In the Studio: Slips vs. Engobes Richard Zakin with Frederick Bartolovic

Appears in the May/Jun 2022 issue of Pottery Making Illustrated.

Slips and engobes are related to one another and sometimes the terms are used synonymously. In the US, we typically differentiate clay finishes that go on before bisque firing as slips, and those that can go on both before and after the bisque as engobes. Yet, in other parts of the world this differentiation is not so pronounced and the word engobe is sometimes used for all of these clay finishes.

Slips

Slips contain the same raw materials as glazes but in very different percentages. Slips are primarily composed of clay, though they often have small amounts of flux in their recipes; this helps keep them adhered to the surface of a clay form during the firing. They differ from glaze only in the ratio of the clay to non-clay ingredients. Slips are generally composed of 50–90% clay, with the remaining percentages for colorants and flux. 

These high percentages of clay encourage viscosity, stiffness, stability, and durability. Furthermore, they discourage fluxing or melting and they tend to stay put where they are applied. Clays naturally have a high alumina content. Since slips are defined by their high clay content, it follows that they, too, have a high alumina content. It is this that keeps slips from running or blurring imagery created with them during the firing. Recipes that are rich in alumina will melt very little, if at all, even at the high-fire range.

Engobes

Engobes are clay finishes that can be applied in both the greenware or the post bisque-fired state. What allows this application versatility is the inclusion of a higher percentage of melters or fluxes, though still not enough to necessarily call them glaze. Engobes contain 25–50% clay and are more likely to contain calcined (pre-fired) clay. This allows engobes to be applied (when properly formulated) to clay in either the greenware or bisque-fired states without any of the cracking or flaking issues that would otherwise plague a slip being applied to bisqueware.

As with slips, the high percentages of clay equate to a high alumina content. This results in a finish that doesn’t move or melt much in the firing, equaling clear, crisp brush strokes or imagery depending on the application method. 

Applying Slips and Engobes

Slips and engobes are usually applied with a brush, though they may also be applied with a sprayer. Ceramic artists often use them to create painted imagery on clay surfaces due to their lack of blurring and flowing. The use of slips and engobes can facilitate further finishing techniques such as sgraffito. 

When applied to wet clay, slips and engobes generally have a soft, natural character. When applied to dry clay or to bisque-fired clay respectively, slips and engobes tend to have sharper edges and a more precise, controlled character.

Note: Use caution when applying slips or engobes to already dry unfired clay, or what is referred to as bone-dry greenware. The re-introduction of water into an already dry piece can cause great stress and tension on the piece as it tries to re-expand from the moisture, resulting in the formation of cracks. If applying a slip or engobe to greenware, it is best to do it slightly after the leather-hard state, while there is still some moisture left inside the clay.

Shrinkage in Slips and Engobes

Clay shrinks; and because slips and engobes have a high clay content, they can shrink a great deal. If you apply a slip or engobe to a greenware piece (which is also going to shrink in the firing), this may not present a problem. It is much more likely to present a problem if you apply a slip or engobe to bisqueware (where the structure of the clay particles has been chemically changed). It is very likely that if applied to a bisque-fired piece, a slip (especially when applied in thick layers) will be marked by a pattern of cracks and perhaps flakes. For this reason, slips are usually applied to work in the wet, leather-hard, or greenware state. Furthermore, engobes usually have calcined clay included in the recipe, reducing shrinkage and giving them the versatility they need. 

Calcining Clay for Slips and Engobes

Calcining is the act of heating a material. To calcine clay, heat it to the point at which all of its chemically bound moisture is driven off. Such clay can no longer hold together nor can it shrink. Calcined clay is a useful addition to slips and engobes because it can prevent them from shrinking or cracking in the firing. 

Calcining most of the clay content in an engobe recipe should allow you to more successfully apply it to bisqueware. This is because the clay used in the recipe is now in the same state of dehydration and shrinkage as the form you are applying it to. But, be cautious about replacing all unfired clay in a recipe. Unfired clay is usually the binder that sticks a slip or engobe to the form (in the unfired state), and the suspension agent that keeps it all in solution. Without a little unfired clay, you once again risk your slip or engobe flaking off the form as it settles in its container. Therefore, it is best to use some (perhaps 10%) of the clay body in its uncalcined form.

To calcine a clay, place powdered clay in an unfired or bisque-fired bowl and fire it to cone 08, 1751°F (955°C). The clay will lose its water content and its plasticity; it will be dry, powdery, and most important, non-shrinking.

Cracking and Flaking

Ceramic artists sometimes view cracking and flaking as appealing testimony to the character of ceramic materials they are using. Though some ceramic artists may see these as faults or defects, others embrace the organic, and at times unpredictable, nature of the material and exploit characteristics such as cracking and flaking. Slips and engobes applied in thick layers (as these are more likely to be unstable) on bisqueware or even on top of glazes can produce some very interesting textured results. 

Gritty Slips

Slips are usually made from fine-grained clays and are sieved before they are used, resulting in a texture that is smooth and creamy. But if some of the clays used in the recipe are coarse and the slip is not strained, the result will be a coarse, sandy-textured surface. Though this surface isn’t practical for tableware, which must be easily cleaned, it can be used on the exterior of some functional pieces.

Gritty slip recipes are simple mixtures of a few clays and usually some materials that encourage melting. They may be applied in a graphic, painterly manner and lend themselves well to surface imagery. They may be used alone or in conjunction with stains and glazes. Gritty slips are reliable and easy to use. When fired, they are surprisingly strong and resistant to abrasion.

Vitreous Slips and Engobes 

Vitreous means glass-like. We expect glazes to be vitreous (glassy), but we do not expect this from slips and engobes, which have a high clay content. If the clay content is rich in alumina, these high clay amounts will normally encourage dry, refractory surfaces. Slips and engobes can, however, be made vitreous if they are carefully formulated. They may contain high percentages of highly melting clays (generally high-iron clays), or they may contain higher than normal percentages of melting materials known as frits. 

Vitreous slips and engobes usually require the substitution of calcined clays for much of their clay content. Once this is done, they tend to be extremely stable and durable. They look and feel much like glazes. They generally lack much visual texture. Their high clay content promotes a highly viscous character and they will not run or blur in the firing. It is thus possible to develop a technique that combines the richness and durability of glazes with the graphic painterly qualities of slips and engobes. 

Excerpted from Electric Kiln Ceramics: A Guide to Clays, Glazes, and Electric Kilns, Fourth Edition, by Richard Zakin and Frederick Bartolovic, published by The American Ceramic Society and available at https://mycan.ceramicartsnetwork.org/s/product-details?id=a1B3u000009udqTEAQ.