• by vinux

The main reason for this difference in color is due to the differences in firing temperature and time. The higher the temperature and longer the time, the darker the clay material will be. Different amounts of raw materials used to make up a batch of clay also has an effect on its color. This can be seen with purple sand, where different types of clays, minerals, and other materials can be mixed together to create a unique hue or pattern when fired. Additionally, certain glazes and different techniques used during firing can also contribute to variances in color.
During the process of purple clay firing, a series of physical and chemical changes occur in the pot blank, its shape is fixed and its performance is exerted, which is called burning.

The prepared blank should be left for two or three days to remove moisture (otherwise it may burst during firing due to moisture in the blank), and then put into the kiln for high temperature calcination under certain gas medium.

Electric kiln firing of purple sand

Throughout the hundreds of years of pottery history, the firing process of purple sand pottery has been constantly improved and perfected. Before the 1950s, Yixing Purple Sand Pottery was fired using Dragon Kilns, then gradually changed to inverted flame kilns, and now usually uses tunnel kilns and electric kilns.

Firing is an important process in the production of purple clay wares. The temperature and atmosphere during firing will affect the colors and density of the finished product, hence a large amount of experience and excellent techniques are required from the firing personnel in order to produce outstanding works.

The bottom trough shows the color effect of different temperatures.

The main process of burning porcelain is to sinter it. Sintering of zisha pottery refers to the physical state under a certain high temperature, when the low melting point substances melt and fill the gaps between the non-molten substances, without reaching the degree of carbonization (coking) and calcium dissolution (pinholes or bubbles appearing).

The sintering temperature range is from the melting point of low melting point to the temperature range at which bubbles, needle eyes, and carbonization phenomena appear. The lower limit of sintering temperature for purple mud is between 1150-1180°C, for red mud is between 1050-1100°C, and for Ben Shan green mud is between 1180-1200°C.

Some high-quality mud materials have a wide sintering temperature range (they won’t spout black after firing), which can be fired at more than 1300°C and even more than 1400°C, such as Huanglongshan No. 4 well bottom groove clear mud, sky blue mud, etc.

The No. 4 Well Bottom Chamber was cleared with a temperature of over 1300 degrees Celsius.

The 4th well bottom groove has more than 1300 degrees of high temperature.

More than 1300 degrees of high temperature in the autumn rhyme of clear sky and mud.

The teapot 1420 Degrees is fired at high temperature with the charming rhyme of sky blue mud.

There is a large amount of ferrous precipitation on the surface, resulting in kiln changes.

Whether the purple clay pot is fired will not only affect practical factors such as body strength and air permeability, but also affect the true quality of purple sand materials, such as color effect, grain feeling, and sand network veins. If the temperature is not controlled properly, there will be flaws of underfire and overfire.

Overfire is when the kiln temperature exceeds the temperature that the clay body can bear, causing the body of the vessel to deform and bubbles to appear on its surface. If the kiln temperature is too high, the pottery body will be deformed into scrap, which is commonly referred to in the industry as “kiln fired white light”.

The teapot that was overheated.

Having discussed the burning temperature, let’s now look at the burning atmosphere. Burning atmosphere refers to the concentration of oxygen and carbon monoxide gases in a burning environment.

In short, under the same firing temperature (of course it must be higher than sintering temperature), more oxygen is an oxidizing atmosphere; more carbon monoxide is a reducing atmosphere. Firing mainly takes place in an oxidizing atmosphere.

The left picture shows an oxidizing atmosphere burning into a weakly reducing atmosphere burning on the right.

The same clay presents different colors under different firing atmospheres. For example, purple clay appears in shades of pink, purple-brown and dark purple in an oxidizing atmosphere, while in a reducing atmosphere, it has various shades of blue-black.

Under an oxidizing atmosphere, the iron in purple mud produces red-brown ferric oxide, while under a reducing atmosphere it forms a bluish-black ferrous oxide.