Recently, a Teapot enthusiast contacted me, saying that they accidentally broke a Purple Clay Teapot recently, only to find that its color differed on the inside and outside. This enthusiast has been quite worried ever since, fearing it might be a chemically-made teapot, which they had been using for some time.

Is this due to substandard clay, or is it indeed a chemically-made teapot? This actually has a lot to do with the firing process of the purple clay teapot!

The firing temperature for purple clay Teapots ranges from 1050° to 1200°. Before firing, the purple clay artisan will determine the upper and lower limits of the firing temperature based on the coarseness of the clay particles. During firing, the temperature must be strictly controlled within this range, following a predetermined temperature curve.

As the firing temperature changes, the color of the teapot body may vary in shade. Even though the temperature is strictly controlled, during the firing process, the surface and interior of the teapot are exposed to different degrees and densities of heat, leading to uneven softening.

This can result in subtle differences in the appearance of the teapot body, with variations in color between the exterior and interior.

The crystallization process of the teapot body during firing often goes through several stages:

1. Evaporation of Combined Water

When the kiln temperature reaches 105°C, the combined water in the teapot body begins to evaporate. This temperature must be maintained for 30 minutes before further heating, otherwise, the teapot body may crack.

2. Release of Structural Water

When the kiln temperature steadily increases to 500°C, the structural water in the teapot body begins to decompose and release.

3. Expansion of Quartz

At 573°C, the quartz particles in the clay expand, increasing in volume by 0.84% and changing crystal form.

4. Oxide Coloration

Between 600°C and 1050°C, the oxides in the clay decompose, and the teapot body begins to take on color.

5. Insulation for Stabilization

Between 1050°C and 1200°C, the teapot body stabilizes, and heating stops while maintaining the temperature for around 30 minutes to balance the temperature difference within the kiln.

6. Cooling for Color Development

After the kiln temperature stabilizes, all flue gates and doors are opened to rapidly cool down to about 700°C before closing them again. This results in a glossy finish on the surface of the teapot, which remains in a molten state without cracking.

7. Contraction of Quartz

Because quartz undergoes another transformation at 573°C, contracting by 0.84%, the temperature must decrease slowly and evenly between 700°C and 400°C. Otherwise, the teapot is prone to cracking.

8. Cooling and Removal from Kiln

Once the temperature drops below 400°C, cooling can proceed more quickly as temperature differences no longer negatively affect the finished product.

During the firing process of purple clay teapots, “overfiring makes them old and unattractive; underfiring leaves them raw, resembling sandy soil.” As the teapot is used, the effects of differences in firing temperature become increasingly evident, one aspect being the color of the clay.

Differences in the “blackening” effect, the type of Tea brewed, and teapot maintenance habits can also lead to varying degrees of “blackening.” In some cases, the difference in color is more noticeable, with parts or the entire teapot darkening, exacerbating the inconsistency in color between the exterior and interior.

In summary, if the color of a purple clay teapot differs inside and out, or if the color is uneven, it does not necessarily mean that the clay is substandard or that the teapot is chemically made. It is everyone's responsibility to approach teapot appreciation, use, and maintenance rationally.

Source: Purple Clay Teapot Encyclopedia, information shared here should be credited, please contact us to remove if copyright issues arise.