Ceramic Glaze Colors
Ceramic glaze, also called stains or underglazes, gives potters an endless array of possibilities. The color of a glaze is determined by its ingredients and the way it is fired. Many glaze colors contain metallic oxides which can be toxic if inhaled or ingested.
Look up the stain chart for your particular glaze to learn what oxides are in it. Most commercial stains also include reference notes to help with mixing and firing.
Ceramic glaze colors come from a variety of sources, most notably metallic oxides. These can produce a wide range of hues, and under different conditions can react differently with the other ingredients in the glaze, resulting in different results. Some of these oxides are toxic and can be hazardous if inhaled or consumed, so they should be used with extreme care and caution when creating functional pieces intended for use with food or beverages.
Colorants can be added to glazes in two ways: as a pigment or a stain. Pigments are a combination of ceramic oxides and may have additional clay components added to them (for example, silica or alumina). Stains are ceramic oxides that need to be combined with other materials to create the desired color.
Zinc oxide is one of the most common pigments, producing white or yellow colors. Manganese dioxide is often used in low-fire glazes to produce brown. Cobalt oxide is used to create blues and purples in high-fire glazes. Cobalt can be volatile, and is highly toxic if not handled properly. It is best to use it in a glaze rather than a slip or clay body.
Other metal oxides may be used to produce a variety of colors in glazes. Copper oxide produces greens and reds; tin produces whites and yellows; iron chromate plus tin yields ceramic glaze colors shades of gray, brown and black; and uranium yields pink, coral, and yellow hues.
Colorants do more than just affect the color of a glaze; they can also effect its melting temperature, thermal expansion and opacity. For this reason, it is important to know how much of each colorant you are using and the proportions in which they are present. These proportions are sometimes expressed as ratios to other oxides, using something called a unity formula or Seger formula.
Typically used for adding color and designs to ceramic pieces, clear glazes can also be poured, brushed or sprayed on. Some are opaque and act like a magnifying glass on top of underglazes, while others are transparent.
No glaze is perfect, and even the best quality will have little bubbles and particles in it. These will be invisible if the glaze is thin enough, but thicker ones may obscure the colors and designs underneath. It is also common for glazes to become cloudy after firing when there are a lot of unmelted particles that are blocking the transmission of light.
This happens when a glaze has not melted and bonded properly with the clay body it is covering. The problem is compounded when the firing temperature varies, or if the glaze has been applied too thickly.
There are several reasons why a glaze may go cloudy or milky, including the chemistry of the mix, its specific gravity, and the firing conditions. These can affect how well it covers the clay and how fast it dries and sets.
Glazes are also subject to crawling when they are cooled too quickly or when the glaze does not fit the clay body properly. This creates small bare spots that are often referred to as “craters” and can cause a glaze to look milky in appearance. Gum solutions can help slow the drying process and reduce shrinkage for a better result. The most common gums used in pottery glazes are talc and carboxymethylcellulose (CMC). They work by absorbing water and slowing the rate of evaporation from the glaze, which improves the consistency and gives better coverage when brushing it on.
When ceramic artists apply color to a piece of pottery it is called underglazes. Underglazes can be applied in a variety of ways to create different decorative and functional effects including mishima, sgraffito and brushing on names or sayings. When a piece of pottery is fired in the kiln, the glaze that covers underglazes forms a protective layer and can be glossy or matte, semi-glossy or satin-like. If the glaze is transparent, colors from underglazes can show through.
Pure stain powders do not melt at typical pottery firing temperatures so they cannot bond to clay surfaces. Commercial underglazes sold in jars dilute these colored materials into a recipe of materials, a base medium designed to impart all the properties that are missing from pure stains. The base medium often contains silica that acts as a glass-forming ingredient and it also includes alumina, which stiffens the glaze so it will hold onto brushes when applied.
The other main component of ceramic glaze is glass formers such as borosilicate and soda-lime silicate. These ingredients form the glass that is melted to make ceramic glazes and they include a wide range of metallic oxides. These metallic oxides are the reason ceramic glazes can have a wide variety of colors and textures. Some, like nickel oxide, produce quiet grays and browns while rutile encourages pronounced streaking in borosilicate glazes.
These metallic oxides can be dangerous if inhaled or ingested so the user should always use gloves when handling them. Some of these metallic oxides can be corrosive so caution should also be taken when applying them to functional pieces such as plates, bowls and cups. The last major ceramic glaze ingredient is gum, which acts as a binder and slows down the rate at which the glaze dries when it is applied to a pot or cup.
Ceramic glaze paints are water-based colors used to create decorative effects, such as marbling and shabby chic. They can be layered or mixed to achieve more color depth. They are not suitable for functional ware because they are too thin and may not withstand the high firing temperatures required to make them durable enough for everyday use.
Glazes are applied by dipping or brushing. Brush glazing is more difficult to do well but it offers potters a lot of options for intricate design elements. Seasoned potters often use a combination of both dipping and brushing to get the best results from their work.
There are many different colored glazes available that are designed to be used on low, mid and high fired clay bodies. These glazes can be clear or formulated with frit that gives them varying degrees of transparency. They can also be made with metallic oxides that give them specific colors when they are fired in the appropriate conditions.
For example, iron oxide in ash glazes will produce greens that can range from olive to apple green. Copper is another common ceramic glaze colorant that gives blue in oxidation and red in reduction, depending on the proportions of copper carbonate used. Magnesia will give pink and warm gray colors. Cobalt with magnesia produces intense blues, and when used with rutile will produce mottled greens and purples.
Some of these glazes are toxic if ingested or inhaled and need to be used with great care. Others are food and drink safe, such as glazes containing Cadmium or Selenium that give brilliant reds and oranges. Lead free and zircon encapsulated stains can be used to provide similar colors but are safer for functional ware.
When it comes to ceramic glaze colors, there are many different ingredients that can affect the finished product. These ingredients can range from clay, slips and underglazes to metallic oxides and deflocculants. Some of these additives can even change the texture and color of the glaze after firing. The best way to control the final results of a glaze is to add or subtract these materials until the desired result is achieved.
One important additive is a gum solution. This material thickens glaze slurries and helps ceramic glaze colors them retain their brushable consistency. It is used especially in dipping glazes and glazes applied over underglaze decoration. When mixed with the right proportions of water and CMC powder, a gum solution can even improve stamping performance in stamped glazes.
There are a number of commercially available gum solutions for ceramics, including AMACO’s Glazemaster Gum Solution. However, it is fairly easy to make your own using dry CMC powder and water in a ratio that works well for your studio. If you use too much dry CMC, the resulting gum solution can clump and become difficult to mix. A simple dilution with more water will solve this problem.
Another ceramic glaze additive is a deflocculant, such as sodium silicate or suspendaid. These materials help reduce the viscosity of a glaze so that it can be easily brushed or poured. They also can help the glaze to adhere better to a bisque surface.
These and other ceramic glaze additives are used to control the consistency of glazes and to modify their visual properties. They can even bring old, hardened glazes back to their original consistency. Most of these materials are nontoxic and safe for functional ware if used in accordance with manufacturers’ recommendations.