I felt my heart sink when I went to open the kiln this morning. Instead of a rainbow of bright colours – lime greens, turquoises, salmon pinks and cobalt blue, I found my series of white earthenware thrown plates had all turned a yellowish off-white. Checking back over the glazes I’d used I realised I’d made some mistakes with the calculations when I tried to double the ingredients. I’d added 1% of coloured stain to the new glazes instead of 10% to the base glaze.
I should have realised something was off by the pale colour of the glazes in liquid form. I was hoping to display these colour experiments on the wall for next week’s corridor exhibition but I’ll have to think of something else instead. The firing itself didn’t really go to plan either. The first time I though I’d put the kiln on, I came back in the morning to find the kiln still on 50C. I hadn’t pressed and held the start button down to begin the program!
Hopefully I’ve learnt a lesson to keep neater notebooks so I’m not cramming illegible glaze recipes into every area of free blank space.
I wanted food-safe glazes to decorate a series of white stoneware mugs I made last month and decided to use a celadon base glaze then adjust the percentages of iron oxide and tin oxide to get a line blend. Celadons originated in Japan and the jade colour is caused by iron oxide being starved of oxygen in a reduction atmosphere during firing.
The original A60 base celadon glaze (1) uses the recipe below. It fits the body well and is a grey-turquoise satin colour.
CORNISH STONE 56
BALL CLAY 20
(RED IRON OXIDE 1.5)
(2) 1.5 IRON OX. & 1 TIN OX.
I thought the addition of tin would make the glaze more matte, but the effect here is the opposite, the colour becomes thinner, more watery. Brown speckles.
(3) 2 IRON OX.
My favourite mug, the addition of 0.5g more Iron ox. to the original recipe gives a slightly darker more yellow-green than blue-green.
(4) 3 IRON OX.
With double the iron oxide in the original recipe, the glaze becomes less smooth and the surface begins to break apart a little. This is a much darker, more ‘forest’ green than the others, breaking to dark brown where thin.
(5) 1.5 IRON OX. & 2 TIN OX.
Again the addition of tin has made the glaze less transparent but the speckling is attractive. Bluer.
(6) 2 IRON OX & 2 TIN OX.
Strangely, despite being the most different mixture to the original recipe, this one looks most similar to (1). The more iron in proportion to tin, the fewer speckles.