Ceramic wares are made by forming a clay body (a mixture of clays or clays and other earthy minerals) into objects of a desired shape. They are fired to high temperatures in a kiln to create reactions that would permanently change their nature. Firing increases their strength and hardens and sets their shape.
Different methods are used for shaping clay into objects. These are: Hand building, wheel throwing, casting and jigger jolleying.
3.4.1 Hand building
This is the earliest forming method adopted by potters. Wares can be constructed by hand from clay coils, flat slabs or solid clay balls. A combination of these can also be used. Potters use clay slip (watery clay) for sticking together joints in a clay objects. Hand building is slower than wheel-throwing, but it offers the potter a high degree of control over the size and shape of wares.
3.4.2 Wheel throwing
Another method commonly used by potters is "throwing" (derived from the word thrawan, which means to twist or turn). The method requires a machine called a potter's wheel. To throw on the wheel, a ball of clay is placed in the center of the wheel-head and it is rotated. As the wheel head turns at controlled speed, the ball
of clay is manipulated to give a hollowed form (Fig. 34). Some wheels use foot power and they are called kick wheels, while others are powered by electric motor and they are called electric wheels. Wheel-throwing usually produces spherical forms and is ideal for mass production because it facilitates faster production of wares.
Fig. 34: A Potter using the Wheel
Source: Wikipedia, the Free Encyclopedia
Other methods of building pottery include casting, jigger jolleying and extrusion.
Jiggering is the operation of bringing a shaped tool into contact with the plastic clay of a piece under construction with the piece itself set on a rotating plaster mould on the wheel. The jigger tool shapes one face and the mould shapes the other. Jiggering is used only in the production of flat wares such as plates, but a similar operation jolleying, is used in the production of hollow wares, such as cups.
Slip casting is often used in the mass production of ceramics and is ideally suited to the making of wares that cannot be shaped by other methods. A slip, which is made by mixing clay body with water, is poured into a highly absorbent plaster mould. The water from the slip is absorbed into the mould leaving a layer of clay body covering its internal surface and taking its inner shape. Excess slip is poured out of the mould, which is then separated and the moulded object removed. Slip casting is widely used in the production of sanitary wares and is also used for making smaller articles such as intricately detailed figurines (Wikipedia, the Free Encyclopedia).
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Self Assessment Exercise 3
Briefly discuss the different methods used in the production of ceramic wares.
3.4.3 Glazing
Glazing, which involves the melting of a thin glass coating over the surface of clay wares is a potter’s means of enhancing the appearance of pottery wares and rendering them impervious to liquid and gases (Fig. 35). Glazing is a technical attribute of contemporary ceramics world-wide, as Okunna (2008:67) rightly notes.
Three principal components are required in any good glaze. These are silica (the glass former), alumina for viscosity, toughness and hardness in the fired glaze and fluxes to enable the silica to fuse and to lower the melting point of the mixture.
The solution to obtaining flawless glazing is to ensure that there is an adequate interaction between the glaze and surface of the body on which it is applied. This is only possible if the glaze is properly compounded with the appropriate materials in the right proportions, suitable clay body compositions and the right firing.
Improperly composed glazes and clay bodies or a disparity in temperature between ware and glaze presents flaws such as crawling, crazing, blistering, peeling and so on, in the fired glaze.
Fig. 35: A Glazed Mug.
Source: Wikipedia the Free Encyclopedia
Glaze may be applied by dusting the unfired composition over the ware, or by spraying, dipping, trailing or brushing on thin slurry composed of the unfired glaze and water. The fired colour of a glaze is usually different from its unfired colour.
During firing, a glaze turns into molten glass and congeals into solid glass after firing, therefore, to prevent glazed wares from sticking to kiln furniture during firing, a small part of the object being fired (for example, the bottom of the ware) is left unglazed. Another way to prevent the problem is to give the kiln shelves a bath wash (a mixture of kaolin and water) or the wares can be placed on special refractory spurs to elevate them from the shelves. There are different techniques of glazing ceramic wares such as: Salt-glazing, where common salt is used, slip glazing using fusible clays and ash glazing using ash from the combustion of plant matter.
3.4.4 Firing
Firing is very important in ceramics. It is a process whereby clay wares are subjected to intense heat in an enclosed device a kiln (see below). It produces irreversible changes in the clay body. Firing is what turns the ware or material into pottery. Ceramic wares are fired to different temperature depending on the function the ware would serve.
Some wares are fired at low temperatures (960oC) and the process is known as bisque or biscuit firing. In lower-fired pottery the changes that occur include sintering, during which the coarser particles in the body at their points of contact with each other fuse together. At higher firing-temperatures, the physical, chemical and mineralogical properties of the constituents in the clay body are greatly modified. Regardless of the temperature to which ceramic wares are fired, the aim of firing is to harden the wares permanently. One important point to note is that the firing temperature chosen for a clay body must be appropriate for the materials used to compound that particular body. For instance, earthenwares are usually fired at temperatures in the range of about 1000 to 1200 °C; stonewares at between 1100 to 1300 °C, and porcelains at between 1200 to 1400 °C. Another important point you must note is that the way that ceramic wares mature (or vitrify) in the kiln is influenced by the peak temperature achieved and by the length of the period of firing. Therefore, it is important that when firing, you hold
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the maximum temperature within a kiln constant for a period of time to produce the maturity required in the body of the wares. This is referred to as “soaking” the wares.
Fired wares have different colours which are determined by the nature of the clay and the atmosphere within a kiln during firing. The potter has the option of creating two types of conditions (atmosphere) in the kiln during firing. They are:
oxidation atmosphere and reduction atmosphere. An oxidizing atmosphere is produced by allowing air (oxygen) to enter the kiln. This creates the oxidation of clays and glazes. A reducing atmosphere is produced by limiting the flow of air into the kiln to remove oxygen from the surface of clays and glazes. Oxidation and reduction can change the appearance of the wares being fired. For example, some glazes containing iron turn brown when fired in an oxidizing atmosphere while they change to green in a reducing atmosphere. You can adjust the atmosphere within a kiln to produce multifaceted effects in glaze for decorative purposes.
Self Assessment Exercise 4
Identify the physical and chemical changes which take place in clay during firing.
3.3.4 Ceramic Kilns
There are different kinds and sizes of kilns used for firing ceramic wares. Kilns may be heated by burning wood, coal, gas or by electricity. Each type of fuel has its advantages and disadvantages. For instance, coal and wood can introduce smoke, soot and ash into the kiln when used as fuels. This can affect the appearance of exposed wares. To counter this problem, wares fired in wood- or coal-fired kilns should be placed in saggars (lidded ceramic boxes) in the kiln to protect them. Modern kilns powered by gas or electricity are cleaner and more easily controlled and fire wares faster (Fig. 36).
Fig. 36: Electric Kilns
There are dangers involved in firing kiln; therefore, the potter must learn how to use the kiln safely. For instance, kilns generate very high temperatures and discharge gases during the firing process, thus, serious burns can occur from the heat and flames of the kiln. It is advisable to use personal protective gear (kiln mitts or gloves) when dealing with hot kilns and pottery wares in order to avoid burns. The power or fuel must be turned off after firing the kiln and the kiln allowed to cool down completely before it is opened.
Kiln firing also releases volatiles into the air, many of which are toxic. It is advisable to stay away from them during firing as much as possible. They also generate a great deal of
glowing heat that can damage the eyes severely. Therefore, it is not advisable to peep with the naked eyes into kiln spy holes during firing. The eyes should be shielded with dark protective glasses specially manufactured for the potter’s use.
Kilns must be properly installed or built and maintained. All kiln components (elements, burners, and fireboxes ,interior walls, exterior walls, bag walls, lids,
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doors, chimneys and venting systems need to be inspected regularly for damages and to ensure that they can function efficiently and safely. Kilns should be elevated at least one foot from the floor. Do not put anything on top of a kiln.
4.0 CONCLUSION
In this unit I have looked at what applied art means and its features. Using the example of ceramics and textile design, which are two major areas in the applied art, I have clearly shown you what this branch of the visual art entails.
5.0 SUMMARY
In this unit, our emphasis has been on the applied art and two of its sub-divisions (textile design and ceramics). The materials, equipment and techniques used in the production of ceramics and textile were examined. In the next unit we shall be looking at another area of the applied art in the basic principles of design.
4.2 TUTOR MARKED ASSIGNMENT