I thought it was just a fluke the first time this happened. I was at a group pit fire last year where I had seven pieces out of a total of thirty some pots in all. Most of my pieces turned out beautifully, so I wasn’t too upset when I noticed that one of them did not have any colors at all — only black and various shades of gray.
Well, it must have been in a corner where we forgot to sprinkle copper carbonate; and the copper mesh and salted raffia wrapped around the piece must have gotten knocked loose by falling logs too — so I thought to myself, and promptly forgot about the whole thing.
Then I started to do a lot of firings in my little backyard pit, and much to my chagrin, these all-black or black-and-gray rocks also began to crop up a lot more frequently (figure 1 above, and figure 2 below.)