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Monday, 14 January 2013

Never judge a clay by its colour

....or, revealing the iron content of clay by the magic of bisque.
These are the clay samples fired relatively quickly to 1000C and I have to say I'm torn. There are some really gorgeous rich red browns there and a lovely range of colour... but I was hoping for something rather paler. Deep red means higher iron content and therefore less chance of it being a stoneware clay (especially in reduction). Surprisingly the darkest red clay (number 4) was the palest clay in its natural state. Where did that come from? The most promising clay is number 1, which was a green/blue in its natural state. lots of sulphur given off during the firing, so the iron probably present as iron sulphide. I was hopeful of the black clay (number 3) and it is relatively pale, however the test bar has swollen dramatically indicating a failure to burn out all the impurities (such as carbon) from the inside. A slower biscuit firing will solve that but the problem is the vitrification of the clay. A stoneware clay will be very open and porous at this temp. but the black clay has become very dense, suggesting it is approaching its maximum useable temperature. Now for some higher firings with the test rings at first to see....How far can I go?


  1. So the black is an earthenware clay, and the colour of the buff, now red clay is amazing.

  2. Interesting! Could the dark color in #4 have come from yellow iron oxide? I know it's much darker than red iron oxide when fired.

    1. I'm not sure Rosa. the clay was almost white when I dug it - though it got a little contaminated by the sandy yellow clay next to it. There are a few different forms of iron in these clays - notably FeS. The smell of sulphur escaping the kiln during bisque was overpowering!