Among the colors that test a ceramicist’s patience, purple at cone 10 oxidation stands as one of the most elusive. Reds and yellows we can coax out of iron and antimony; blues are a reliable gift from cobalt. But purple — a blend of red and blue that the kiln seems eager to unpick — often arrives as a washed-out lilac, a brownish smear, or simply vanishes into the glaze melt. The problem is not the fire itself, but the chemistry of the colouring oxides at high temperature. With careful selection of colorant combinations and a glaze base that supports them, it is possible to produce purples with strength and clarity even in oxidation firing. This guide walks through the known reliable routes and the adjustments that make them work.
Understanding the Challenge
A vibrant purple requires two things: a blue component (usually cobalt) and a red component (manganese, chrome-tin pink, or iron in specific conditions). In oxidation firing at cone 10 (about 1300 °C), several difficulties arise. Manganese, the classic red-purple source, tends to produce brown or grey tones when it dissolves fully in the melt. Cobalt alone is forcefully blue, and its tint can swallow any red companion if the ratio is off. Chrome-tin pinks, often stable at lower temperatures, degrade in many high-fire base glazes because the chromium reacts with zinc or high alumina. Iron can push the colour toward maroon, which may be acceptable but is not a clean purple.
Beyond colorant choice, the glaze melt itself plays a role. A base that is too fluid allows colouring oxides to dissolve completely, diluting the purple. Too stiff a melt can leave streaks or insufficient blending. The key is to balance the chemistry so that the chromophores remain partly suspended as fine particles or crystals, giving the glaze depth and that elusive purple hue.
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Colorant Combinations for Stable Purples
No single recipe guarantees success, but three main approaches have proven reliable in oxidation high-fire. Each requires careful control of percentages and base composition.
Manganese and Cobalt
The most direct method blends manganese dioxide (or manganese carbonate) with cobalt carbonate or oxide. The classic starting point is about 3–5 % manganese dioxide and 0.5–1 % cobalt carbonate in a transparent high-fire base. This yields a medium to deep purple, leaning slightly toward aubergine. Increase cobalt for a bluer purple; increase manganese for a more reddish tone — but beyond 6 % manganese the glaze often turns grey or brown. Firing temperature and soak time matter: a slower cooling can help manganese re-crystallize as manganese-alumina pink, adding back some of the red.
Chrome-Tin Pink Plus Cobalt
Chrome-tin pink stains (calcined blends of tin oxide, chromium, and often silica) can survive cone 10 if the glaze base is low in zinc and not too high in alumina. Add a commercial chrome-tin pink stain (around 5–10 %) together with a small amount of cobalt carbonate (0.25–0.5 %). The result is a clearer, brighter purple than manganese can give, though it may be somewhat pastel if the base is too white. The key is a base with moderate calcium and strontium, and no zinc oxide – zinc kills chrome-tin colour. A good starting base is a standard cone 10 transparent with feldspar, silica, whiting, and kaolin, omitting any zinc.
Cobalt and Iron
For a darker, more wine- or plum-coloured purple, combine cobalt with red iron oxide. Start with 1–2 % red iron oxide and 0.5–1 % cobalt carbonate. The iron provides a deep reddish-brown background, and the cobalt shifts it toward a very dark purple. This combination is more robust than manganese, though the colour is less bright and more suited to matte or semi-matte glazes. A higher alumina base will mute the iron’s brown and push the colour toward aubergine.
Below is a comparison of the three approaches, listing typical additions and the resulting character.
| Combination | Typical Additions (in a clear base) | Resulting Purple | Stability at Cone 10 | Notes |
|---|---|---|---|---|
| Manganese + Cobalt | 3–5 % MnO₂, 0.5–1 % CoCO₃ | Deep aubergine, slightly earthy | Good with careful cooling | Avoid overgrinding; can go grey |
| Chrome-tin pink + Cobalt | 5–10 % chrome-tin stain, 0.25–0.5 % CoCO₃ | Bright, clear purple, may be pastel | Good in zinc-free base | Requires stable stain; test for leaching |
| Cobalt + Iron | 1–2 % Fe₂O₃, 0.5–1 % CoCO₃ | Dark wine-plum, opaque | Very stable | Best for matte glazes; less vibrant |
Adjusting the Glaze Base
The base glaze’s chemistry influences how colouring oxides behave. For purple, a few adjustments are particularly important.
Alumina and Silica Ratio – A higher alumina content (e.g., from added kaolin) makes the melt more viscous, reducing the dissolving of chromophores. This helps keep manganese particles suspended, producing a more mottled, saturated purple. However, too much alumina (above a 1:6 alumina-to-silica ratio) can make the glaze opaque and dull. A balance around 1:5 to 1:7 works well.
Calcium and Strontium – Calcium supports the development of pink and red tones from manganese and chrome-tin. Strontium, often used to replace calcium, can brighten colours and reduce the tendency to haze. A small replacement of calcium with strontium carbonate (2–4 %) can improve clarity in chrome-tin purple glazes.
Zinc – Omit It – Zinc oxide is common in many high-fire glazes to promote matteness or to adjust thermal expansion. In purple glazes, however, zinc can discolour manganese (turning it brown) and destroy chrome-tin pink entirely. If your chosen base contains zinc, either find an alternative or test the purple with a small batch first.
Tin or Zircon as Opacifier – A small amount of tin oxide (2–4 %) can help whiten the base and make a clear purple stand out. Zirconium silicate is cheaper but may produce a colder, slightly blue-tinged white that can shift the purple toward violet. For bright purples, tin is preferred.
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Application Techniques for Depth
Even with a well-formulated glaze, the way it is applied can make the difference between a flat, muddy surface and one with luminous depth.
Layering Underglaze – One of the most reliable techniques for bold purple is to apply a commercial or homemade purple underglaze (slip) to the bisque ware, then cover it with a clear high-fire glaze. The underglaze provides a concentrated layer of colour that is less likely to dissolve into the glass. Use two or three coats of underglaze, letting each dry thoroughly. Then apply a thin to medium coat of clear glaze. The purple will appear more saturated and less likely to shift toward brown.
Double Dipping – Instead of a single layer, try dipping the piece first in a lighter purple glaze, then a darker one. The overlap creates subtle gradations. This works especially well with a base that is not too fluid, so the layers do not fully blend.
Application Thickness – Purple glazes often benefit from a slightly thicker application (but within the glaze’s recommended range). Too thin, and the colour appears washed out. Too thick, and you risk running or blistering. Weigh test tiles to find the dry thickness that gives the best colour.
Controlling the Firing Atmosphere – Although this guide focuses on oxidation, a slight reduction at cone 10 can actually help manganese purples by reducing manganese oxide to a pinker state. However, if you strictly fire oxidation, a slow cooling (soaking at 1100°C for 30 minutes, then natural cool) allows manganese to re-form as an alumino-silicate pink. This recrystallization enriches the red component and can dramatically improve the purple.
Common Mistakes to Avoid
- Overgrinding the glaze – Colourant particles should be fine but not dust. Over-milling can cause complete dissolution into the melt, killing the purple. Grind just until the glaze passes through an 80-mesh sieve.
- Contamination from other glazes – Even tiny amounts of copper, iron, or chromium can muddy purple. Thoroughly clean mixing tools and sieves between batches.
- Ignoring the clay body – White stoneware or porcelain gives the truest purple. A buff or dark clay body will darken the colour considerably. If you must use a tan clay, consider a layer of white slip under the glaze.
- Assuming commercial stains are interchangeable – Different brands of chrome-tin pink stain have different calcination temperatures. Test each new batch before relying on it for a glaze.
Photo by sina jahadi on Unsplash.
Final Thoughts
Developing a reliable purple for cone 10 oxidation is a matter of patient experimentation. Begin with the manganese-cobalt or chrome-tin-cobalt approach, using a simple base. Keep meticulous notes on percentages, mixing methods, and firing schedules. A single variable — such as a 0.5 % addition of strontium or a change in cooling rate — can shift a disappointing brown into a satisfying violet. The effort is rewarded with a colour that, once pinned down, becomes a signature in your work. As with all high-fire colour, test, adapt, and respect the firing. The kiln will teach you more than any recipe book.