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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. UPDATE: I've created an improved version, which is slightly easier to install (I hope), and runs faster. I've only tested it on Windows, but in principle it should run on other operating systems. However, the installation process is probably still too complicated for most people, and I still haven't figured out how to write an installer.
  2. PV clay? https://digitalfire.com/4sight/material/pv_clay_1152.html
  3. Was the unknown ingredient Plastic Vitrox clay? Sounds like it might be this recipe: https://glazy.org/recipes/2875 Be aware that the analysis in glazy uses the theoretical composition of Colemanite (https://glazy.org/materials/15119) whereas what's actually sold may have a different composition, for example https://glazy.org/materials/37220 or https://glazy.org/materials/20787 If the 35 year old piece is functional and has been in use for most of its life, I wouldn't worry about any theoretical durability concerns.
  4. It looks like the proportions in first one are correct, but it's been normalized with Al2O3 = 1, instead of the sum of the fluxes = 1.
  5. Are you sure the tests didn't involve glazes in slight compression? I would think that, given the relative thicknesses of glaze and claybody, the compressive force exerted by a glaze on a claybody would be minimal. I did a bit of digging, and came across this Alfred Masters thesis by Jennifer Benson. After skimming through this, it's still 100% clear to me why an increase in the compression of a glaze should increase the strength of the ware (up to a point), but I think it may be due to the fact that this reduces the likelihood of cracks originating in the glaze.
  6. A non-crazed glaze is (probably) not under tension, and may be under slight compression. But if the glaze is under compression because the clay-body contracted more than the glaze, wouldn't the clay-body be under tension? I've also heard the claim that glazes under slight compression strengthen ware, and while I don't doubt that it's true, I can't figure out why this is.
  7. 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.
  8. 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.
  9. 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/
  10. 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.
  11. 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
  12. 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.
  13. 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?
  14. Mary, Ian Currie published his book on the grid method under a Creative Commons licence, so you can obtain a free electronic copy here.
  15. 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
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