourished. Then, ash-glazed pottery, also known as proto-celadon, began to appear. Later, hard ash-glazed pottery and ash-decorated pottery were fired. Then, gray pottery with or without colored decoration was produced in large quantities. Then, low-fire lead-glazed pottery was developed. Then, lead-glazed pottery and celadon were produced. Later, the blue and white pottery technique was developed, and copper-red pottery painted under the glaze using copper oxide pigment began to appear. Blue and white pottery and Longquan pottery were exported to the Middle and Near East and other countries.
Did you know?
China was the first to produce light, translucent porcelain. Kaolin, a pure clay, was combined with feldspar to make porcelain. After each color was applied, it was fired separately to achieve the vibrant decorations of porcelain.
6. Japan
In the 17th century, porcelain was first made in Japan in the Arita region, using the techniques of potters from the Korean peninsula. It was known as Imari ware, after the port from which it was shipped to other regions. The Dutch East India Company exported Imari ware to European nations between the second half of the 17th century and the first half of the 18th.
One particularly significant kind of pottery is rakuware. In Japan, where tea bowls were handmade from extremely coarse clay, the crackled glaze known as Raku first appeared.
7. Iran
The earliest evidence of Iranian pottery dates to the Neolithic period around the 7th millennium BC. Early pottery forms lacked decoration but were functionally efficient, indicating an early awareness of manipulating clay into functional containers. Painted exterior decorations based on organic oxide in geometric patterns, animal representations, and symbols appeared around the 5th century BC. Pottery was hand-made using the coiling or casting method. The style and form of early pottery on the Iranian plateau resembled those of its neighbors in Mesopotamia and Anatolia, indicating cultural affinity across the region.
Clay Deposits
There are two types of clay deposits: primary or residual deposits, which are found in the same location as the deposit, and secondary or sedimentary deposits, which are found where the clay was transported after formation.
Types of Pottery Clay
Many types of clay are used in making pottery.
- China clay,
- Ball clay,
- Fire clay,
- Talc,
- Shales,
- Stoneware clay.
1. China Clay
China clay is used to make traditional ceramics when the finished product’s color and resistance to high temperatures are important considerations. China clays usually contain large amounts of kaolinite but can contain considerable amounts of other clay minerals. China clay is mostly found in secondary and residual deposits.
China clays have a medium particle size (1-2 micrometers) and produce a moderate amount of flexibility during shaping due to the particle size. Shrinkage during drying and firing is usually moderate. China clays are used in making pottery, stoneware, and porcelain.
What is Kaolinite?
Kaolinite (Al2O3⋅2SiO2⋅2H2O) is the most common clay mineral in the ceramic industry. The degree of crystallinity of kaolinite present in clay is highly variable. It depends on the conditions of origin and the content of impurities that enter the crystal lattice.
Kaolinite group clays undergo many phase changes when heated in air under atmospheric pressure, including grinding, drying, metakaolin, spinel, platelet mullite, and needle mullite.
2. Ball Clay
Clay balls are highly elastic when mixed with water. This elasticity is due to the fine particle size of this clay (0.1–1 micrometer). Due to the fine particle size, the water demand for ball clay is high. The fine particle size also gives ball clay bodies a higher firing strength than other clays. The color of ball clay ranges from light brown to almost black, depending on the organic content. Because ball clays are strong and workable, they are used in traditional ceramics.
3. Fire Clay
Fire clay refers to minor types of clay that can produce refractory bodies. Fire clays are often found in coal deposits. Plastic fireclay, flint fireclay, and high-alumina fireclay are the three basic subtypes of fireclay, ranked according to increasing alumina content. Due to their low cost, high corrosion resistance, and exceptional thermal stability, fireclay refractory materials set the performance standard in metal processing applications.
Did You Know?
Volcanic ash is a mixture of minerals, rocks, and broken glass released during a volcano eruption. The diameter of these particles is less than two millimeters.
4. Talc Clay
Talc is a magnesium silicate found in secondary deposits. It is formed when magnesium silicate minerals, like pyroxene and peridot, erode. Talc has long been used in the ceramics industry, where it is incorporated into glazes. Commercially, talc is used in tableware, paper, rubber, pesticides, and paints.
5. Shale
Shale is a term that describes sedimentary deposits that have undergone compaction. One way that clays are formed is through the weathering of shales. Shales frequently have high iron content, which gives them a red color when fired.
6. Stoneware Clay
Stoneware is a vitreous or semi-vitreous ceramic made from stoneware or infusible fire clay. Applications include tableware and decorative vessels. Stoneware is fired at temperatures ranging from about 1,100 °C to 1,300 °C.
Traditional Clay Pottery Processes
There are two main methods for traditional clay pottery.
- Hand-Building
- Wheelwork
The steps of the traditional clay pottery process are:
- Hand-Building and wheelwork methods,
- Trimming,
- Bisque Firing,
- Glazing,
- Firing of Glaze.
1. Hand-building and Wheelwork
Hand-building is the oldest individual method of clay forming. It is slower than wheel-throwing but gives the potter a high degree of control over the shape of the pots. Hand-building techniques are coil, slab, and hollow-ball.
The coiling is done by rolling on a flat surface. You can make pots by forming coils, placing them on top of each other, and binding them with a liquid mixture of clay and water. The pot’s base can be prepared by making a circular plate, and the gaps between the coils should be filled with wet clay. Two adjacent rings can be better connected by placing vertical lines between them.
Clay can be rolled into thin slabs using a rolling pin on a flat surface. Slabs are usually made on a piece of plastic or cloth so that they do not stick to the table surface. Pottery can be made by combining these slabs in different ways.
The hollow ball technique involves cutting a clay ball in half, extracting the clay from the two halves, and joining them together to form a whole. The walls of the hollow halves are perforated, and a lubricant is applied to them to seal them together. Afterward, they are cleaned with sponges and smoothed to make them look good.
Wheelwork can form items with radial symmetry on the vertical axis. The process can be summarized as follows:
- Place the clay ball in the center of the wheel head.
- Press the clay ball down and inward to achieve perfect rolling consistency.
- Make a cavity in the center of the solid ball of clay.
- Inside the pot, make a flat bottom.
- Shape the walls to obtain an even thickness.
- Give the pot the desired shape by applying different pressure on the inside and outside.
What is hump throwing?
The hump is thrown to make many small pots from a large lump of clay. The steps involved are the same as for throwing a larger pot. They are pulled out one by one from the lump using a string. This process requires a lot of practice but is a faster and time-saving method.
2. Trimming
The goal of this procedure is to get the desired shape by removing extra clay. This is done by holding the pot in the middle of the wheel or by holding it by hand. Because the rim is typically well-finished during throwing, the pot’s base is worked on, and its foot ring is marked to obtain a clear contour.
Fluting is engraving, embossing, and creating textures on the pot. This is done after trimming, while the pots are hard as leather, neither too wet nor too dry, and after that, not much can be done because they are too dry and prone to breakage.
3. Firing
The pots are then left to dry naturally. Typically, there is a shrinkage of 4% to 10%. They are then fired in kilns heated with wood, coal, gas, or electricity. Modern gas or electric kilns are cleaner and easier to control. Firing involves heating to a high temperature. It is important to heat them very slowly.
Remember
If pots are heated too quickly, the water will turn to steam inside the clay body, causing the clay to explode.
When the kiln temperature reaches about 350°C, the chemically bound water begins to come out. At 500°C, the clay is completely dry. At this point, the clay changes and becomes ceramic. Firing continues until the kiln reaches about 850-900°C. At this temperature, the pots become less brittle while still being sufficiently porous to accept paint. The kiln is shut off when the temperature reaches the desired level.
4. Glazes
Before glazing, a layer of wax is applied to the base so that the glaze does not leak and stick to the pot on the oven rack. The glaze is applied to the pots by spraying, dipping, or brushing.
What are the components of glaze?
The glaze contains five main components: silica, additives, alumina, coloring materials, and modifiers.
- Silica is the fundamental component of glass and ceramic glazes and melts at about 1710 °C. It can be introduced into glazing in the form of silica oxide, silica sand, and flint.
- Additives lower the melting point of silica, making it usable in ceramic glazes such as feldspar.
- Aluminum oxide is used as a hardening agent, allowing the glaze to stick to the vertical surface of the pot without flaking off when melted, such as china clay.
- Coloring materials such as metal oxides are added to the glaze to produce a variety of colors, such as manganese dioxide, cobalt oxide, and copper oxide.
- Modifiers can alter the glaze’s unfired characteristics, such as its gloss or opacity. Titanium dioxide and barium carbonate are examples of polishing agents.
5. Firing of Glaze
The clay is transformed from a soft, brittle material into a rock-hard material impervious to water, wind, and time. Modern potters most frequently use electric and natural kiln designs. A heat sensor is fitted through the kiln’s wall to read the varying temperature on a pyrometer.
Sources
- Clays… Trade Data.
- Kilns and firing structures
- Athenian Vase Painting: Black- and Red-Figure Techniques
- Clay and Pottery – Brief History
- A Brief History of Chinese Ceramics
- The Early Beginnings of Iranian Pottery
- uomustansiriyah.edu.iq/media/lectures/pdf
- Craft of Pot-Making
©Eman Abdallah Kamel, 2024
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