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Underfired bisque and Black Specks: The cause and cure
by Teresa Desantis
The term “mildew” is applied to black specks which show up during the china paint firings on porcelain bisque. Mildew specks generally have a grayish hue and don’t seam to have a distinctiveness of specks which are due to slip contamination. Mildew is generally limited to the area in which the china paint was applied and its immediate edges. So far, mildew has only been documented on bisque pieces which do not have glaze on them.
There are two basic types of mildew:
- Mildew which is evenly distributed in the china painted areas. Two current theories of why this occurs are: that slightly underfired bisque creates mildew OR that too rapid a firing creates mildew. One school of thought explains that underfired bisque creates mildew by trapping carbon from the media or the china paints in its pore structure. A second school of thought believes that a long firing cycle is necessary to prevent mildew with an especially slow rise in temperature in the range of 1200-1800 degrees Fahrenheit.
- Mildew which is limited to areas which have been touched by the dollmaker (e.g. in areas on the rim or neck of the head which were held while china painting). This latter type of mildew is uncommon. At least one study has been done in relation to this type. According to this study, some people poorly metabolize foods high in sulphur. The excess sulphur is then excreted through the skin onto the head while painting. The cure for this type of mildew is to wear latex gloves while painting and/or to abstain from hoods high in sulphur (e.g. garlic).
Cures for mildew:
- Some dollmakers use a caustic compound to remove china paint from the head and then refire the piece to bisque. (One brand is called “Wink”) Seeley’s does NOT endorse this technique, as working with the caustic material is very dangerous and can be damaging to the skin. Look below for better solutions.
- Bisque fire again and repaint the piece. If you have a very light overall wash on, you may be able to refire the piece to bisque to remove the mildew. After the bisque fire, sand the piece well, and use Matter in your china paint to compensate for the slightly glossy surface of the bisque.
- Start with a fresh bisque piece. If your original bisque piece was slightly underfired, adjust your kiln so it will be firing properly.
- Try an extra firing. Some dollmakers report a lessening in mildew with an additional china firing upon completion of a piece. Try a long and slow firing to one cone below your preferred china fire cone.
- If you notice any black specking which does not seen to be attributed to any of the above types, please let us know. We would be happy to work with you to determine the cause of your problem.
If for any reason your porcelain is contaminated, you may see black speck on the inside of a bisque piece. Specks on greenware or soft-fired ware are most likely “lignites”, a natural occuring organic material which is present in all clays. Lignites help give the slip some of its plasticity. Lignites are burned up in the firing.
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