January 28, 2013 § 13 Comments
Writing the date for the heading of this post I can’t quite believe it, very nearly February already… I think it’s age, time is flying so much faster these days. I seem to be growing quickly into my workshop and it has a really good feeling about it; I feel happy there, at home, and productive. I have been working on my glazes.
Last year, I had three different glaze bodies for my three different colours. For a number of reasons, including the overall consistency of the range, I have decided to develop my colours out of the same base glaze – and I have chosen the blue/grey one.
The reasons for choosing this one are that it gives a lovely satin smooth surface that is perfect for tableware, it fires very reliably (so important), and the glaze from it has been popular. It is a simple and common recipe: Potash Feldspar 40, Whiting 20, China Clay 25, Flint 15. The oxide percentages that I added to get this blue are: 1% Nickel Oxide, 3% Cobalt Carbonate and 1% Copper Oxide Black.
I have been doing some tests with other oxide combinations and have found two others that I like very much.
The white has 1% Red Iron Oxide and 10% Rutile added; the green, 3% Copper Oxide Black. I love them both and will mix up enough of each to see if they fulfill their promise on a pot. I am also trying to make a black.
There’s too much grey here, so I am going to try a few variations. Currently, it has the addition of 2% Nickel Oxide, 2% Cobalt Carbonate and 2% Red Iron Oxide. I am going to increase the proportion of Nickel and Red Iron Oxide in various permutations. I have found that reducing the cobalt doesn’t help, as it needs the cobalt to preserve the black. Developing the colours of my range is incredibly exciting and I’m really looking forward to seeing the glazed pots.
Another reason for developing the new glazes is that I had some issues with one particular glaze towards the end of last year, with the most likely reason for the problem being varying temperatures within the kiln. So, in the last firing I decided to use Orton cones to see if I could discover what was happening.
This was the result. The nearest shelf was at the bottom and the furthest was the top – they are cones 8, 9 and 10. The hottest part of the kiln is one shelf up from the bottom where cone 9 is right down and 10 is bent quite far over. The top shelf is the coolest with cone 8 touching the shelf and cone 9 not. I set the controller to fire the kiln to 1260 degrees C with a 45 minute soak. So, going by the cones, the top of the kiln isn’t quite making it to 1260 degrees C (cone 9), even taking into consideration the soak, the bottom and next to top shelves are pretty much on cone 9, and the one up from the bottom is half way to cone 10. There’s probably a 20 degree difference across the kiln after having a 45 minute soak. So, it seems taken as an average the kiln is firing to approximately cone 9. If you need reliability in your glaze having a wide-firing one can be quite important.