Book: Ceramics - Art or Science? Author: Dr. Stan Jones

11. Pottery Technology 2

1061 Page: 301 of 418  Go To Page:
Click to Go To the Specified Page
◁◁ First ◁ Previous Next ▷ Last ▷▷

If hard-paste porcelain had a first reduction firing to biscuit at 1400 degrees C, then was glazed with a low temperature bright glaze having a relatively high expansion and refired, the low temperature glaze would shrink more than the body and craze. With a high temperature, low expansion, less brilliant glaze, the first firing can be lower but the glazing firing has to be above 1400 degrees C to avoid damage to the glaze called chittering caused by its excessive compression. The latter process is how hard-paste porcelain is fired and glazed today.

Hotel ware has a high coefficient of expansion because of its high, undissolved quartz crystal content, so it needs a high expansion glaze. Low temperature glazes are high expansion so this ware is fired at 1100 to 1250 and the glazing or “glost” firing is 1000 to 1100 degrees C. If this ware had a high temperature glaze and high temperature second firing, the glaze would put the body under excessive tension that could disrupt it to the point of causing it to break up during cooling or shortly after.

As we have discovered previously, underglaze colours are limited in range, as prolonged heating causes most to fade. Also overglaze enamels are vulnerable to scratching and wear, so cannot withstand dishwashers. However, modern techniques of “fast firing” of overglaze colours at a higher than usual temperature causes them to sink into the glaze and protect them from wear, and the shorter time at the higher temperature reduces the fading of the conventional metal oxide colours. Subsequently a range of higher temperature (900-1000 degrees C) overglaze enamels and special gold preparations have been developed to suit these temperatures.

11.8 Decorating

Colour for decoration is provided by metallic oxides that are mixed with refractory material such as alumina, quartz or kaolin for higher temperatures, or a flux for lower temperatures.

Egyptian pot containing pigments prior to grinding UC59746 - Copyright of the Petrie Museum of Egyptian Archaeology, UCL

Egyptian pot containing pigments prior to
grinding UC59746 - Copyright of the Petrie
Museum of Egyptian Archaeology, UCL

The high-firing underglaze colours used on stoneware and porcelain usually give more sombre colours compared with the lower temperature overglaze enamels. Varieties of iron oxide can give a range from greys, greens, lavender blues, browns and yellows, depending on the composition and firing conditions. Black is produced from a mixture of iron and manganese oxides. There are two “bright” exceptions, cobalt for blue and copper for red, but they need very careful reduction firing conditions. The copper is used to produce the flame-like “Sang-de-Boeff” decoration. Since the 19th century uranium has been used for yellow, orange and red, but after 1940 the supply was cut off for the atomic programme. Now depleted uranium is used. Chromium compounds can produce red, brown and green.

Painting was entirely carried out by hand before the introduction of transfer printing, and even now hand painting is carried out to embellish transfer printed items or to decorate special or low volume pieces.

Hand painting pottery. Image courtesy of the Potteries Museum and Art Gallery, Stoke-on-Trent

Hand painting pottery. Image courtesy of the
Potteries Museum and Art Gallery,
Stoke-on-Trent

Interestingly in the early days of soft and hard-paste European porcelain there were often small firing faults and blemishes on the surface of the pot. These were covered by careful placement of small painted motifs over the blemish, usually small insects or flowers, typically seen on early Meissen and Chelsea wares.

Blemishes hidden by painting, tip of berry in Chelsea gold anchor cup and small leaf in Derby barrel mug

Blemishes hidden by painting, tip
of berry in Chelsea gold anchor cup
and small leaf in Derby barrel mug

Another interesting feature of overglaze enamels, particularly earlier ones, is that some of them change colour on firing, so artists have to use considerable skill to achieve the required final colour. In particular, getting details such as the skin colour of a face must have required great experience. This problem also existed in painting Majolica. Today it is made a little easier as stains are used in paints to give closer to true colours.

Page: 301 of 418  Go To Page:
Click to Go To the Specified Page
◁◁ First ◁ Previous Next ▷ Last ▷▷


Author: Dr. Stan Jones  © Copyright 2010 -
   Copyright © 2005 - 2021 Eic Content Management System Version 5.0 from Edge Impact Websites www.edgeimpact.co.uk