Slips and engobes are more or less the same thing, and some confusion exists over the use of the two words. 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 refiring.
To produce slips for casting into plaster molds, the slip ingredients are mixed with water, to which a deflocculant, such as 0.25–0.5% sodium silicate or 0.4% Darvan 811 has been added. Slips made with a deflocculant require much less water to achieve the same degree of fluidity as those without deflocculants.
Casting slips give extremely good properties for use as a drawing medium in a fine-to-medium aperture slip trailer. 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 low-relief, raised-line slip drawings, the trailed slip 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 are traditionally 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 imagery.
Excerpted form Making Marks by Robin Hopper, available on the Ceramic Arts Daily Bookstore at http://ceramicartsdaily.org/bookstore.