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 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. Slips can be divided into two categories: Casting slips and decorating slips.
To produce slips for casting into plaster molds, the ingredients for slip are mixed with water to which 1–2% of a deflocculant, such as Darvan and/or sodium silicate, 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 with a fine-to-medium-tipped slip trailer (1–6). 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 (7) by rolling the surface with a small rubber-coated roller or printmaker’s brayer.
Slips used for decorating usually are mixed with water only (no deflocculant), 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. 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 potential imagery (8–10).
Excerpted form Making Marks by Robin Hopper. This title, along with Functional Pottery and The Ceramic Spectrum, also by Robin Hopper, are available from the Ceramic Arts Network Shop at https://ceramicartsnetwork.org/shop.