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Pieter Mostert

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About Pieter Mostert

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    Cape Town, South Africa
  1. Ian Currie Test Tiles Forums?

    Please keep the tiles coming. The quote from Tom Coleman that Min shared seems to contradict my suggestion that reduction makes copper a more powerful flux. The only explanation I can think of is that perhaps with the finer mesh sizes of SiC, all the SiC is reduced earlier in the firing, so there's some re-oxidation happening.
  2. Ian Currie Test Tiles Forums?

    Reduction makes iron a more powerful flux, so if the same is true for copper, that would explain why the SiC tiles appear more fluxed. But I don't know if this is the case.
  3. Ian Currie Test Tiles Forums?

    There's a nicer online Currie grid calculator here, made by Tom Demeranville. The only downside is you're restricted to 4 fluxes.
  4. Ian Currie Test Tiles Forums?

    Clara Giorello has done a lot of testing with SiC copper reds. If you're on Facebook, it may be worth joining the Ceramic Recipes group, since SiC glazes come up from time to time.
  5. When it comes to optimizing a variable / variables in your glaze subject to a number of constraints, things get a bit more complicated. Solving a linear programming problem in many variables is not something I'd want to do by hand. I'm not suggesting that you have to do this - usually a good working knowledge of your materials and some common sense will get you in the right ball park - but if its possible to get quantitative bounds by letting a computer run through some algorithm, that's information I'd want to know when I'm playing with a recipe.
  6. I agree that it's good to know how to do the calculations by hand, but having software take care of the maths frees you up to juggle other variables. When I use the software I'm working on (not that I've done much work lately), I can specify a particular oxide composition, and then play with the ingredients to make sure the glaze has enough clay for suspension, or minimal LOI, or minimal soluble ingredients, etc. That said, the UMF only tells part of the story. The ingredients you source the oxides from make a difference. I picked up some schorl in Nambia earlier this year, and used it to make a glaze with the same oxide composition as another glaze I'm working in, but which obtains its boron from a frit. It turned out quite differently. Then again, I was using a generic analysis for schorl, so perhaps the oxide compositions were different too.
  7. Ian Currie Test Tiles Forums?

    Curt, I don't see any reason why you couldn't get oilspots the way you suggest. If you fired in oxidation up to the point where the glaze was melted enough to hold bubbles and then reduced, you'd be reproducing the thermal reduction of Fe2O3 (I think...). However, I suspect you'd be limited in how much higher you could fire before the spots dissolved back into the glaze. Concerning the boron, it's worth keeping in mind that on the old thread of mine you mentioned, Neil posted a cone 6 oilspot recipe which only has 0.024 B2O3 (UMF). In my Currie grid, B2O3 is 0.09, which is still on the low side for cone 4, although the other families of spotty glazes I'm working on are higher at 0.32, 0.33 and 0.38. Not sure what to make of that. Joseph, I've started making a bunch of small bud vases to test glazes on, like you do. For most of my pottery life I've been more interested in form than glazes, so now I'm in the unusual position of having to decide on pots for my glazes, rather than glazes for my pots (although I've been using terra sigillata for my large coiled pieces for a while now). I think oilspot glazes really suit the shape of Jian tea bowls, but I'd need to get myself a wheel if I wanted to do something like that.
  8. Ian Currie Test Tiles Forums?

    It could be, but I'm a little hesitant to draw conclusions about glazes based on phase diagrams.
  9. Ian Currie Test Tiles Forums?

    Thanks LT! I'm looking forward to going through those notes. Joseph, it's a mystery to me why I'm getting the spots, since I fire really low and slow to get to cone 4. My max temp was 1129 C, whereas the iron bubbling in traditional oilspots is only supposed to start at about 1230 C (depending who you ask). The major difference is that my glazes have boron, so that might somehow be catalysing the process, or it could be some other process happening. But the fact that glazes like the ones I'm getting are quite rare at mid-fire temps, seems to indicate that there's something about the way I fire that makes a difference. I've seen papers that mention high iron boro-silicate glasses are prone to phase-separation, but until I brush up on the necessary chemistry, I can't say if this is relevant.
  10. Seger cone 4 + RIO

    You're right. The left hand one has 13.5% RIO added.
  11. Ian Currie Test Tiles Forums?

    Joseph, that's good to know, although ideally I'd like all the bubbles to heal in the first firing. Someone's mentioned holding the temperature on the way down to allow the holes to smooth other, but I only held at 960 C in this firing to try get some iron reds, which is probably too low for the oilspots. I know you slow down the cooling, but is this only after first dropping quickly from the top temp, or do you go slow all the way down? The glaze this grid was based on shows the effect of different thicknesses: https://glazy.org/recipes/8010 I dipped these 4 times, each time covering less of the tile, so that thickness increases from left to right. Here's my theory why thicker glaze layers produce larger bubbles: Microscopic bubbles join up with each other to form larger and larger bubbles, until the bubbles extend beyond the surface of the glaze, at which point they burst. The thicker the glaze, the larger the maximum radius of a bubble that can fit inside the glaze layer, and thus the larger the maximum spot you get. Curt, I'll have to get hold of a copy of Hamer and Hamer. For this test, I think the striding man may have obscured what was happening with the spots, but I can see how using it and the 6 o'clock mark would be useful in general, especially if the glazes aren't applied as thick as they were here. Looking forward to your next oilspot test.
  12. Seger cone 4 + RIO

    Do you mean percentages of iron oxide added?
  13. Dtc 600C Keypad Buttons Not Working

    I had a similar problem with the DTC 800C controller on a Paragon TnF 82. I bought an old kiln that had been in storage for about 18 months, and got 4 firings out of it before I had to replace the controller.
  14. Ian Currie Test Tiles Forums?

    I'm afraid my knowledge of glass and glazes doesn't expend that far. Do you have a reference where I can read up on this?
  15. Ian Currie Test Tiles Forums?

    High Bridge, I'm fairly certain that the spots are due to bubbles bursting. This paper illustrates how bubbles form spots in traditional oilspots (see figure 1). That said, I'm not sure if the bubbles in my glazes are because of the thermal decomposition of iron, given that I fire 100 C lower than the temperature at which this is supposed to happen.