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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. 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.
  5. 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.
  6. Mary, Ian Currie published his book on the grid method under a Creative Commons licence, so you can obtain a free electronic copy here.
  7. 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
  8. 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 t
  9. @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
  10. 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 ir
  11. 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
  12. 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 ar
  13. Well, on Glazy you can specify the firing type and type of surface, although not everyone does this. I agree that the other variables are important, and should be included if the goal is for other people to replicate your glaze. On the other hand, I think of recipes I find online as starting points for tests, since most of the time I have to (a) tweak them to work at cone 4 instead of 6, and (b) reformulate them in terms of the ingredients I have available. Also, I'm limited by how fast and how high I can fire, so even if someone gave a firing schedule for cone 4, I might not be able to follow
  14. I'm going to have to start ditching my test tiles at some point too. I don't fire that often, but they're already staring to clutter up my studio. I'm just a bit wary of relying on photos, since there are some aspects of glazes that are hard to capture in a photo. I use Glazy for recording most of my tests, but a system that naturally caters for recording things like application thickness and firing cycles would be ideal. I know that Derek Au, the creator of Glazy, has thought about this, but at the moment he's busy rewriting the current version, so I don't think it'll happen any time soo
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