I had hoped that UcA6 would be my final base glaze but there are a number of minor problems with it. Firing Temperature. Ideally I would prefer to fire them all at cone 7 or even 6. The clay is still well within maturation range. Ashraf Hanna fires between 900 and 1280 deg C and James Oughtibridge only takes his large outdoor pieces to 1100. Lower temperatures are better for the environment, kinder to heating elements ( I am thinking about my own kiln here!) and give a slightly lower risk of cracks, though most of those are at the bisque stage. In addition some colours (8red, 22yellow, 11b purple, still look slightly under fired at cone 8 despite adding lithium, whilst 3(teal) needs only cone 7. Rather than fire them at different temperatures I think I need to abandon the idea of using the same base glaze for all. A small change in recipe (UcA7), back towards version 5, with a little more whiting and 1% less china clay has reduced the firing temperature for the yellow glaze; now I will adjust the others as I make more. Colour. 9(Chartreuse) has had a tendency to look a bit thin and brown so I added more yellow stain. Interestingly this does not seem to have affected the firing temperature. 22 (High temp yellow from CTM) has looked really custard like on pieces. As I had to make a new batch I replaced the stain with 'sunshine yellow' from Northern Kilns, reassured by the label stating it fired to 1280. Tin and other oxide content remained the same but I also added 1% bentonite as explained below. these are fired to cone7, 1250 degC Settling All of the glazes settle a bit, but some, especially yellow, purple and chartreuse do it really badly. When discussing a different base glaze, Dave had previously recommended using bentonite 5%, but I found that glaze clumped even as it passed through the sieve. This time I looked it up, and cautiously replaced 1% of china clay with 1% bentonite.
A new pot of purple is required so I have made a further adjustment and used 2%. The test tiles are in the kiln.... I suspect my adjustments to glaze will continue. Its a good job I made plenty of test tiles!
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16/11/2022 20:49:05
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AuthorI am indulging my passion for ceramics by undertaking studies for an MA at UCLAN Archives
August 2021
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