These are made from Plainsman Polar Ice translucent NZ porcelain. The one on the right was used in the coffee room of the plant and washed between uses in a common manner (which is: not very much!). The stains are obviously not nearly as visible on a stoneware mug.
Pages that reference this post in the Digitalfire Reference Database:
This post is one of thousands found in the Digitalfire Reference Database. Most are part of a timeline maintained by Tony Hansen. You can search that timeline on the home page of digitalfire.com.