If you’ve seen anything I have made in the last several years, you know that I’m a little bit obsessed with pale-turquoise and pale-green glazes. I can’t get enough of them.
So today, I thought I would share some samples of the glazes I obsess over. Linda Bloomfield explains the chemistry behind glazes ranging from the palest yellow-greens to some terrific teals in this excerpt from her book Colour in Glazes. Plus she shares loads of pottery glaze recipes (for all firing ranges). There are many ways to get greens and blues in ceramics, but if you’re looking for a specific hue, this will help you find the right combination. – Jennifer Poellot Harnetty, editor.
Today we still use copper oxide to make green pottery glazes. We now are more acutely aware that the color is affected by the kiln atmosphere and the flux used in the glaze. In reduction, copper reds can be achieved in alkaline glazes containing tin oxide. Copper is volatile, occasionally causing pink flashing on surrounding pots. In oxidation, alkaline glazes with low alumina will give bright turquoise glazes with copper. Other oxides can be added to change the color: iron for yellow-green, nickel for olive green, or ilmenite for blue-green. Turquoise can also be obtained using vanadium zircon stains, which are opaque.
Chromium oxide gives a reliable green, unaffected by kiln atmosphere, although it is volatile at high temperatures. Cobalt and chrome combine to make teal blue-greens. In alkaline glazes, a bright chartreuse green may be obtained using a small amount (0.2%) of chromium oxide. Zinc should be avoided in chrome glazes, as it turns the chromium brown. Cobalt and titanium or rutile can produce a pale green in high-alumina glazes and slips. Nickel and titanium will give green in magnesium matte and zinc crystalline glazes.
Ash glazes are often green, when fired in reducing conditions, the color coming from iron oxide present in the ash. Ash from different trees contains varying proportions of minerals, which result in a range of colors in the glaze, from olive to apple green. Iron oxide is also used in celadon glazes, which are gray-green, or blue if there is no titanium present in the glaze or clay body. Low alumina and the addition of boron or barium carbonate also promote blue celadon colors.
Copper oxide (CuO) and carbonate (CuCO3) are used to give green in oxidation and oxblood red in reduction. Copper oxide is volatile, and will cause a pink blush on surrounding pots in reduction. In alkaline pottery glazes with low alumina, a bright turquoise may be obtained in oxidation from 1–2% copper oxide. If a glaze becomes oversaturated with copper oxide (more than 5%), it turns a matte, metallic black.
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Chromium oxide (Cr2O3) also known as chrome oxide, gives a reliable green and is used, together with varying amounts of cobalt, in commercial green stains. Chromium oxide is not very soluble in glazes, except in high alkaline glazes, where it turns bright yellow-green. In the presence of tin oxide in calcium glazes, chromium turns pink. Only a very small amount of chromium is needed for this (0.1–0.5% with 5% tin). In glazes containing zinc, chromium forms brown zinc chromate. Chromium oxide is refractory and toxic. It is volatile at high temperatures.
Stains are made by fritting coloring oxides together with silica, alumina, and opacifiers in a kiln. They are heated, then cooled and ground to a powder, which can be dispersed in a glaze and remain suspended in the glaze during firing. Some stains have the spinel structure and are very stable and highly refractory, so they do not dissolve in the glaze. Spinel (MgAl2O4) is a mineral with a cubic, close-packed crystal structure in which the atoms are packed as densely as possible. The magnesium and aluminium can be replaced by cobalt, zinc, iron, or chromium to make colored spinels—cobalt zinc alumina chromite is a blue-green spinel. This color also can be obtained by adding the same oxides directly to a suitable base glaze.
Other stains are based on the zircon (ZrSiO4) structure, with some of the zirconium replaced by coloring oxides. These include a vanadium turquoise stain, among others. The oxides in these stains need to be heated together to produce the stain before adding to a glaze (often termed zirconium inclusion pigments). Stains based on the same system are compatible and can be blended together to produce secondary colors; for example turquoise and yellow combine to make green.
Chrome and tin are combined with calcium and silica to make a chrome-tin pink stain widely used in glazes, although this is not as stable as zircon-based stains. Commercial stains are often stabilized with opacifiers such as tin or zirconium, and are refractory. They opacify glazes and increase viscosity, so glazes sometime appear under-fired. If this occurs, the silica content in the glaze can be reduced. It is not necessary for studio potters to use stains to make greens, as they can easily be obtained using cobalt, copper, and chromium, which dissolve in the glaze, giving transparency and depth. However, stains have some advantages over oxides, including stability and consistency, but they often look flat and opaque.
Excerpted and adapted from Colour in Glazes by Linda Bloomfield. Co-published by A&C Black, London, England and The American Ceramic Society, Westerville, Ohio.