Perhaps the most coveted of all kinds of clays for Yixing teapots is zhuni. Information on zhuni is very thin in English, and often they contain questionable or conflicting viewpoints. What exactly is zhuni anyway? I don’t pretend to know all the answers, but I do have some theories and ideas from observations as well as written and material sources. I’m going to try to see if I can get at least a few things straight here. I will try to make this a multi-part posting, since it is too much material to cover in one go. Comments are more than welcomed.
First of all, let’s start with the name. Zhuni 朱泥 is, literally, vermillion clay. Vermillion is really a orangy reddish colour, not quite red in our normal sense of the word, but somewhere in between. It’s certainly not a darker red such as crimson or even maroon. Strictly speaking, according to old sources from the Ming and Qing dynasties, vermillion clay only comes from one place, Zhaozhuang 趙莊 mountain in Yixing. There are two types — soft huangni, and stone huangni. The difference between the two is that the soft kind has been exposed to weathering, whereas the stone variety has not and is still in pretty solid form, and needs extra processing to get it to workable condition. The stone variety, when fired, is what produces the vermillion colour.
I think it is already pretty obvious from the above description that not all red clay is zhuni. In fact, red clays of all stripes exist, but only one kind should be called zhuni. There are those out there who will say “anything red can be called zhuni” or some such; that’s simply incorrect. It’s like saying all green teas are longjing — that’s simply not true.
However, because you never really use 100% pure zhuni to make pots (for structural reasons, I’m told) there is always a mix going on, and the mixture you use and the formula involved, the preparation work done, and the firing procedure all influence the eventual colour of the clay, colour alone is not a reliable and definite indicator. It does, however, give you some sense of what’s going on. These days a lot of so-called zhuni pots have a very dark reddish colour — that’s usually a good indication of the use of some pigment to achieve such a colour, rather than a natural outcome of the firing process. It all depends, of course, but that’s at least a start.