Clay or ceramic?
‘A vase of unbaked clay, when broken, may be remoulded, but not a baked one.’
Clay transformed
Clay is a naturally occurring material. It is stiff and sticky and comprised primarily of fine-grained minerals found in the earth. Clay has a high water content, so is impermeable and can be easily moulded when wet.
Ceramic is the name for non-metallic materials that are formed by the use of heat and that remain solid after being heated. When clay is heated to a high temperature in a kiln, it becomes ceramic.
Different clays can be heated, or fired, to differing temperatures, depending on their density and water content. An average firing temperature is approximately 1100ºC, it takes several hours for the kiln to reach this temperature. The water evaporates in the firing causing the clay body to shrink.
Clay variety
Clay can be found in a range of colours, from white to grey, from brown to deep orange. This variance is dependent on the mineral content of the earth. Terracotta clay, for example, contains a higher proportion of iron oxide, so giving it its rich red hues. Porcelain’s bright white appearance is due to a high content of the mineral kaolinite.
Artists working in clay often use blends of different types of clays according to the colour, texture and porosity they require. These mixes are called clay bodies. There are combinations of clay bodies that are generally known as earthenware clays, stoneware and porcelain clays. Earthenware clays are usually fired at a lower temperature than stoneware and porcelain.
Ancient craft
Ceramics is one of the earliest crafts in human history. As early as 24,000 BC animal and human figurines are known to have been made using clay and other materials, and fired in partially buried ovens. It was approximately 10,000 BC that functional pottery was first used, as storage vessels and as tiles. Babylonians produced bricks, initially using them in their unfired form, and then using fired bricks in construction from around 2,500BC.
Pottery arrived in Britain with the first farmers, between five and six thousand years ago. Some of these were the first pottery vessels to be decorated with patterns made in the soft clay, before firing.
Clay tablets were the earliest known writing blocks. Clay tablets were the earliest known writing blocks. Clay utilised medicinally goes back even further, to prehistoric times. It was recognised even then as having properties that aid digestive distress.
Diverse medium
Ceramics, in varying forms, are fundamental to our daily lives. Ceramic tiles, bricks, crockery, sanitary ware are evident examples. Less so apparent is the use of ceramic components in watches, cars or telephone lines. Ceramics can be formed to serve either as electrically conductive materials or as electric insulators.
Clay is also used in many industrial processes, often acting as a filter. Many clays, being relatively impermeable, are used as natural seals in the cores of dams, on in landfill management.
Clay is inexhaustible. It is a cheap and natural material that is, at once, both expendable and precious in its different ceramic forms.
Techniques
Just as clay can be found in great variety in its raw state, in its blends and refinements, so too can a myriad of treatments be applied to it. Ceramic artists may explore a breadth of techniques and combinations of processes, and will hone skills very specific to their creative endeavours. Thus, the diversity of techniques and how they’re applied, adapted and specialised mean that the forms that can be realised in clay are extensive. A few of the more widely used techniques include:
Throwing
Throwing is done on a potter’s wheel. It is the act of shaping a piece of clay as it revolves on a spinning platform. Be it a manual or automated wheel, controlling the clay takes considerable expertise. Horizontal throwing lines in the clay are indicative of the form being shaped by hand. The thickness of the clay may vary throughout the thrown form, as artists vary the pressure they apply. Each thrown piece is intrinsically unique, though an expert thrower may repeatedly be able to create seemingly identical forms.
Moulding
This enables the same form to be made repeatedly. Model making is integral to moulding. A model of the desired form can be made using clay or plaster, this may be seen as a prototype and the more precise the model is, the more exact the moulded form will be. Press-moulding is when clay is pressed into a rigid mould, hump moulding is when clay is draped over a mould, both shaped whilst the clay is wet. Bowls are an easily identifiable form that can be made with hump moulds. 3D press-moulded tiles are often used architecturally.
Casting
Slip casting is used in both studio and manufacturing environments. It can be used to make batch productions of particular pieces and is used, on a greater and refined scale, in industry. Slip is liquid clay that is well blended and smooth. The slip is poured into porous moulds and left to harden to the consistency of leather before being removed and trimmed. Moulds are porous to allow the water in the clay to evaporate. When clay is cast in this way it bears a fine and often delicate quality.
Decoration
There are numerous techniques for decorating ceramics. The clay can be patterned when it is still soft, either freehand or using tools that impress a specific shape. Sgraffito is produced by applying layers of colour to leather-hard clay and then scratching into the layers. Intaglio is when the pattern is scored into the surface of the clay; a colour may be applied and the excess wiped away leaving just the scored lines to hold the colour. Moulded pieces of clay can be applied to give textured decoration, this is called sprigging. Slip trailing is a versatile means of surface decoration as it’s also possible to use coloured clay slips.
Hand building
There are several different methods for building ceramic forms by hand, often a combination of techniques contributes to making each individual form. These are some of the most common means of hand building:
Slab building
This is making ceramic forms using slabs of clay. Slab building enables the construction of sculptural or functional forms, potentially on a large scale. The forms can be angular or cylindrical. The finish of both the slabs and joins is critical to the overall strength of the final piece.
A cutting wire can be used to slice slabs of clay from a block. Each slab needs to be rolled out to a consistent thickness. Wooden guides and a rolling pin can be useful for this. The slabs may then be cut to a more precise size or shape, sometimes using templates. Clay slabs offer a flat surface that can be decorated prior to the form being constructed; patterns can be impressed into the clay using stamps for example.
The finished slabs are easiest to join when they are leather hard. This is achieved by scoring the edges of each slab and applying slurry (slurry is a mix of clay and water) before pressing and holding the edges together. Pressing a rolled coil of clay into the inside of the join can reinforce it, the coil should be smoothed completely into the adjoining slabs. Use a tool to smooth the outside of the joins together as well, ensuring that there are no cracks or creases. A completely smooth finish contributes to the overall strength and integrity of a fired form.
If the slabs are joined or shaped whilst the clay is still soft then it is important to support the slabs with either pieces of clay, cardboard or rolled paper during the making and drying processes.
Justine Allison uses slab building to make very fine vessels. The slabs of rolled porcelain that Justine uses are only 1-2mm thick and so are extremely delicate to handle.
Coiling
This is the use of long ropes, or coils, of clay to make pots. A key advantage of coiling is the malleability of the clay. The coils are often rolled out by hand but they can also be mechanically extruded. They should be rolled to a uniform thickness in order to ensure consistency in the finished piece. If coils vary too much in thickness then they will be more difficult to join effectively onto adjoining coils. Also, there is an increased risk of cracking during firing as the clay will be different thicknesses throughout the whole form and so will dry at different speeds. The coils can be rolled as the form is being built, the coils are not usually all made in advance, otherwise they would start to dry out and not be malleable.
A solid base of clay is often rolled out, on which to start coiling. Each coil is joined by pressing the inside edge firmly into the clay base beneath it, the joined surface is then smoothed using fingers or tools. Should you want the finished form to be smooth on the outside then the coils need to be pressed together and completely smoothed on the outside as you build each layer, ensuring that a consistency of pressure is applied when pressing and smoothing the clay. Often the piece being worked on will need to be wrapped up in plastic and allowed to dry a little before the next coil is added. This allows the piece to build up an inherent strength, so that it is able to support the next coil of clay. Coiled forms can be quite heavy owing to the weight of clay used
Coiled forms are often organic in shape as, unless using a mold, it is difficult to shape the coils symmetrically. The overall shape of the form can be affected by placing adjoining coils off-centre from one another. For example, if the form is to curve outwards then each adjoining coil is placed slightly to the outer edge of the previous one, if the form is to curve inward then adjoining coils are placed slightly to the inside edge of the previous coil, and so on.
Forms can be made from coiling and can then be modelled to further shape the final piece.
Modelling
Modelling is used to make sculptural or functional forms. There is a wide variety of modelling tools available, though often artists will fashion tools from everyday or found items. Hands are effective modelling tools as well of course.
Anna Noël models her figurative ceramics using a range of techniques, tried and tested over many years in the studio. Modelling weight-bearing parts of a piece, such as a horse’s legs, requires the clay form to be supported whilst it is being shaped and whilst it is drying.
Details of vessels are often added by modelling. A lip of a jug can be modelled into the rim of a thrown vessel, for example. Or a handle will be modelled and shaped before being attached onto a cup. Modelling is used both decoratively and to add function to ceramic pieces.
Processes
Clay is processed in both its raw, malleable state and as a heat-hardened piece of ceramic. Depending on which clay body is being used, different processes can be undertaken to vary aesthetic effects and practical qualities.
Firing
The first time that a piece of clay is heated up in a kiln is known as the bisque or biscuit firing. This hardens the clay and renders it permanent as a ceramic form. This first firing is often around 1000ºC. Following this, subsequent firings, to lesser or higher temperatures and with varying mediums applied, will further develop the ceramic surface.
Different kilns can render variances of ceramic form. Electric and gas kilns are relatively controllable, in temperature settings and duration of firings. Wood fired kilns afford tremendous variation, in temperature within different places inside the kiln, in effect on the surface of the clay from flames, smoke and ash, and in response to other elements that may be introduced during the firing, such as salt. The often unpredictable conditions of firing, particularly wood firing, result in the most expressive of qualities. Effects on the clay can be accidental and wonderful, or they can be orchestrated and disastrous.
One of the most exciting methods of firing is raku. Raku is traditionally a type of Japanese pottery originally used in tea ceremonies. Raku translates as enjoyment or comfort. The ceramic forms are removed from the kiln whilst they are still glowing hot and allowed to cool in the open air. Western interpretation of raku sees the hot ceramics placed in a container of combustible materials. The ceramic surface can become unpredictably coloured and crazed.
Vitrification
Vitrification comes form the Latin word for glass, vitreum. Vitrification is the process of melting that clay goes through when it is fired to high temperatures. It is the point at which clay is no longer porous after firing. The temperature at which this is reached varies, depending on the clay body being used.
Vitrification can be understood as the transition of water from a liquid state to a solid state, glass. As clay is fired hotter and hotter, it reaches a point at which, when cooled, it is sufficiently dense and strong to be able to hold liquids. Clay that has been fired sufficiently for its intended purpose is said to be mature. In a fully matured clay body, the spaces between particles are completely filled up with glass, fusing the particles together and making the clay body waterproof. This process also makes the clay body more brittle, as there are less or no spaces between the clay particles to absorb any movement.
Justine Allison works with porcelain clay to make vessels that are waterproof. Justine fires her work to 1400°C. When porcelain is fired to this high temperature it shrinks because all the clay particles fuse together and there are no longer spaces between them. Firing porcelain to this temperature means that the clay actually melts. This is why you can see waves and bumps occurring in the sides of Justine’s porcelain vessels. The clay has melted but has been held in place because it is joined to other slabs of clay; several slabs make up each vessel. Once clay melts it can warp and form blisters or bubbles, particularly if any air is trapped within the clay or if the joins between each slab aren’t strongly formed.
Glazing
Many ceramics are functional. To be durable, watertight, resistant to mold and disintegration, ceramics are often glazed. Very simply, glaze is a glass coating that is fused to the ceramic form. Glaze can waterproof, colour and decorate surfaces. Essentially it comprises three key components; silica, aluminium oxide and flux. Silica could transform into glass itself if fired at about 1700ºC, this is too high for ceramic kilns so flux is added to facilitate the transformation at lower temperatures. Aluminium oxide makes the glaze more viscous, preventing it from slipping off during the firing.
Glazes can be coloured, often with oxides, and textured. They can also be applied in numerous ways, including pouring, brushing, spraying, each gives a different surface finish. There is a complex chemistry to glazing and glaze making, it is the source of much experimentation.
Salt glazing is one process that has remained similar over hundreds of years. Salt is introduced into the kiln during the highest temperature of firing. The sodium and chlorine separate. The chlorine disappears. The sodium sticks to whatever is inside the kiln, forming a glossy, translucent texture. Salt glazing results in diverse and surprising effects, it is an exhilarating ceramic practice.
Diversity
Ceramic forms are multifarious, multi-functional, highly valued, or, can go unnoticed. Its longevity as a useful and creative medium is testament to how reliant we are on clay and ceramics. There are potters, ceramic artists and ceramic industrialists the world over that research and specialise in innumerable processes and outcomes for clay. We use, admire and express ourselves through ceramics. There are many descriptors of different types of ceramic forms therefore, these are a few common ones.
Functional
Functional ceramics are designed to be used. Be this in domestic, public or industrial contexts. From washbasins to teacups, tiles to insulators, the ceramics are designed to be utilitarian, to perform specific tasks. Functional pottery is familiar to us in its domestic scale and the majority of potters creating functional work achieve considerably more than serviceable ware. Considering the ergonomics of use, the process of interaction between a jug, for example, and the person using it, is key in designing a piece of work that not only functions effectively but will also be a pleasure to use. Aesthetics of form and finish not only contribute to the character and individuality of a piece of functional ware, but are also a means of creative conversation between the maker and the user. Holding a vessel, feeling its weight, sensing its shape, appreciating its texture, colour, or decoration are all elements that will contribute to the experience of using that piece of work.
Sculptural
Sculpture is three-dimensional, it is usually designed by the artist to be viewed from different angles. It is often highly textural and evocative. Sculptural ceramics is an ancient and expressionist artform. It permeates any perceived division between fine art and craft. Sculptural ceramics encompasses a vast diversity of work. Scale, style, technique, materials are all explored to myriad effect. There are ranges of technical approaches in using clay to make sculptural forms too. Carving, coiling, slab-building, modelling, throwing are all used to differing effects in sculpture. Work might be representative, of specific shapes or ideas or narratives; or it may be entirely abstract. Sculpture may be seen as a form of three-dimensional sketching. Artists may make one continuous ceramic object. Alternatively they may combine a series of ceramic objects or pieces made from a combination of mediums and present them together as an installation or an assemblage.
Figurative
Figurative work falls under the broader umbrella of sculptural work. Figurative ceramics are usually representative of forms that are recognisably derived from life. Although figurative work can often be representational, they do take real life as their source. Finished works can also include abstract elements. Indeed, making styles are so varied that figurative work ranges from faithfully realistic renderings to quite indeterminate forms. Figurative ceramics have, over the millennia, been used to transform the human form and to explore an indeterminate range of ideas and issues pertinent to humanity.