Purple Clay, also called purple sand or zisha, is not a clay at all but a rock that is ground, filtered and made into a paste by adding water. This rock is very rare, but there are several deposits of it near Yixing City. The resulting paste is called Zisha Sand and is used primarily to make teapots. The sandy particles in the paste create a porous surface when it is fired that is ideal for making tea and also for taking paints, patterns and etching.
Instead of being made from one solid piece on a potter's wheel, the artists of Yixing assemble their teapots piece by piece. Each part of the teapot is made separately using several different tools such as wooden mallets, metal knives and small spoons or paddles. The spout, body, handle, base and lid are all shaped by hand and with selected tools. Attention to balance, form and geometry are an essential components of this technique -- otherwise the finished teapot will have a design flaw.
The people of China claim that teapots made from zisha are the best for making tea. This is due to a combination of the special properties of the purple clay and the techniques of assembling the teapot in a perfectly balanced design. The tea breathes easily in the porous surface, resulting in the perfect brew. Older teapots that have been used many times have soaked up so much flavour they can be used to make tea with only hot water and no dried leaves.
The techniques and traditions of Yixing carving were developed and perfected by certain famous masters. Ren Banting, also known as Foushou, was proficient with knife carving and adorned many zisha teapots with poems and inscriptions. Pei Shimin made zisha figures and implements like plates and ink blotters out of the paste as well. His work was so refined his pieces were compared to similar implements of bronze.