A teapot enthusiast asked the other day, “Why are purple Clay Teapots called purple clay teapots?” What does the term “clay” specifically refer to in this context, and what role does it play within the teapot?
1: What Exactly Does the Term “Clay” Refer to in Purple Clay?
1. Quartz particles (as shown by the white dots in the image below). This is according to academic sources;
2. The physical cohesion of purple clay, as per experienced potters.
When discussing these concepts with others, it's essential first to clarify which definition you're using. Otherwise, conversations can become heated, with experienced potters criticizing academics for not understanding the craft and academics regarding potters as ignorant. The “double porosity structure” of purple clay is closely related to its quartz content. The “purple” refers to color, while the “clay” denotes its nature.
When a purple clay teapot breaks, you can examine its fractured surface to clearly discern its unique structure. During firing, carbonate ions decompose, releasing carbon monoxide and carbon dioxide, forming bubbles. These bubbles can be interconnected or isolated, creating a dense “chain-like cluster of pores” that open up, or they can become trapped between quartz and silicate particles, preventing interconnection and thus remaining closed. The coexistence of open and closed channels is known as the “pore-void structure” or, more commonly, “dual porosity.”
Observing the cross-section of a fired purple clay teapot reveals open pores between cohesive bodies where gases can pass through, and closed pores within cohesive bodies that block water.
Due to the surface tension of Tea, the liquid cannot penetrate these pores to reach the teapot's exterior. However, since gas molecules are much smaller than water molecules, water vapor can permeate through these pores. This is often referred to as “breathable but not permeable to water,” a unique characteristic of purple clay teapots.
2: What Role Does “Clay” Play in Purple Clay Teapots?
(1) Decorative Function, typically through the application of sand (mature sand) technique.
This is somewhat akin to sprinkling sesame seeds on a bun. Sesame seeds enhance the taste of buns; “sesame seeds” add visual appeal to teapots. However, the sand application technique should not be overused. If improperly executed, it can detract from the natural style of the purple clay teapot.
If the distribution of sand is too regular or too chaotic, it doesn't embellish the teapot but rather mars its Beauty. As shown in the image below, the Purple Jade Golden Rhombus Teapot is clearly more aesthetically pleasing.
(2) Increasing Firing Temperature to Achieve Desired Coloration.
The color and degree of vitrification are two opposing factors in the production of purple clay. At a certain temperature and under specific firing conditions, purple clay will exhibit a particular hue.
However, an issue arises when the color is excellent but the teapot is not fully vitrified. This would not be considered a finished product because a purple clay teapot must fulfill practical functions: it must be breathable but not permeable to water and have sufficient body strength (resistance to breaking). Here, the addition or reduction of sand plays a crucial role. Increasing the firing temperature involves adding sand; decreasing it means reducing sand.
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