Zi Sha ore belongs to the class of sedimentary rocks, which form through a gradual layering process. During mineral deposition, minerals with higher iron content and coarser particles tend to settle first. Therefore, in the same ore layer, the upper part has relatively lower iron content and finer particle composition, while the lower part has slightly coarser particle composition and higher iron content. This is why there is a distinction between “old” and “tender” within the same Zi Sha ore.
Huanglong Mountain formed 200 to 400 million years ago, so the deeper layers of ore are approximately 200 million years older than the surface layers. The Zi Sha ore undergoes a gradual petrification process, meaning that the longer the time, the more abundant, larger, and harder the particles become – this is what we refer to as “older.”
The clay from the lower part of the same ore layer is older than that from the upper part. Among different ore layers, the deeper ones yield older Zi clay.
However, “old” and “tender” are relative terms; there is no absolute standard for oldness or tenderness. For example, Hong Pilon (Red Skin Dragon) belongs to the surface layer of purple clay, whereas Di Cao Qing (Bottom Groove Clear) comes from the bottom layer. The tender part of the upper layer of Di Cao Qing is much older than the lower part of the older Hong Pilon.
Duan clay is a mixed ore, and within a single ore layer, there is no distinction between old and tender. However, there is an age difference between deeper and shallower layers.
Red clay forms a deposit weathered by wind and rain, and most of its layers are relatively thin. Within a single layer, the distinction between old and tender is not obvious. Red clay typically resides at the very top layer of the mine, and although it does have a distinction between old and tender, its formation pattern differs from other types of clay.
The layers of green clay are too thin for a noticeable difference between old and tender within a single layer, though there is a distinction between deeper and shallower layers, albeit not very pronounced.
Therefore, we have “old” Zi clay, “old” Duan clay, and “old” red clay, but not “old” green clay.
Zi clay becomes older the deeper the layer, so the difference between old and tender Zi clay essentially lies in the depth of the ore layer. Old Zi clay has relatively higher iron content and a greater proportion of particles, thus requiring a higher firing temperature.
During firing, iron undergoes oxidation-reduction reactions, producing ferric oxide (red-brown) and ferrous oxide (black). The higher the firing temperature, the more reduction occurs, resulting in a darker color for the finished Zi Teapot.
Which is better, old Zi clay or tender Zi clay?
In general, deeper layers of Zi clay are considered better, but depth cannot be the sole criterion for judging the quality of Zi clay; it is merely one of several reference points.
However, Duan clay, green clay, and red clay do not follow this rule. Gold Duan (Golden Duan), Zhu Ni (Red Ochre), and other varieties are all surface-layer clays, so the depth of the ore layer is only particularly important for Zi clay. This is why most surface-layer Zi clays are entry-level ores for Ben Shan Zi clay, while deeper Zi clays, especially Di Cao Qing, are considered premium ores. There are exceptions, such as the legendary Tian Qing Ni (Sky Blue Clay), which belongs to the surface layer.
Moreover, if one were to claim that deeper is always better, some older clays might be too sandy, lacking plasticity, and producing a dry texture after firing, with more white mica flakes. Some people pursue old Zi clay and prefer darker Zi Teapots. The market capitalizes on this preference by adding manganese to Zi clay to darken the pot's color. Therefore, any standard must be applied with moderation, and the quality of clay cannot be judged solely based on one concept.