Stone yellow mud also known as goethite, is a type of natural coloring mineral with a remarkably hard texture and high iron content. It is generally found in interlayers of soft clay but in relatively small quantities.
This mud typically appears in the form of egg-shaped, round, cylindrical, or grape-like nodules of varying sizes, with the egg-shaped nodules being the most representative. The outer shell is very hard, resembling iron, and has a yellowish-brown color, while the core resembles egg yolk, hence the name "Stone yellow."
The iron content of this mud mineral is high, but its firing temperature is relatively low. Pure Stone yellow mud is difficult to shape into zisha teapots and has a low yield rate, so it is often ground into a slurry and combined with other minerals to improve the color.
Some zisha clays have a color after firing that lacks vibrancy and appears dull. In such cases, a small amount of this mud is added. By incorporating natural Stone Yellow into certain zisha clay materials, the firing temperature can be lowered, resulting in enhanced color and smoothness.
