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Pieter Mostert

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Pieter Mostert last won the day on August 5

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About Pieter Mostert

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    Cape Town, South Africa
  1. 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.
  2. 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.
  3. Hamada, Hamada, and Hamada

    This worked for me, although the sound is out of sync, and it repeats a part at the end: Thanks for sharing, Sputty.
  4. 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.
  5. Ian Currie Test Tiles Forums?

    It could be, but I'm a little hesitant to draw conclusions about glazes based on phase diagrams.
  6. 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.
  7. Seger cone 4 + RIO

    You're right. The left hand one has 13.5% RIO added.
  8. 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.
  9. Seger cone 4 + RIO

    Do you mean percentages of iron oxide added?
  10. 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.
  11. 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?
  12. 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.
  13. Ian Currie Test Tiles Forums?

    Thanks for your comments, Curt. To answer your questions, what I'm calling earthenware clay is sold as 'Smooth Red Earthenware', but I've fired large coil pots made from it to cone 4 without any sign of overfiring. I'm not sure what cone the stoneware is designed to be fired at, but cone 10 would be a good guess (unfortunately my supplier gives temperatures instead of cones, if they give any info at all). Both tiles were fired to a bit above cone 4, so the stoneware one is definitely underfired. I'm in the process of formulating some clay bodies that are vitrified at cone 4, which I hope to use these glazes on, and while I really should be testing the glazes on these clay bodies, at least I can get an idea of how things change when you vary alumina and iron (among other variables I've looked at) and then apply that knowledge once I've settled on a clay body. Because I used the wrong kaolin, the glaze I have on Glazy doesn't correspond to any of the ones on the grid. The closest one, both visually and in terms of the oxide composition, is tile 33, but this has SiO2 2.27 and Al2O3 0.33, as opposed to SiO2 2.48 and Al2O3 0.28 for the one on Glazy. Of course, that's assuming the info in the spec sheets is correct. Iron is increasing across each row, so as a percentage of the glaze, everything else is decreasing. If you include iron as a flux, this would mean all the other oxides in the UMF are decreasing, although their ratios remain constant. Iron can definitely act as a flux in oxidation, as this series of test bars shows: Here I just added increasing amount of red iron oxide to a recipe that had the same UMF as Seger's original cone 4 composition. What makes things tricky is that you'll get different numbers of molecules of Fe2O3, Fe3O4 and FeO from the same weight of iron oxide, and since there's probably a mix of these oxides in the glaze, how do you convert from weight to UMF? Regarding iron making the glaze more viscous, I think that this is similar to adding more and more calcium or magnesium to a glaze; at some point the fluxing action will stop and the glaze will become more refractory. I've noticed that excessive iron will make a glaze crawl, although that may also be because of its effect on the glazy slurry, not just the surface tension. I agree that the surface tension is highest in the top right corner. For the unhealed bubbles, I think high viscosity must be playing a role, since I'd expect a high surface tension to smooth out any dimples in the surface. You may be onto something with the white tile contributing more alumina to the glaze. If you moved the tiles in the white grid up one or two rows you'd get a better match with the red grid, although they'd still be slightly darker, so that can't be the whole story. If the glazes on the red tile are getting iron and fluxes from the body, I'd expect them to be darker, particularly since alkalies reduce the solubility of iron, according to Currie's experiment Fe4 on pg 161 and 162 of Stoneware Glazes. There is some variation in the sizes and densities of the spots, though not nearly as large as in your oilspot grid. I suspect that the oxide composition of the cells in your grids vary more than in mine , but even taking that into account, my spots seem quite persistent. I'm working on the theory that the reason I'm getting spots has to do with firing very slowly as I approach the peak temperature. So far I have 4 families of spotty glazes; besides this one, there's the one I posted about in this thread, which has now morphed into this, one with multicoloured spots: and this one. So either my kiln god has a spot fetish, or there's something unique about how I fire.
  14. Seger cone 4 + RIO

    From the album Glaze tests

    Fluxing effect of iron oxide in oxidation.
  15. Glaze tests