Appalachian Center for Crafts - Clay Studio

Vince Pitelka, 2006

Art 3520 - Advanced Clay Studio - Surface Design
Engobes and Underglazes

In working with surface relief and slip decoration we have explored two of the primary areas of surface decoration which are available as our work moves through the stages of forming, drying, and firing.  As mentioned in earlier handouts, it is critical to think of the cumulative effect of surface decoration, as multiple effects are layered onto one piece.  This is normally done with deliberate planning and anticipation, but in this case we are doing experiments to find out just what the possibilities are.  We will now explore the range of surface decoration options after the bisque firing, including underglazes, stains, patinas, and glazes. 

For our experiments with these techniques we will use the bisque-fired surface-relief experiments completed earlier in the semester, but you will no doubt also want to make additional forms in order to fully explore the possibilities.  As you look at the surfaces of these forms, try to anticipate what might happen with the use of underglaze, oxide stain, ceramic stain patina, or glaze, and plan the outcome accordingly.  Keep a written record of what your intentions were with each experiment, and make sure that your pieces are marked to correspond with this written record. 

As we have discussed before, when looking at ceramic work, both amateur and professional, it is common to see pieces where different stages of surface decoration conflict or negate each other.  For example, a delicate surface texture might be unintentionally concealed with a thick slip or glaze, or an opaque glaze might be applied over a painted or stamped slip pattern, concealing it completely.  In some cases, the artist carelessly applies one form of surface decoration over another without considering the consequences.  This should never be a careless decision. 

Some of us carefully plan each piece before beginning, and then execute the work without deviating from the plan.  Others prefer to allow form and surface to evolve as the piece is realized.  Either approach is fine, as long as we achieve our desired results.  In all cases it is essential that we carefully consider the visual and narrative implications of each successive stage of the forming and decorating processes.  Make sure that each new application of surface decoration works with what is already there.  In some cases, a simple use of one surface option might be all that the form needs.  In other cases, multiple overlays of surface decoration can build up a richness and intensity associated with the accumulated patina of age or the weathering of ancient surfaces.  These effects can have very powerful narrative implications. 

Utilitarian vessels need to be glazed on food contact surfaces, but for non-utilitarian vessels or sculpture or on non-food-contact surfaces, glaze is just one option to be considered.  In some cases, the abrasiveness of bare clay or a precarious, flaking slip surface may be just what you want, while in other cases a very thin patina mix of coloring oxide and flux might be the ideal finish. 

Engobes/Underglazes
We have explored plain claybody slips, applied to wet or leather-hard work.  As you know, slips made from unmodified claybody recipes are not appropriate for application to bone-dry or bisque-fired work.  We have an exciting opportunity for experiments with underglazes or engobes.  Engobes refer to any slip formulated for application to bone-dry or bisque-fired wares, while underglazes specifically refers to slips that are mixed thickly and are very strongly tinted with stain, so that even a thin application gives strong color.  The nature and performance of an engobe or underglaze lies somewhere between a plain clay slip and a glaze.  Glazes are of course formulated to be applied to bisque-fired wares without peeling or flaking as they dry on the surface.  Engobes or underglazes are simply specially-formulated slips containing higher percentages of non-plastic materials (calcined clay and/or additional silica and feldspar), which reduce drying shrinkage, and create a strong bond with the surface during the firing. 

Below you will find the Mason underglaze recipe.  Mason Color Works manufactures Mason-brand ceramic coloring stains, the standard of the industry.  These stains are fritted, and thus are very stable, and the unfired color is at least indicative of the fired color.   Mason has been giving out this recipe for decades, and thus it is proven in wide applications.   The use of non-toxic antifreeze is a more recent and very sensible modification of the recipe.  We have mixed up a quantity of this underglaze base, and you have only to add the stain.  It is important to realize that underglazes are rarely used in any sort of thick application, and as mentioned are tinted much more strongly than normal, so you will only use small quantities of underglaze base when you add stains.  

Mason Color Works Underglaze Formula
Components in Underglaze Base:
F-4 Soda Spar -------------------- 20
Kaolin ----------------------------- 10
Ball Clay --------------------------- 5
3124 Frit ---------------------------10
Zircopax ---------------------------- 5
Underglaze Liquid --------------- 75 

Add:
Stain -------------------------------- 40
(Or for a lighter tint use a mixture of stain and opacifier [Zircopax] to total 40) 

Components in Underglaze Liquid:
Propylene Glycol Anti-freeze --- 10 parts by volume
Water ------------------------------- 10 parts
CMC Liquid ------------------------ 5 parts (To make CMC liquid, stir 30 grams dry CMC in 1 qt. hot water and let sit for 2 days to break down)

Note #1: Normal antifreeze is ethylene-glycol and is toxic. Propylene-glycol antifreeze is the non-toxic substitute.  Such antifreeze is sold in most auto-supply stores, and is much more friendly to the environment and to people and pets. 

Note #2: The above underglaze mixture will allow you to brush-paint directly on dry greenware or bisqueware without your brush dragging.

Note #3: The presence of CMC gum will give a tough surface that is slightly more impervious to water, and thus glazes will not apply as thickly over underglaze.  When overall color is needed as a base for glaze application, an engobe is more appropriate than an underglaze. 

Adding Stain to Pre-mixed Mason Underglaze Base
As mentioned above, underglazes contain a much higher concentration of colorant than the slips we use on damp greenware, and thus they are rarely used in any sort of thick application.  When you purchase underglazes commercially they normally come in 2 ounce (60 gram) jars, and those people using large amounts of underglaze sometimes purchase them in pint jars.  Unless you have a specific project in mind where you know you will need a larger amount, measure out 100 to 200 grams of the underglaze base for each color you plan to mix. 

The combined total of the dry materials and the liquid in the above underglaze base is 125, and the addition of stain is 40.  That means that the stain to be added is 32% (40÷125) of the weight of the underglaze base.  For the darkest or most-saturated colors, add 32% stain.  For lighter tints, add a mixture of stain and opacifier (Zircopax).  For a darker shade of a color, add a combination of that color stain and black stain.  The combination of stain and opacifier, or the combination of multiple stains should always add up to 32% of the weight of the amount of underglaze base you are using. 

To figure out the proper amount of stain (or stain/opacifier) addition, weigh the container you are going to mix the underglaze in, add the amount of underglaze base you want to use, weigh again, subtract the weight of the container, multiply by .32.  That will give you the amount of stain (or stain/opacifier) you should add.  Stir the dry powdered stain (or stain/opacifier) into the underglaze base, and if it starts to thicken excessively, thin it with plain water.     

Application of Engobes and Underglazes
Engobes are often applied by dipping or pouring, but underglazes are a more specialized, concentrated decorating medium, and are generally mixed in small quantities, and thus dipping and pouring are not appropriate.  Both engobes and underglazes may be applied by brushing, spattering/flicking, stamping, or spraying, and for the latter, underglazes generally need to be thinned a little with water. 

In some cases an engobe can provide an opaque ground for additional decoration using a different engobe, an underglaze, or an oxide stain.  In other cases an opaque underglaze or engobe coating, either applied overall or selectively, can serve as a foundation for a glaze, affecting the outward appearance of the glaze.  Even an opaque glaze usually looks quite different when applied over a slip, as compared to application over the bare claybody.  The same effect will occur with an engobe or underglaze. 

An engobe or underglaze may also be applied to the textural surface of a bisque-fired piece and sponged off the high spots, leaving an opaque coating in the recesses.  As was the case with slips, an engobe may be applied using resist techniques including tape, adhesive labels, latex, and/or wax in order to achieve hard-edged design work.  All of these media except wax can be peeled off after the engobe or underglaze is applied, and thus a glaze could then be applied overall.  If you brush on wax resist and then apply an underglaze and then a glaze, the wax will of course resist both the underglaze and the glaze, leaving bare clay in the resisted areas.  

Underglazes are particularly suitable for painted imagery.  The color is strong enough that it will often come through an opaque glaze, but for greatest clarity of design, use a transparent glaze.  Because the medium is clay-based, the imagery will usually remain distinct through the glaze firing, unless the overglaze is especially solvent or shifts significantly in the firing. 

With a properly formulated engobe or underglaze, you can achieve some of the effects possible with thick slips on damp or leather-hard wares.  For example, some engobes and underglazes may be trailed onto the surface with a slip bulb, producing raised relief decoration.  This can be used both for color contrast and for relief effects under a transparent glaze, or can be used for surface relief under an opaque glaze.