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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

    Electric Reduction Firing

    I'd love to hear what you learned. It's unlikely I'll have the opportunity to build a reduction electric kiln in the near future, but I'm still interested.
  2. Pieter Mostert

    Irregular shape pot

    If you want to start with a solid cube with 40 cm sides, you'd need over 100 kg of clay (assuming the density of clay you're using is similar to that given here). I suggest starting with a shape that's more like a box with thick walls. With 30 kg of clay, you can make a cubical box with 40 cm sides and walls 8 - 9 cm thick. This will also make it easier to join the separate pieces of clay together. Having groggy clay helps, but I think you'll need to be very careful about how you dry the pot to stop it cracking.
  3. Pieter Mostert

    india ink or what else ?

    I wonder if a higher temperature simply makes the ink less viscous, allowing it to penetrate the cracks better, even if they're narrower. This link doesn't specify what type of 'ink black' they're measuring, but it shows a dramatic decrease in viscosity between 20 C and 50 C: https://wiki.anton-paar.com/en/ink/
  4. Pieter Mostert

    india ink or what else ?

    If the clay body expanded more than the glaze when heated, it would shrink more than the glaze when cooled, so the glaze would be under compression, and wouldn't craze.
  5. Pieter Mostert

    india ink or what else ?

    I've found that heating does the opposite. In one crazed vase I have, the cracks disappear when you soak it in hot water, but then reappear once it's cooled. This makes sense to me, since there's no tension in the glaze at the temperature that it solidifies, but then the tension increases as you lower the temperature, due to the difference in thermal expansion between the glaze and body. So heating the pot should reduce the tension, and therefore shrink the cracks. But you aren't the only one who suggests heating a pot expands the cracks; Nigel Wood talks about this in his book on Chinese Glazes.
  6. 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.
  7. 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?
  8. 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.
  9. 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.
  10. 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.
  11. 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.
  12. 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%)
  13. 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.
  14. Pieter Mostert

    Rocket Stoves & Kilns

    Have a look at Marvin Bartel's webpage.
  15. 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/
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