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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

    Hudson River Clay

    That last tile looks like a nice fake ash glaze. If your aim is to work with mostly local materials, you could try replace the Whiting with washed wood ash, but that's a whole other rabbit hole.
  2. Pieter Mostert

    Home Made Kiln Controller

    I came across this project recently: https://github.com/ZachJMoore/smart-kiln-web-interface Remote control, monitoring, and logging of your firings. What more could you ask for?
  3. Pieter Mostert

    Hudson River Clay

    Mary, Ian Currie published his book on the grid method under a Creative Commons licence, so you can obtain a free electronic copy here.
  4. Pieter Mostert

    Hudson River Clay

    Curt, I gave a description of the tile in my last post: Silica increases from left to right, and Whiting increases from top to bottom. So it isn't a Currie grid, but the bottom left corner still has the most flux. This is a useful test when your starting glaze doesn't have much clay, but has relatively high alumina. Mary, I fired the tile flat. This is not an example that shows increased fluidity, but I can dig up some if you're interested. I should add that the results of firing flat vs vertically can be fairly different. There's some discussion in the Currie thread about how to get the most info from flat tiles, including some indication of fluidity. By the way, the reason Insight gives different UMF numbers than what you calculated is that it doesn't include iron as a flux. Glazenerd, I haven't heard of iron being involved in shivering. I thought lithium was the main suspect.
  5. Pieter Mostert

    Hudson River Clay

    That's not always the case. I've done several line blends (at cone 4) where I just added increasing amounts of Whiting to a glaze, resulting in an increase in fluidity, up to a point. If you keep adding Whiting, sooner or later your glaze will become underfired, since Whiting on its own has a pretty high melting point. What sticks out for me from the Insight analysis is the high UMF value of alumina. Have a look at the glazes plotted here (you can refine them to show only cone 6 if you like. I assume that's what you're firing to). There aren't many with Al2O3 as high as 0.99, and most of those that do, have high KNaO. Adding Wollastonite will bring down the Al2O3 UMF value, in addition to increasing to Al2O3 : SiO2 ratio and getting the flux ratio closer to 0.3 : 0.7 (Insight rounds off the numbers). But it might be more informative to do a biaxial test where you add both Whiting and Silica. I've attached a test I did where I increased Silica going left to right, and increased Whiting going top to bottom. I'm not claiming you'll get a stable glaze this way, since I think Matt recommends some boron for cone 6 glazes. But I also think the tests he did on stability didn't involve iron, so I'd be reluctant to extrapolate from them. I should point out that I haven't taken his course, so I could be completely wrong about this.
  6. Pieter Mostert

    changing specific gravity of glazes

    This is great! I made a similar form using google sheets, but it's almost impossible to use on my phone, so hopefully something like this will work (I haven't tried this on my phone yet). Do you mind if I fork your repository to adapt it to my version? You can find my spreadsheet here: https://docs.google.com/spreadsheets/d/11WFZMq3A6ZtTCneMBRO_5DCOBuNJ3SICMvPkA2YtgB4/edit#gid=0 Actually there are two versions. The second one uses a two-step procedure for when it's impractical to weigh the glaze.
  7. 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%)
  8. 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.
  9. Pieter Mostert

    Rocket Stoves & Kilns

    Have a look at Marvin Bartel's webpage.
  10. 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/
  11. 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).
  12. 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.
  13. 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.
  14. 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.
  15. This has come up before; see this topic.

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