We asked people if they cared to guess
what finished color the glazed but un-fired bowl on the left would be when it came out of the kiln. (We offered a 15% discount coupon code they could use or giveaway if they got in the right color family). We said that the primary colorants used were red iron oxide and cobalt carbonate.The replies were in each instance, “Black”. The actual result is a blue/blue-green hue.
This is only a mild case of difference between the raw glaze color and the fired color and we’ll have more examples again. The thing to realize, if you care about this sort of thing, is that glazes are a combination of minerals which combine to form compounds and it is the interaction of the minerals with one another that determine color. It has nothing to do with colors as we normally think of them as a consequence of light. You can take a green glaze and overlay it with, say, a blue glaze and you will probably get a color that is blue-green but only because you are seeing both those glazes, one on top of the other. If you combine those glazes and apply the combined glaze, you will likely not get a blue-green because the minerals will interact to form some completely different, and consequently differently colored substance.