Zhou Gaoqi, the Purple Clay researcher from the late Ming dynasty, said: “Among clay Teapots, those with crystalline luster are the best; those with a glossy sheen are superior; and those with a monk's head-like shine are of the poorest quality.”

This succinct 25-character statement is remarkably comprehensive. It means: “A Purple Clay Teapot with crystalline points is the best; one with a lustrous sheen is very good; and one that shines like a monk's bald head is the worst.” This is a scientific and effective method for discerning the quality of a Purple Clay Teapot.

Illustrations of crystalline luster, glossy sheen, and monk's head-like shine:

The above images show an equal-grain Purple Clay teapot and a microscopic view. The sand grains are basically uniform, and the crystalline surfaces are relatively large, producing larger, stronger points of light, which are referred to as crystalline luster.

The above images show a Dicaoqing material Purple Clay teapot and a microscopic view. There are large, scattered points of light and small, dense points of light, which visually create a crystalline and glossy appearance.

The above images show a Zhuni Purple Clay teapot and a microscopic view. The points of light are fine and densely packed, creating a visual impression of glossy sheen.

The above images show a microscopic view. Although the surface is uneven, the grains and spaces between them are covered in fatty paste, creating continuous reflective surfaces. As a result, after polishing and maintenance, these teapots become shiny like a monk's bald head, which is referred to as the monk's head-like shine.

Crystalline luster and glossy sheen are distinctions based on visual perception. Large, strong points of light are called crystalline luster, while fine, dense points of light are referred to as glossy sheen. For example, when observing a teapot from 30 cm away, it may appear to have crystalline luster, but from 1 meter away, it might appear to have a glossy sheen. This is due to the different intensities and densities of the crystal reflections perceived by the eye.

The difference between crystalline luster and the monk's head-like shine is that crystalline luster consists of dense, thick points of light, while the monk's head-like shine is a flat shine. When the points of light become increasingly dense, the depth of the shine becomes shallower. When approaching a flat surface, it transforms into the monk's head-like shine.

Purple Clay teapots do not have purely crystalline luster or purely monk's head-like shine.

Crystalline luster corresponds to crystallinity, while the monk's head-like shine is commonly known as vitreous. The more crystallinity a teapot has, the better it is, whereas the more vitreous it is, the poorer the quality.

Conditions for achieving a crystalline and glossy appearance:

Firstly, the raw material must be pure, high-quality sand. Only clay with high-purity sand grains will allow the sand grains to bond firmly, enabling the burnishing tool to effectively smooth the grains. If the clay is of inferior quality, the grains will sink in upon application of the burnishing tool, and the powder-like clay will rise to the surface.

Furthermore, the sand grains must be uniformly sized. If there are many impurities or uneven vitrification, the firing temperatures will differ, resulting in varying colors and making it difficult to achieve a crystalline and glossy appearance. Therefore, teapots with a crystalline and glossy appearance are definitely made from high-quality clay.

Secondly, the craftsmanship must be excellent. Without superior burnishing skills to level the sand grains and remove the fatty clay, it would be impossible to achieve a crystalline and glossy appearance.

Even the best clay without proper burnishing will result in a clay slurry color after firing, or if too much water is used during production (as in slip-cast teapots), the finished product will exhibit a watery shine. Therefore, teapots with a crystalline and glossy appearance are undoubtedly well-made.

Thirdly, the teapot must be fired properly, meaning it is fully fired and vitrified. Purple Clay belongs to the kaolinite-quartz-mica type and contains high-quality granules. A Purple Clay teapot that has not been fired to the point of crystallization cannot have a crystalline and glossy appearance.

A truly high-quality Purple Clay teapot does not look garish and is not glaring under strong light, giving off a warm and smooth feeling. Under a microscope, the reflection of light from the crystalline surfaces would be quite apparent.

Thus, Zhou Gaoqi's method is profound in its simplicity. One only needs to learn how to recognize crystalline luster and glossy sheen. However, what appears simplest can also be the most challenging.