Decorating with slip (I)
SLIP, a sieved mixture of clay and water, can be use in many ways to make attractive decorative patterns. This article explains the principles and techniques involved.
Using slip
Slip can be specially mixed or blended, but start by exploring the potential of the natural red and white clays for slips. These clays can be stained with metal oxides to produce a wide range of colours for decoration. Black slip for example, is made using red clay with the addition of 8-10 per cent marganese oxide. There is endless potential in the basic earth colours and these can be mixed to produce contrasting tones and colours. It is only after much practice and experience that the majority of potters achieve attractive results with other than red, white or black slip. The painting of slips onto ware can be very beautiful and subtle giving an effect like oil or water colour painting, as well as making bold, incisive patterns.
Slip is made by mixing clay and water to a given consistency and then sieving it through an 80 mesh sieve. It may be poured, bushed or sprayed over the surface of the pot, or the pot can be dipped into the slip. According to the desired effect, the thickness of application is relevant. If the slip is too thin, the clay body will tend to show through, especially when it is glazed. Even this can become a feature.
Spotting slip
Spotting slips is a traditional technique, used in parts of Portugal. A piece of wood or metal is dipped into a variety of colored slips and used to ‘spot’ the slip onto the soft clay. The articles can then be biscuit fired and glazed or raw glazed before firing.
Slip trailing
Slip trailing is a fluid medium for drawing on ceramic ware. The slip is trailed from a rubber or clay container with nozzle attachments of varying sizes to limit the flow of slip. The technique is very similar to icing a cake. The article is given a coat of slip and, while it is still wet, a trail of contrasting coloured slip is applied to the surface.
Burnishing and scratching slip
This technique is used for decorative, rather than functional ware. The slip is applied when the clay is soft or leather hard and burnished when the surface loses its shine. Slips can be coloured with basic oxides or stains and according to the design or effect required, more than one colour can be brushed or applied at a time. Painting the slip onto the article is done before biscuit firing or sometimes on the biscuited ware, and it is usually more satisfactory to apply two coats. When the slipped pot is almost dry (or is bone dry) the design can be lightly sketched on the surface with a pencil and then scratched through the slip to the body of the ware with a suitable tool. All surplus clay should be dusted gently from the surface of the pot. For the best results with this method, use a fine terracotta. When this body is fired it gives a good colour and tone contrast with the applied coloured slips. Red slips generally contain more iron than the red clay body.
Sgraffito
This is the process of scratching through a pigment slip or glaze applied to a fired or unfired ceramic surface, to refeal the clay body underneath. The clay body is coated with a contrasting slip while is damp and when it becomes leather hard, the coating is selectively scratched or scraped away with ascribing tool or scraper to produce a decorative designs. A different quality of line can be achieved if the coating of slip is left until bone dry. It is also traditional to add washes or areas of basic oxides to the raw clay with a brush when using this technique, and sometimes splashes of coloured glaze can be added at the biscuit firing stage, especially when a white slip is used.
Resist
Areas of slip can be prevented from adhering to the clay body using resist techniques of decoration. Pieces of shaped are dampened slightly and placed on the surface of the moist, firm clay. A coating of slip is then poured over the whole surface and when it has dried to a matt state the paper can be peeled off. If required, more paper and slip can be used to build layers. Wax resist can also be painted on to the leather-hard clay and a slip coating poured over it. For this method the slip must not be too thick or it will not resist. It is quite possible to combine several of these resist techniques to produce a more complex form of decoration.
Painting and casting slip moulds
The painting of coloured slips onto the surface of a plaster mould can be combined with the incising scratching or carving of a design into the plaster. When slip cast this becomes relief pattern on the surface of the article.
It is important that the slips are made from the same clay as the body of the article in order to reduce the chance of variable shrinkage. The surface to be decorated must first be dampened with a sponge so that the slip, when applied, does not dry too quickly or flake off. When the application of one or more coloured slips is complete, the mould is filled with prepared casting slip and left until the necessary thickness is attained. Drain in the usual way and remove the mould when ready. Such articles are glazed with a transparent malt or transparent coloured glaze. The technique is best done in a two-piece skin mould if the articles is a bowl or dish, and cast through the top of the mould (the base of the article) rather than in an open mould. It is impossible to stick thin areas of patterns of coloured clay to the surface of the mould and then fill the mould with casting slip.