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Pieter Mostert

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About Pieter Mostert

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    Cape Town, South Africa

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  1. Pieter Mostert

    Theoretical Glaze Components?

    Joel is right. You've probably seen this in recipes that came from Glazy, which used to describe certain ingredients as theoretical, although this seems to have changed recently. The problem is that some recipes just call for something like 'potash feldspar' without specifying which brand. In order to work out the Seger formula for that recipe, you need to know (or make assumptions about) the composition of oxides in each ingredient. 'Theoretical' potash feldspar just meant that the composition of pure, i.e. "theoretical", potash feldspar was used to crunch the numbers. In practice, think of it as a warning that your brand of potash feldspar (or whatever) may give different results than the brand used by the author of the recipe. (Also, don't trust the given Seger formula 100%)
  2. I have this model, but it came with the older DTC 800C controller, which lasted 4 firings before it went completely haywire. I got an electrical engineer friend to replace it with a PID controller, which hasn't given any serious trouble. According to the previous owner, the original controller had been working fine, but the kiln had been in storage for 18 months before I bought it. If possible, I'd test the controller by programming a short ramp with a hold, and checking that the kiln follows this. If you do this, make sure you read the manual beforehand. I need an extended firing to reach cone 4, but that's probably because the floor is cracked and the bricks are out of alignment on a couple of corners, so there's a gap that opens when the kiln heats up. Some of the element grooves are quite close to the top of the bricks they're in, and for a number of these bricks, the part above the groove has broken off. This was because the previous owner transported it with shelves (wrapped in newsprint) inside, and lifted it out of the car at an angle . I should have know better too, and told them to only put balled-up newsprint inside. Anyway, I don't think this will be a problem if you don't do anything stupid, but just be aware that that's something to look out for. If the kiln and elements are in good condition, I don't think you'll have trouble reaching cone 6.
  3. Pieter Mostert

    Rocket Stoves & Kilns

    Have a look at Marvin Bartel's webpage.
  4. Pieter Mostert

    Solar Powered Kiln?

    Turns out it took less than 3 years. And you can use some heavy 3-phase machinery at the same time. https://tonightmyfingerssmellofgarlic.com/2018/03/20/firing-on-sunshine/
  5. Pieter Mostert

    Question on mixing colorant batches

    It would be close enough if you were dealing with water (assuming volume is measured in mililitres and weight in grams). If you want B4 to be the volume of the glaze, all you have to do is multiply by the sg to get the weight of the glaze. Then the formula in F4 becomes (13/8)*B4*(E4-1).
  6. Pieter Mostert

    Question on mixing colorant batches

    Based on the formula you use for F4 in the Glaze Measuring spreadsheet, cell B4 should be the weight of glaze per cup.
  7. Pieter Mostert

    Question on mixing colorant batches

    Maybe this is where the mistake is? Specific gravity is the ratio of the weight of the glaze to the weight of an equal volume of water.
  8. Pieter Mostert

    Question on mixing colorant batches

    If your s.g. is 1.4, then assuming the relative density of the materials in your glaze is 2.6 (see the pdf Min linked to), you'll only have 65g of dry materials in 140g of glaze. You can work this out as follows: Let D be the weight of the dry materials, and W be the weight of water (in grams). Then since the relative density of the dry materials is 2.6, the volume of the dry materials in the glaze is D/2.6, and since the relative density of water is 1, the volume of water is W/1 = W (in mililitres). Therefore the weight of the glaze is D + W, and the volume of the glaze is D/2.6 + W, so its specific gravity is sg = (D + W) / (D/2.6 + W) With a bit of algebra, you can express D in terms of sg and W, or W in terms of sg and D. However, in practice, you usually only know the specific gravity and the total weight, T, of the glaze. In this case, since T = D + W, you have W = T - D, which when you plug into the formula above, gives sg = T / (D/2.6 + T - D) = T / (T + (1/2.6 - 1)*D) Now you can solve for D in terms of T and sg: D = T * (sg - 1) / sg / (1 - 1/2.6 ) = (13 / 8) * T * (sg - 1) / sg Plug in T = 140 and sg = 1.4, and you'll get D = 65.
  9. This has come up before; see this topic.
  10. Pieter Mostert

    Crackle Blue .. Recipe Please

    I found this on Glazy by searching by colour. Looks similar.
  11. Pieter Mostert

    Ian Currie Test Tiles Forums?

    @Joseph F When I increased the phosphorus in my original glaze (see link in my reply to curt) there was a more abrupt transition between the orange centre and the background colour. (The application on this tile was way too thick, but you get the picture) I think you'd have a similar result if you increased the phosphorus in #9. @High Bridge Pottery Yes, alumina increases up and iron to the right. If the glazes in my grid behave similarly to the ones in the paper I mentioned, then for a given column, the ones lower down start softening at the same time as the ones higher up, but they're more fluid at the top temperature, so the rate at which the viscosity changes with temperature is greater. I think this sudden stiffening of the glaze as it cools is trapping the bubbles. After looking at the paper again, I realised that the conclusion I quoted was for clear glazes only (so alumina can't be too high, as in the glazes I tested). Even here, the data isn't totally convincing.
  12. Pieter Mostert

    Ian Currie Test Tiles Forums?

    Sorry for not responding sooner; I've had internet trouble at home over the weekend. @Marcia Selsor Funny, I was looking at that paper on crystalline glazes yesterday. I know that the knowledge of crystalline glazes has advanced alot since then, but it's still a great source of data. I think Ferro 3191 is reasonably similar to the frit I was using, but I'd have to import it. Will try some local options first. @glazenerd I've noticed that in general, my tests on the red earthenware are more likely to pinhole than the ones on white stoneware, so that could well be the result of iron sulphide. But in this case I think the source of most of the gas and subsequent bubbling is from the decomposition of Fe2O3 to FeO. I can't explain why it would happen at 1130C instead of 1230C, which is the usual temperature given, but it still seems to me to be the most likely explanation. @curt The oxides other than iron and alumina are the same as in this recipe. The left-hand column has 5 - 6 % red iron oxide, which translates to 0.09UMF. The reason I think the bubbles may still be forming (or at least expanding and bursting) as the kiln cools, is that the later ones sometimes have different colour developing. For example, in the first close-up, the squashed bubble in the centre is more orange than the two adjacent to it, which must have formed later. This is assuming that the crystals giving the orange colour only form at lower temperatures. I may be (probably am) completely wrong about the effect of changing viscosity on the bubbles. I guess I was thinking something along the lines of what Tony Hansen talks about here: I really need to go back and read the bubble thread again.
  13. Pieter Mostert

    Ian Currie Test Tiles Forums?

    Here's what I think may be happening with the bottom left corner. There's a 1914 BSc thesis by Sidney Sewell where he measures the viscosity of a family of glazes as a function of temperature. All glazes have 0.3 K2O, 0.7 CaO with Al2O3 ranging from 0.3 to 1 and SiO2 from 1.8 to 6 UMF. (Well, that was the plan, but the little slacker didn't finish testing all of them in time). He found that for fixed SiO2, decreasing Al2O3 increases the rate at which viscosity changes with temperature. So assuming this is also true for the family of glazes in my grid, as the kiln cools, the ones near the bottom increase in viscosity more rapidly than the ones near the top. So if there's a window of viscosities in which the glaze is too stiff to allow bubbles to grow, but fluid enough to heal over burst bubbles, the low alumina glazes may not be spending enough time in that window. Of course, this doesn't explain why adding iron makes the bubbles heal over. Maybe in this situation it's acting like alumina? I'd be interested in hearing any other explanations.
  14. Pieter Mostert

    Ian Currie Test Tiles Forums?

    Here are the results of a Currie-type grid where I again varied iron and alumina. The proportions of everything else are fixed, and the same as my previous test, except that silica is now at 2.48 UMF (previously it was between 2.12 and 2.27). Based on the results of the first test, I reduced the range of iron slightly, to run from 0.09 to 0.22 UMF from left to right (previously 0.07 to 0.22). Alumina goes from 0.22 to 0.34 from bottom to top (previously 0.32 to 0.46) Below is a screenshot from Glazy showing the corner glazes (with Fe2O3 not regarded as a flux). The ones circled in red are my first test, and the ones circled in blue are the second. I kept essentially the same firing cycle (slow firing to 1132C, with a 45 min hold at 950C on the way down. Cone 4 buckled but not flat on the shelf the tiles were on). However this time I fired the white stoneware above the red earthenware tile. This time I also made a little hole in the glaze in each square (based on Curt's suggestion of scoring a 6 o'clock line), but I can't see any evidence of them. On red earthenware: On white stoneware: As in the first test, the tiles get progressively darker as you move from the bottom left to the top right corner. Unlike the first test, which had unhealed bubbles around the top right corner, here the bottom left corner had the most unhealed bubbles, which I don't know how to explain. The sort of stringy, collapsed bubble thing reminds me of another series of iron glazes I'm working on which are also relatively low in alumina, and form ugly blisters when thick. I'd have thought lower alumina would mean lower viscosity, and therefore give the glaze a better chance of healing over. While I was writing this, I thought of a possible explanation, but I'll put it in a separate post. The orange background appears with similar levels of Al2O3 for both tests. It would be interesting to do a grid varying silica and alumina around these points to see how far this region extends. I love the variety of colours and combinations of colours that this tile produced. It's hard to pick a favourite. The area in the middle is the most striking (and is what got me started on this series of tests), but I'm also drawn to the yellow-purpley combination in bottom right corner. In fact, if you look closely, the purpley background has tiny areas of red, which might be encouraged by changing the cooling cycle. The top left glaze and the one to the right of it (#1 and 2) are really interesting, even though I'm not mad about the dirty yellowish - orangey background. I'm intrigued by the blue though, and will probably play with the amount of phosphorus to see how it effects this. But the glaze I keep coming back to is the third from the top in the left-hand column (#11). I really love the subtle blue halos. Unfortunately the frit I used for these tests has been discontinued, and the replacement my supplier claims is the same is slightly different. Time to buy a big batch of frit from a supplier that provides the composition, if only I can find one.

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