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April 22, 2009

Slip, Engobe, or Underglaze? Robin Hopper Demystifies Three Common Pottery Materials

by Robin Hopper Read Comments (3)

Dotted slip decoration.

Slips, engobes and underglazes are very similar materials used in pottery, but some confusion exists over the use of the terms. Today we turn to expert potter Robin Hopper for some clarification and some ideas for how to use these materials to decorate pottery and ceramic art. Plus we’ll throw in a basic engobe recipe for various firing ranges. - 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.

This post is excerpted from Robin Hopper’s Making Marks: Discovering the Ceramic Surface, available in the Ceramic Arts Daily Bookstore.

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.

Various Methods of Slip Decoration:

Pouring on basic iron slip Trailing slip of contrasting color Combing through trailed slip

 

 

 
Dotting before feathering Feathering Trailing for raised decoration

 

 

 
Preparing dotting Flattening dots by settling Combing

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); and 6) Colorants.

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.

Basic Engobe Recipes:
Raw Material Cone 04-3 Cone 4-6 Cone 6-10
Kaolin 20 15 15
Calcined Kaolin 10 20 35
Talc 25 10 5
Calcium Borate (or Frit 3110) 15 10 —–
Nepheline Syenite —– 10 15
Silica 15 20 15
Borax 5 5 5
Zircopax 10 10 10
Total 100% 100% 100%
Download the Ceramic Workshop Handbook: Pottery Tools and Ceramic Studio Resources now to get more great pottery ideas like these!

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3 Comments

  1. ruth | July 6th, 2009 at 2:35 pm

    fantastic recipe; i used it with RIO and chrome ox in cone 6 ox with great results

  2. Gail | July 24th, 2009 at 1:26 pm

    what percentage of RIO or Chrome ox would you suggest for the cone 6 engobe? thanks for your advice.

  3. diaa | October 2nd, 2009 at 1:35 pm

    (Engobe) thes word means that the interface between the body and glaze and mainly mex between body &glaze batches in between 50% to25 %glaze 50 to 75 %of body batch

    diaa elsayed

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