If you’re looking at a “Heini” 黑泥 (Black Clay) Yixing Teapot, chances are you’re looking at a teapot that has either had chemical pigments mixed into the clay or one that has been fired in a low-oxygen (reduction) environment.
Heini Teapots that have been colored by mixing a pigment into the clay are traditionally made by adding Manganese Oxide to clay (usually zini) before shaping and firing the teapot. The chemical additive gives the clay its black color.
Reduction Firing / 焐灰
Firing Yixing Teapots in a low-oxygen environment can also produce black teapots. Firing a teapot in a low-oxygen environment is called “wuhui” 焐灰 which can be loosely translated as “smother with ash to fire.” Typically, the teapot is stuffed with dry rice straw, placed in a clay box and surrounded with more rice straw before being covered. When it is fired in this way, the straw uses the oxygen when it burns to ash, reducing the oxygen in the clay box while the teapot is being fired. This creates the reduced oxygen environment.
Making a Wuhui Heini Teapot
The Teapot is fired first in an oxidation environment at a high temperature (depending on the specific firing requirements of the clay). Once the clay cools and the teapot is removed, it can be fired again in a reduced oxygen environment.
Once stuffed and surrounded by rice straw, the pot is placed in a clay box and the box is placed in the kiln. The temperature of the kiln is lowly raised until it reaches 1000°C. As the temperature increases the straw catches fire, sucking the oxygen out of the environment, releasing CO. When the temperature reaches 900°C the CO will react with the Fe2O3 on the surface of the clay to produce FeO on the clay surface and CO2. The temperature of the kiln is then slowly lowered. Once the temperature drops to between 400 to 600°C, the unstable FeO molecules will react to produce Fe and Fe3O4. Whereas Fe2O3 is red-orange in color, Fe3O4 is black.
The color of a Wuhui teapot will depend on the amount of iron oxide in the clay. Less iron oxide generally means a lighter color clay after being fired this way.
Wuhui fired teapots will have a lot of ash in the pot and on the surface of the clay when they are first removed from the kiln. Wuhui teapots will need to be cleaned more than regularly-fired Yixing Teapots to remove the ash. The water will appear dirty from the ash until it rinses out after many soaks/rinses. Chemically-colored heini will not have this ash dust.