Tennessee Tech University - Appalachian Center for Crafts - Clay Studio

Vince Pitelka, 2006

Art 3520 - Advanced Clay Studio - Surface Design
Patinas and Glazes

Reading : In Chapter 6 of the text, study pages 127 to 137

Oxide Stains and Other Patinas
The terms oxide stain and patina are often used interchangeably in ceramics, but to clarify, oxide stains are a variety of the broader category of patinas.  Normally, in ceramics the term “stain” refers to a fritted colorant, as in Mason stains.  But in this case, when we refer to an oxide stain, we are using the term stain as it is used in painting houses and furniture, where it implies a transparent, colored patina that allows the texture and some of the color of the substrate to show through.  An oxide stain is usually a simple mix of ceramic coloring oxide and water, although it sometimes contains a small amount of frit to help flux the colorant to the substrate.  Similar stains can also be made by mixing ceramic stains like Mason stains with water, often with a frit included as well.  These media are different from glaze in  that they are primarily just colorant in water, and thus are applied very thinly.  Oxides along on a surface are never food safe. 

It is easy to be confused by these terms.  The term oxide stain is easy to recognize, but how do we refer to a similar mixture containing a commercial ceramic stain colorant?  For clarity’s sake, we will call them ceramic stain patinas, but remember that oxide stain uses the term stain in the sense of a thin, transparent coating, while ceramic stain patina uses the term stain in the sense of a commercial fritted ceramic stain colorant. 

Oxide stains and ceramic stain patinas can enliven the ceramic surface and introduce color with little or no glaze.  They can be applied overall to modify the appearance of a glaze.  They can also be applied selectively with a stamp, brush, or airbrush to create imagery under or over the glaze.  On textural surfaces, an oxide stain or ceramic stain patina can be brushed overall on the raw bisque surface and then sponged off the high spots in order to accentuate the texture, with or without a glaze.  An oxide stain or a ceramic stain patina applied overglaze by spray, brush, spatter, or sponge-stamp can modify the glaze color locally and create interesting effects. 

When applying a patina to a piece that will otherwise remain unglazed, it is a good idea to use a fluxed patina that contains some clear glaze or Ferro 3134 frit as a flux or melter, which will make the colorant fire hard and adhere to the clay surface.  This is especially important at low-fire temperatures, as otherwise oxides or stains will remain dry and powdery on the surface after the firing.  Up to 50% frit or glaze may be added, but the more you add, the shinier the surface will be, especially where it remains thicker in recessed areas.  In some patinas, a zircon opacifier or tin oxide may be added to give a more opaque deposit of patina in the recessed areas.

Even without coloring oxides or ceramic stains, a very thin application of a watered down glaze or a watery mixture of 3134 frit can act as a patina to produce a wetting effect and even a slight sheen on the surface, intensifying color and visual texture.  While inappropriate for utilitarian surfaces, this may be an ideal finish for sculpture or non-utilitarian vessels, especially over complex slip, engobe, or underglaze decoration, since glaze can often mute the effects of painterly slip application. 

An interesting variety of patina in ceramics are those which simply approximate the accumulation of dirt and debris found in much ancient claywork.  Varying proportions of zircon opacifier or tin oxide and coloring oxides with some 3134 frit (to flux the patina onto the surface) will give a range of “dirt” colors from light to dark brown or black.  Following are a range of sample patina recipes using oxides, opacifiers, and frit. 

Dry Patinas - for a dry, earthy effect, intended to imitate the accumulated residue in the recesses of ancient wares.  Should not be used on any surface which comes in contact with food.  Amounts are proportional by weight.  Mix all patinas with water to a thin milky consistency. 

            Black - 3 manganese dioxide, 3 red iron oxide, 2 cobalt carbonate, 2 Ferro 3134 frit.
            White - 8 tin oxide, 2 Ferro 3134 frit.
            Dirt - 5 tin oxide, 1 copper carbonate, 2 red iron oxide, 2 Ferro 3134 frit.
            Copper - 4 tin oxide, 4 copper carbonate, 2 Ferro 3134 frit.
            Iron - 8 red iron oxide, 2 Ferro 3134 frit.

Gloss Patinas - for a glossier finish, especially where thicker, as in corners and recesses.  Should not be used on any surfaces which comes in contact with food.  Amounts are proportional by weight.  

            Black - 2 manganese dioxide, 2 red iron oxide, 1 cobalt carbonate, 5 Ferro 3134 frit.
            White - 5 tin oxide, 5 Ferro 3134 frit.
            Dirt - 2 tin oxide, 1 copper carbonate, 2 red iron oxide, 5 Ferro 3134 frit.
            Copper - 3 tin oxide, 2 copper carbonate, 5 Ferro 3134 frit.
            Iron - 5 red iron oxide, 5 Ferro 3134 frit.

Wax-based Patinas
Oxides and/or glazes may be mixed with wax resist and used for brushed or stamped decoration on the bare clay or over a previously applied glaze.  In applying a subsequent glaze, this mixture will both provide color and resist the glaze.  If this mixture is to be applied over bare clay, add a little dry powdered glaze or 3134 frit to bond the oxide in place in the firing.  When used on bare clay, the amount of glaze or frit added will determine how shiny the oxide surface is after the firing.

A lesser-known use of wax-based oxide stains or ceramic stain patinas is found in the technique known as Cuerda Seca. In this technique, oxides or ceramic stains and a small amount of glaze or 3134 frit are combined with wax resist as described above.  This mixture is painted onto the surface of a piece to create a line drawing of a pattern or image.  Different colored glazes are painted onto the intervening areas of bare clay, and the wax in the Cuerda Seca mix resists the glaze, simplifying application.  After the glaze firing the tinted oxide lines are generally much less shiny than the glazed areas, and the resulting effect looks similar to tile work with a colored grout line between individual ceramic tiles.  

Glazing
When you decide to glaze a piece, you still have a myriad of possibilities to consider.  In approaching glazes in general we immediately think of color and shine, but these are just two of many considerations. 

Glaze Color
Glaze color alone involves a range of variations including hue, shade, tint, intensity, and saturation.  This is the language used by painters, and for the most part it is applicable to glaze color.  Hue is the actual color, such as red, blue, orange, etc.  A shade of a color is darkened from the median point of light and dark, usually by the addition of black pigment.  A tint of a color is lightened from the median point, usually by the addition of white.  Intensity is the brightness of the hue; the degree to which it commands attention and seems to jump off the surface.  Saturation is the depth and richness of the color, resulting from the combination of hue, intensity, and shade or tint.

Glaze Transparency and Surface 
When we think of glazes in general, we often assume a gloss surface, but again this is just one of many options.  Glazes are just silica glass with various modifiers added to affect their behavior and appearance.  Depending on the glaze materials and modifiers present, the glaze can be gloss, semi-gloss, semi-matt, or matt.  These are all qualities of reflectivity and visual/actual texture, but variables in the glaze appearance may also include transparency/opacity, translucency, iridescence, thickness, and mobility (viscosity at maturation temperature).  The most transparent glazes are invariably glossy, because anything other than a high-gloss surface interferes with transparency.  However, high-gloss glazes often tend to be a bit garish, and will show every surface flaw.  Most potters usually prefer semi-gloss or semi-matt glazes, which can give a very appealing silky surface texture.  Even on a semi-gloss or semi-matt clear glaze, transparency is hardly diminished, and such glazes can work extremely well over slip-decorated surfaces, showing the variations of slip color while maintaining a softer surface quality. 

Effects such as translucency, opacity, and iridescence result from either of two phenomena, or a combination of the two.  First, they may result from particles or globules of inert materials floating within the glaze.  Opacity is usually achieved by adding highly refractory materials such as zircon opacifiers, which simply remain suspended in the glaze, interfering with transparency, without effecting any other quality of the glaze.  However, if too much of this material is added it will interfere with proper glass-formation, and the glaze will be dry and under-fired. Total opacity may be achieved in almost any glaze by the addition of 10% zircopax (zirconium silicate opacifier) or 5% tin oxide.  The latter is a very expensive material, so we generally use zircopax except in situations where glaze color responds positively to tin.

The second phenomenon which can produce opacity or translucence is the growth of crystals within and upon the glaze during the cooling ramp.  If the glaze is cooled too quickly, crystal growth will be minimal.  When wishing to accentuate crystal development, it is a good idea to close the damper entirely at the end of the firing for at least a few hours and/or soak the kiln between 1900 and 1800 degrees Fahrenheit during the cooling ramp. 

Iridescence can result from the presence of an immiscible material such as phosphorus (usually from bone ash), which produces small globules suspended within the glaze.  As long as the glaze is otherwise transparent, the reflective surface of these globules can create iridescence, such as is often found in Chun blue glazes. Iridescence can also result from crystal growth in the surface of the glaze, as in many “khaki” saturated iron red glazes. 

Glaze Effects Resulting from Thickness of Application
Behavior of glazes obviously depends greatly on thickness of application.  Viscosity is a key issue here, and we must anticipate the mobility of a glaze to insure that it does not run excessively.  We know that a thick application of an opaque glaze will conceal any subtleties in actual texture, and may run all over the place.  Inversely, a very thin application of glaze will never run, but it tends to cover the entire surface without any variety, and may leave a surface texture feeling too raw.  Between these two extremes, an average thickness of glaze will deposit most heavily in recesses, moderate thickness on flat surfaces, and thinnest on outer edges and corners.  A colored transparent glaze, such as a celadon, will always appear darker where it is thicker, especially wherever there is a recess in the surface.  Such a glaze, when applied thickly, may completely eliminate the actual tactile qualities of the original surface, but may retain considerable visual texture because of the variations in glaze color in thick and thin areas.  The classic Chinese celadons are the prime example of this effect.  When applied over a carved relief surface, the celadon glaze tends to settle more thickly in the recessed areas, producing a darker green accentuating the effects of light and shadow in a way that can be very dramatic.

For heavily textured surfaces, glaze may be applied and then sponged off the high spots.  The piece can be fired like that, or it can be quick-dipped or sprayed with the same glaze or a different glaze to bring back a thin glaze coating on the high spots. 

One of the most important and interesting glaze effects is color-breaking, where a single glaze produces a variation in color dependent on glaze thickness and the effects of oxidation and reduction.  The most dramatic color breaking always occurs in reduction high-firing.  The classic East Asian brown-black temmoku glaze is a perfect example.  Most of the iron color we encounter in rocks and in ceramics is red or ferric iron, which is the oxidized form of iron, as in ordinary iron rust.  But in a reducing atmosphere in the latter stages of the firing, the iron in a temmoku glaze loses oxygen atoms and reduces to black ferrous iron.  Where the glaze is thick, it will permanently retain this black coloring, because the molecules of black iron are impacted deep in the glass, protected from re-oxidation.  Once the kiln is shut off, all remaining carbon or hydrogen in the kiln atmosphere immediately combust, and we are left with a strongly oxidizing atmosphere.  Anywhere the glaze is thin, the iron molecules are exposed to this oxygen-rich atmosphere, and will return to the red ferric form.  So a temmoku glaze characteristically is black where thick, breaking to brown or red-brown where thinner, usually on rims and outside corners and edges.  If a temmoku is applied too thin, the entire surface will re-oxidize to red-brown.  If the glaze is applied fairly thick overall and if there is no breakup in surface relief, the entire piece might be pure black except for where the glaze breaks to brown along the rim, and this can be very striking. 

Similarly, in a well-managed reduction firing, a properly applied copper-red glaze will develop a bright red color, and the color will be retained where the glaze is thick, but will re-oxidize to a colorless transparent where it is thin.  This is especially striking on porcelain and white stoneware bodies, where the rims and outside edges are white against a deep red.

Some of the most interesting glaze effects involve layering glazes and patinas, and there are many ways to do this.  A piece can be coated with a thin patina, and wax resist decoration can be applied with a brush or sponge-stamp, followed by an overall glaze coating.  Where the wax decoration has been applied, there will only be the patina on the bare clay.  The same technique can be used with multiple glazes.  Decoration with wax resist can be brushed or sponge-stamped over a raw glazed surface, and the piece can then be quick-dipped into a second glaze. 

Any glaze can be dipped, brushed, or sponge-stamped over another glaze.  Keep in mind the general guidelines for glaze applications and avoid excessive buildup of glaze.  Double dipping is rarely a good idea overall, unless your glazes are adequately thinned and you do very quick dips.  Sponge-stamped patterns are one of the most interesting ways to achieved rich layered pattern effects at all firing temperatures. 

Additional glazes can be brush-painted over a raw glaze coating, but the brush will tend to drag unless the second glaze is thinned with water a bit.  Use a soft, watercolor brush with good reservoir capacity, and load it up with glaze.  Use quick, fluid strokes. 

Glazes can be trailed and/or dripped over a raw glaze coating with a slip-trailing bulb.  This can create very interesting effects, due to the additional depth of glaze applied with the bulb, as compared to glazes brushed on a raw glazed surface. 

A technique known as glaze sgraffito can create interesting effects.  Paint wax over a dry raw glaze surface.  Carve or scrape a design through the wax.  Leave as is to expose the claybody beneath, or else dip the piece in a second glaze, or brush an oxide stain or a ceramic stain patina over the carved/scraped areas. 

Underglaze/Patina/Glaze Assignment
Examine ceramic work in books and magazines in the library and look at work in the gallery and around the studio.  Consider the underglaze, glaze, and patina effects and the possible mechanical (rather than chemical) causes of those effects.  Next semester we will study the chemistry of clays, glazes, and glaze materials in depth, but for now just think about the surface qualities discussed above.  For now, you do not need to understand glaze chemistry in order to use engobes, glazes, and patinas effectively.  But you do need practical knowledge and experience concerning surface effects created by these media. 

Using your surface relief experiments, slip-decorated pieces, and additional test pieces (tiles or small thrown vessels or sculptural forms), explore the use of underglazes, oxide stains, and ceramic stain patinas.  Explore as many of the following technique as you can.

Underglaze Effects

Oxide Stain/Ceramic Stain Patina Effects

Glaze Effects

In some of your tests use clear glazes, and in others use opaque glazes.  Try a good range of our glazes on both flat and relief surfaces.  On slip- or underglaze-decorated work, if the slip/underglaze effect is primarily textural or relief (as in high-relief slip-trailing), then both transparent and opaque glazes will work.  A slip/underglaze effect involving color contrast would require a transparent or translucent glaze, a patina, or an unglazed surface.  With each test piece, and with all the work you are firing for the rest of the semester, carefully plan and record all of the effects you use, so that you (and the rest of us) can learn as much as possible from your results, successful or not.

Examine all the high-fire glaze tiles in the glaze room and any glazed wares around the studio, and decide which glazes give good surface variation, and think about why this variation occurs.  In some cases the surface variety will be caused by opacifiers or inclusions which render the glaze opaque or translucent.  In other cases crystal growth will give interesting color variety, or varying degrees of transparency and/or mattness.  And finally, many of the glazes will break from one color to another, dependent on texture and relief in the clay surface, the thickness of glaze application, the mobility of the particular glaze, and the tendency of the glaze to change color where oxidized or reduced.  Be thinking about all these phenomena when glazing your work, and as mentioned above, be sure to record the specifics of all of your experiments carefully in a notebook or sketchbook.