Slips, engobes and underglazes are a lot of fun to use for ceramic decoration and there are endless ways to use them.
Today, Robin Hopper explains how you can achieve many different types of surface decorations and patterns in a short period of time using only a simple slip trailer. The post is an excerpt from Making Marks: Decorating the Ceramic Surface, a must for every potter’s ceramic library. – Jennifer Harnetty, editor.
Slips and Engobes
Slips are predominantly liquefied clay; they usually are applied on wet to dry greenware. Engobes usually have a lower clay content and also can be used on bisque-fired ware. The word slip generally is used to describe any clay in liquid form. All slips and engobes can be colored with oxides, carbonates and stains. Sometimes very crusty surfaces can be made by applying slips and engobes over the fired glaze surface and then refiring.
Casting slips give extremely good properties for use as a drawing medium in a fine-to-medium aperture trailer. To produce slips for casting into plaster molds, the ingredients for slip are mixed with water to which 1 percent to 2 percent of a deflocculant, such as sodium carbonate (soda ash) and/or sodium silicate, has been added. Since there is much less water in the deflocculated slip, it will leave a crisp, raised line drawing when applied to leather-hard surfaces. To remove sharp points or develop a low relief, raised line slip drawings can be flattened slightly by rolling the surface with a small rubber-coated roller or printmaker’s brayer.
Slips used for decorating usually are mixed with water only, unless specific qualities of fluidity or viscosity are desired. For these qualities, a flocculant such as vinegar or Epsom salts can be used for increased viscosity or thickening. Or a deflocculant, as mentioned above, can be used for increased fluidity. Decorating slips traditionally are used to coat the surface of clays in a variety of ways. They can be made from naturally occurring clays or from mixed materials and colorants to provide a range of decorative effects. They can be applied to wet, leather-hard or dry clay bodies, depending on the technique being used and the dry strength of the body. The slip decoration usually is covered with a glaze after bisque firing, although many people prefer to leave the slip patterns unglazed.
Slips can be used to coat another clay to make it lighter, darker or colored. They also can be used as a coating through which designs can be cut or scratched, resisted with wax or latex or layered with other slips to create a wide range of potential imagery.
7 Methods of Slip Decoration
1. Trailing on top of a base slip offers many decorative options. You could simply trail slip and let it sit on top of the base color as applied.
2. Combing through trailed slip makes a simple design very complex in short order. This takes practice to do smoothly, without hesitation.
3. The more complex you get with your initial slip design, the more options become available, but balance the trailing with the combing.
4. Because this trailed design was a bit more complex, a single line is pulled through selective areas with a single reed or feather quill.
Engobes and Underglazes
The word engobe is used most often in North America and describes a wider range of uses in the development of the decorative surface. Underglaze is basically the same thing, and it can be colored with any colorant or stain. Many commercially made underglaze products are available, offering a wide range of color potential. They are applied easily by brush or spray. Whereas the simple liquefied slip commonly is used to coat greenware, an engobe can be formulated for use at any stage, including over bisque-fired ware. Engobes also often are used without a covering glaze, giving a wider potential for experimentation with the surface. An engobe or underglaze is more like a glaze in structure and may contain very little plastic clay.
Materials for making engobes fall into six groups:
1) Clays, with kaolin or calcined kaolin usually used in place of ball clay to counteract shrinkage
2) Fluxes as used in glazes;
3) Fillers (usually silica);
4) Hardeners (borax, calcium borate, and various gums)
5) Opacifiers (tin, zircopax, titanium, superpax)
The following three recipes for basic engobes give a good starting point for further experimentation. The engobes can be colored in any of the usual ways.
Click to enlarge!