Guide to Ceramics - part two

This is the second part of a Guide to Ceramics, written by ceramicist Kevin Andrew Morris. Part One covered the essentials of working in ceramics, whilst this guide will cover ceramic finishing, surface and decoration techniques.

Ceramics have a range of surface, decorative and finishing techniques that can be used at different stage of production. Similarly to the forming process it is important to understand the various changes in material that can occur throughout these stages as well as the variety of processes that can be used. These can be grouped as follows; techniques used in the making process, biscuit stage and a range of firing, multiple firing and post firing techniques.

Techniques during the making process

Techniques used during the making process can be viewed as a marriage between form and decoration; working directly into or onto the raw clay gives a strong chance of a harmonious result and allows for little intervention at later stages, one of the main techniques associated with decorating clay at the making process is making additions to the clay at the raw stage. This can take the form of incombustible and combustible materials, which can be mixed or pressed, into clay before or during the making process. The final effect on the surface will depend on the reaction the material has when it is fired. 

Paper Clay

Paper clay is a good example of this and has become increasing popular as people discover the wonderful versatility of this material. The addition of paper pulp (or flax) changes the working properties of clay dramatically, cellulose fibrous tubes in the paper increase flexibility and help to knit the clay together as it dries. Paper clay can be used in very thin sheets, and offers amazing freedom in the building of sculptural work, its adaptability solves many of the constructional problems presented by regular clay.

Colorants

Colorants are another good example of additions to a clay body. While clay is found in many geographical locations and in a variety of naturally occurring colours (the colours achieved depend on the influence of metallic oxides during its formation and the amount or carbonaceous material that it contains) it is also possible to colour the clays through oxides and stains. One of the main techniques that use different coloured clays to involves mixing and marbling-different coloured clays, this is called agate ware. Agate clays can give strata effects and be used with a variety of making techniques including hand building and throwing, there are many ways of making agate clay including coiling, rolling, stacking, slicing and rearranging to give a whole range of effects, however it is worth being aware that clays have different shrinkage rates and can split apart before they dry or are fired so it is important use clays of similar firing ranges when making and agate clay.

Slip

Slip has also been used as a means of colouring and decorating ceramics for centuries, as soon as people discovered clays of different colours it was found that clay mixed with water made a useful coating for a surface. Slip is a mixture of clay, water and sometimes pigment-such as an oxide or stain-though sometimes other materials are occasional added to help it fit the clay body to which it is applied. 

Terra sigillata is also a type of fine slip refined by the process of levigation. A levigated slip is made by using defoloccuant to keep the finest clay particles in suspension so they can be separated out. The resulting fine-grained slip is applied to clay surface, then usually burnished and fired up to 1000 to give a rich gloss.

When slip is mixed to a thick consistency it is possible to change the surface colour of a piece with one application. In this way it provides a solid coloured background for applying glaze and is an inexpensive way of making common dark clay appear white. When slips are thinner, it gives interesting effects allowing the base clay to show through and different layers of colour and texture can be built up. The possibilities for decoration using slip at this stage are varied and include the following:

Pouring; is a simple and effective way of applying slip (and glaze), which enhances the forms it coats, the object is held firmly over a bucket while a pitcher of slip is poured over it. Dipping like pouring is also a useful way of applying a dense and even coating of slip, and can also be used for decorative effects.

Spraying comes into its own when pouring or dipping would be unsuitable and can provide some delicate effects. Spraying is the method to choose when applying slip to large, fragile or complex pieces, when you have too little slip to pour or dip or when only a thin coating is needed, it is also useful in applying decorative effects and fades.

Resists also allow for the build up of several layers, there are many ways to mask off defined areas so that slip doesn’t contact the clay, these include, hot wax, latex and newspaper.

Slip trailing

This is usually done with a rubber tool in the form of a bulb with a detachable nozzle, where patterns of slip are built up using fluid lines and a sure and steady hand. Feathering (named for its feather like pattern) is a delicate technique that uses slip trailing. With a background of slip, parallel lines are trailed onto the background using a contrasting slip, a feather or tool can then be used to drag the slip in different directions to create a feather like effect, a technique similar to that used for frosting on cakes. A technique very similar to feathering is marbling which has its roots in traditional country pottery. 

Sgraffito

Sgraffito refers to scratching, or drawing into clay-the term derives from the Italian graffiare, to draw, usually sgraffito is done through an application of surface colour to reveal the clay body beneath. The colour coating can be a slip or other pigment and the marks can be made with a variety of instruments. Combing is another direct method of decorating that could also be regarded as a form of sgraffito through wet slip; it enables the creation of free and fluid marks by dragging tools layer of slip. Incising is a similar technique where leather hard clay is incised with a knife or sharp tool to create a groove, which is later, filled with slip and scraped back to reveal crisp line.

It also goes without saying that the range of marks made by drawing, printing and painting on clay is vast can of course be applied to different stages of the making process. The use of a banding wheel or potter wheel for example is also a good way of applying stripes, a flat coat of slip or another colour to a piece, applying slip with a brush is also useful when decorating a single piece with a multitude of colours.

There are also a range of techniques used to apply texture to clay to create a decorative surface, these include impressing, piercing, rouletting , incising, stamping, making marks made on the wheel and burnishing. Burnishing involved the compressing of the clay particles in a polishing action when the surface of the clay is leather hard. While decorative it can also help to make the clay less porous. 

Moulds also offer a large potential for decoration, wildly used in the making process as a means of forming clay they can also be used to create a surface. Sprigging makes use of shallow moulds into which clay is pressed; these moulds can be made from biscuit fired clay or plaster. A classic example of sprigging is the blue ad white Jasperware made by Wedgewood, where the white raised patterns applied to the blue body are sprigs. The method for making sprigs is similar to making bas relief and moulds for relief printing.

Techniques used in the Biscuit Stage

It is commonplace to have two firing cycles: a biscuit firing and a glaze firing. Compared to once firing a piece can be handled more easily during glazing and a biscuit firing reduces the chance of glaze faults occurring. Underglaze colours, as the term suggests are intended to be applied beneath a glaze, usually onto fired biscuit ware. There are many other ways to treating of then, and can be in fact used on top of a unfired glaze or without a glaze at all. They come in variety of forms as a powder to be mixed with water or medium, as ready to use mixtures, as crayons or pencils. Underglaze painting can be done directly onto biscuit ware and glazed with a clear glaze; similarly under glaze crayons can be used with a clear glaze or in combination with other techniques. Underglaze colour can also be printed onto biscuit.

Glaze

Glaze is a form of glass, which becomes fused to the ceramic surface during firing, glaze can be shinny, matte, opaque, transparent, smooth or textured and is stained with a whole range of colours. Glaze can be used for several reasons, functionality,strengthening the clay body as it becomes fused to it, making it non porous smooth and hygienic to use, it can also be decorative, admired for its rich depth of colour gloss, or sometimes so textured or dry as to render a piece completely non functional. Glazes can be bought ready made or mixed from a recipe which is often more economical.

Pouring and dipping glaze are often used when glazing and very similar to the methods for pouring and dipping slip, but with some additional points to bear in mind most importantly that glaze should be wiped back a little from the base to allow for any runs, and to avoid fusing to the kiln shelf.

Techniques used in the Firing Stage

Firing is the essence of the ceramic process, during firing an irreversible transformation takes place; the transformation from clay to ceramic. Clay once fired, loses plasticity and with an increase in temperature, the clay particles form a strong bond creating a new and durable material.

The modern electric kiln is thermally efficient and convenient; often it is the he only kind of kiln available, and with proper care is safe and highly controllable, used in conjunction with a sophisticated controller. The controller regulates the rate of temperature rise and fall (the heating and cooling curve, often known as the ‘ramp’) or soaks the kiln at a required temperature for as long as it is programmed to do so, such devices make electric kiln firing reasonably accurate. A gas kiln is usually more expensive and less convenient than an electric kiln but can achieve results and electric can’t such as reduction firing.

During stoneware firings the atmosphere inside a kiln is either oxidising or reducing. An electric kiln firing is always mildly oxidising, and although certain agents can be introduced during the firing to induce a reduction atmosphere, this is likely to shorten the life of the elements. Kilns that burn fuel (usually wood oil or gas) are therefore normally used for reduction firing. In oxidation firing there is a unrestricted flow of air into the combustion chamber, so the fuel is fully burned. In reduction firing the air intake is restricted and so some fuel remains unburned. The result of this inefficient combustion is carbon monoxide. This unstable gas takes oxygen from metals in clay and glaze in order to achieve a stable form, carbon dioxide. The oxides most affected by this are iron and copper, the low oxygen (reduced) forms of iron and copper oxides display different colours from those produced by an oxidised firing.

Salt and Soda Firing

Both salt and soda firings can bring a distinctive ‘orange peel’ quality to the surface of a piece. During the firing salt or soda is introduced into the hot kiln, the sodium reacts with silica and alumina in the clay to produce colourless glaze sodium aluminosilicate. The surface texture comes from the relative coarseness of the silica particles in the clay, these effect the formation of the glaze in an uneven way to produce the bead like, mottled, orange peel effect sometimes referred to as a tiger skin surface. Any colour comes from the oxides in the clay and from application of colours in the making and decoration stages. Salt and soda firings usually require a kiln specifically for this purpose.

Raku Firing

With its roots in 16th century Japan, the word raku freely translates to enjoyment. The technique of raku has been adapted and developed in the west to cover a variety of firing methods. As a general rule it is a low temperature earthenware firing, with rapid firing cycle. Pieces are drawn with tongs from the red-hot kiln and while they are still hot they can be treated in a number of ways. Often they are plunged into sawdust or other combustible material or they can be quenched directly in water. Raku firing is appealing for its rapid results and appears to break all the rules because it is such a direct process it also brings the maker very closely in touch with fire and its effects on clay.

Most ceramic process require one or two firings, but sometimes it is necessary to fire a piece several times. On glaze enamels and metal lustres need at least a third firing. Glazed and fired ceramics is usually regarded as finished, but this is not the case.

Enamels and Decals

Additional pigments can be applied, demanding a third firing. Enamels (on glaze or on glaze enamels) are low-fired glazes that are usually applied to a glazed surface and then fired to 700-850 C, a glazed surface is not however essential as a base for enamels. They can be painted and printed or used in decals and transfers. These can be made using enamels in conjunction with silkscreen printing process. Making decals is a time consuming process and demands certain facilities but there are also companies that will make up decals for you or decals can be bought ready made sheets.

Commercially prepared lustres can be applied to the surface of a fired finish glaze and then fired again to a low temperature in a oxidising kiln. Lustres are a means of developing a wide range of decorative colours and metallic effects on matured glazed surfaces; they are reliable and easy to apply. Lustres can also be achieved by using a combination of key materials and tight control of a kilns atmosphere, which must at some pint be reducing. Like on glaze enamels lustre fires just bellow the softening point of the glaze to which it is applied thus ensuring adhesion to the glaze.

Written by Ceramicist Kevin Andrew Morris 

If you've enjoyed this Guide to Ceramics finishing, here's another production guide for Ceramics explaining the essentials of working in ceramics. 

If this has inspired you to start a Ceramics project take a look at our Ceramics listings to find out more about local ceramicists and ceramic studios.