Firing Process of Yixing Zisha Teapots

The grey-smothered firing technique for Yixing Zisha teapots is a special firing method that creates artificial kiln transformation effects by introducing external media. It enables uniform colour changes on teapots during firing. When firing in a dragon kiln, incompletely burnt thatch residues and charcoal ash keep accumulating, especially in the lower and bottom sections of the kiln. This creates a local mixed reducing atmosphere, which triggers colour transformation on the teapot body. After continuous experimentation, observation and summarization of rules by later craftsmen, this natural kiln transformation phenomenon was gradually mastered and applied. In the early grey-smothered method, teapots were placed inside saggars, filled with rice husks and other materials both inside and outside the pot body, then tightly sealed. The whole set of saggars was placed at the very bottom of the dragon kiln for firing.

Since the natural mixed reducing environment at the bottom of dragon kilns is difficult to replicate, fine coal powder, wood chips and other carbon-rich fillers are mostly placed inside saggars today to artificially create a local reducing atmosphere. Low-temperature firing (approximately 1100–1150°C) is adopted to strengthen the carbon adsorption capacity of the clay body surface.

Zisha raw material has strong adsorption properties and contains abundant organic matter and carbon components, which do not burn completely in a reducing environment. Furthermore, when the temperature ranges from 400–600°C and ferrous oxide exists, carbon monoxide decomposes violently. The carbon generated by the reaction gets adsorbed onto the surface of the clay blank. This adsorbed carbon delays oxidation and only fully burns out in the late firing stage and early cooling phase. This is the main reason why the sintering temperature of grey-smothered ware is lower than that of conventional firing. The densely packed carbon-rich filler inside the sealed saggar reduces free oxygen from combustion products within the saggar and isolates the teapot body, keeping the Zisha teapot steadily in an optimal local reducing atmosphere. Carbon from the filler continuously penetrates the clay body to enhance colouring effects. Due to differences in the chemical composition of various Zisha raw materials, as well as variations in firing temperature and atmosphere, finished grey-smothered pieces exhibit diverse layers of blue-black tones.

Principles of Grey-Smothered Firing

The grey-smothered craft mainly uses high-iron Zisha clay as raw material and fires it under a reducing atmosphere to alter the structural form of iron oxide in the clay. During conventional firing, iron oxide in Zisha mostly exists in the crystalline form of hematite. Under reducing firing conditions, hematite is reduced to magnetite crystals. For this reason, finished grey-smothered teapots have slight magnetic attraction on their surface and can be weakly pulled by a magnet.

Low-iron Zisha clays such as green clay are generally unsuitable for grey-smothered treatment. After firing, they usually produce a dull, lifeless pale black colour with poor visual appeal. If a grey-smothered Zisha teapot is refired in a neutral or oxidizing atmosphere kiln, it will gradually revert to its original colour. As early as the Ming Dynasty, the famous Zisha artisan Chen Zhongmei already used sand blending and sand inlay techniques to enhance the decorative beauty of Zisha wares.

Sand Blending Technique

According to design requirements, sand grains of specific particle size and proportion are mixed into kneaded clay or clay powder to enrich the granular texture layers of the clay body. If the blended sand grains are from the same mineral source as the base clay, it is called same-colour sand blending. If sand grains come from different mineral deposits with contrasting colours, it is known as contrasting-colour sand blending.

When same-colour sand is mixed into the base clay, consistent material properties prevent obvious colour difference after firing. It mainly reinforces the skeletal structure of the clay body and improves the strength of the clay blank. Soft sand grains must first undergo biscuit firing at 600–800°C for solidification; otherwise, they tend to be crushed by tools during shaping and leave trailing streak defects on the surface.

For example, Zhuni clay has a high shrinkage rate and deforms easily, making it hard to produce large vessels alone. Sand blending effectively increases the strength of the raw blank, reduces overall shrinkage and deformation rates, and raises the yield rate. For this reason, most modern Zhuni teapots are mixed with coarse sand grains or stabilized with pre-biscuit-fired clay powder.

Contrasting-colour sand blending creates rich visual effects due to differences in hue and texture between sand grains and base clay. Hard blended sand grains protrude slightly after firing, forming delicate pear-skin texture. Softer sand grains leave tiny pits on the surface; the softer the grain, the more obvious the pits, presenting a natural orange-peel texture.

Sand Inlay Technique

Sand inlay is performed while the Zisha clay blank retains moderate moisture during shaping. Sand grains of different colours are sprinkled, dotted or scattered onto the blank surface, then lightly pressed with tools to embed the sand shallowly into the exterior of the clay body.

Sand inlay focuses primarily on visual decoration. After firing, contrasting sand grains complement the base clay colour, forming strong colour layers and striking aesthetic contrast.

Sand grains used for sand inlay must have density and hardness equal to or higher than the blank particles. Overly soft sand will easily cause dragging marks during fabrication. Large shrinkage gaps after firing lead to poor bonding with the base clay and compromise quality.

Sand Sieving Technique

Sand sieving is the process of separating and screening particles of specific mesh sizes from ground clay powder or clay slip.

Example: Sift 60–80 mesh fine sand from 40 mesh clay powder. The remaining coarse particles and fine clay form an interleaved coarse-and-fine tactile texture after firing, creating unique and rich surface grain details.