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Pieter Mostert

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Everything posted by Pieter Mostert

  1. I've reformulated the recipe using the Potash Feldspar I use and EPK for kaolin, since I don't know the composition of the ones you use. So this may not work for you, but could be a starting point if you can't find more info about your potspar and kaolin. Kaolin 17 Silica 22.5 Wollastonite 21 BPS Standard Borax Frit 26 Potash Feldspar 13.5
  2. Dick, I've always assumed that the reason a short hold at a higher temp equals a long hold at a lower temp is that the rate at which the chemical changes occur in the glaze or clay-body increases with temperature, above some minimum thresh-hold. I don't think thickness is the critical issue. If the rate at which the reactions occur is comparable to the rate at which cones soften, then a glaze or clay-body rated at, say, cone 6 should be mature regardless of the firing speed, provided the cone 6 cone is bent. Of course, this is a big 'if', and assumes the temperature of the cone is the same as the temperature of the glaze / clay-body. Also, that's not to say the glaze will look the same, since things like crystalization and smoothing over of pinholes depend on the firing profile. The Orton website does state that The starting premise of cone development was that a blend of ceramic materials compounded to exacting proportions should behave similarly to the ceramic ware being fired in the kiln. A great deal of work later the starting premise was confirmed... It would be nice to know what 'similarly' and 'ceramic ware' mean in this context. By the way, according to this presentation by Dave Finkelnburg, fritted glazes melt from the outside in, while feldspar-based glazes melt outwards.
  3. This is a fascinating thread. I've also been under the impression that iron and titanium are the main things to control for whiteness. Is it possible that one of the ingredients in the grey test bars has an incorrect analysis? What makes me doubt magnesium as the culprit is that Macaloid is about 22% MgO by weight (apparently Veegum T is similar). In a 50% Kaolin, 25 Neph Sy, 25 Silica body, 1% Macaloid contributes about 0.4% molar MgO. If you're adding 2% Veegum T, shouldn't all your tests be grey?
  4. I think the Desert Stone AGB is being flocculated by the high Neph Sy content. From the digitalfire entry on Nepheline Syenite: However, the place where you may note the solubility of nepheline the most is in glaze slurries containing significant percentages, they can gel over time and the addition of more water to thin the slurry can wreak havoc with application performance (try adding a few drops of deflocculant instead). It's really weird that the specific gravity goes up when you add water. Do you have the numbers? In my (limited) experience, glazes with borax tend to be thicker, so it's strange that adding borax thins your glazes. Hopefully someone who understands flocculating / deflocculating better than I do can explain this.
  5. Diesel Clay's right, of course. You don't need Brongniart's formula to work out how much water to add, and in fact this isn't why Brongniart came up with his formula. The usual way in which the formula is presented expresses the weight of the dry ingredients in terms of the weight and specific gravity of the glaze. This is useful if you have a batch of glaze that you've been using for a while, but want to modify the composition.
  6. I think the formula Celia referred to is Brongniart's formula, which effectively says that the specific gravity of a glaze is the weight of the dry ingredients plus the weight of the water (in grams), divided by the volume of the dry ingredients plus the volume of water (in ml). Really, it's just saying that specific gravity is the weight divided by the volume. For most glazes, the volume of the dry ingredients is equal to the weight of the dry ingredients divided by about 2.5, so if D is the weight of your dry ingredients and W is the weight of water (which equals the volume of water, in our units), then SG = (D + W) / (D/2.5 + W) Using D = 1000 and W = 631 (the volume of 1 pint + 2/3 cups in ml, if I've converted it correctly), you get SG = 1.58. It's a little on the high side, so I'm a bit surprised you need to decant water, but perhaps the value of 2.5 isn't quite right for the glaze you're using. If you want to find out how much water to add to a batch of dry ingredients to achieve a certain specific gravity, you can solve the formula above for W. You should find that W = D * (1 - SG/2.5) / (SG - 1) So if you wanted a SG of 1.5, for example, you'd need to add 800ml of water to your 1000g batch of dry ingredients. EDIT: I've fudged over the fact that the volume of the dry ingredients isn't the actual volume you'd measure, it's the volume you'd get if you could remove all the air between the particles. But this isn't something you need to worry about.
  7. Just make sure that whatever stain you use is compatible with your clear. For example, some stains require zinc-free glazes.
  8. If you're looking for a more yellow glaze, it might be easiest to use a stain. An example is Transparent - T8 Yellow, though it might not be quite what you're looking for. Perhaps Transparent - T5 Orange with less red stain would work. Whatever recipe you decide to use, you'll probably have to try a bunch of variations to get something you like using your kiln and on your clay body.
  9. If you're looking for glaze recipes, have a look at Glazy, which is a free online database of ceramic recipes. Honey Amber looks reasonably close to the glaze in your link, but be aware that manganese dioxide has its own health and safety issues.
  10. I think I've mentioned this before, but you can simplify the measuring method Currie describes by doing two 7-glaze line blends from A to C, and B to D, followed by seven 5-glaze line blends between the glazes along the vertical edges of the grid. Not only will this cut down on the number of measurements, it also means the smallest volume you need to measure is 12 ml (assuming each of the 35 cups holds 48 ml)
  11. Jed, dividing the radius by 0.6185 isn't the same as dividing the volume by 0.6185. You'd need to divide the radius by 0.88 to get the same effect. For any 3-dimensional object, not just a sphere, dividing each length by 0.88 is the same as dividing the volume by 0.6185. So for a cylinder, you'd divide both the height and diameter of the base by 0.88 to have the same effect as dividing the volume by 0.6185. Maybe you meant that you should divide the height by 0.88 and the surface area of the base by 0.7744.
  12. There's a potter called Dan Weaver who has a series of on how to de-water slip using a filter-press bag inside a vacuum bag attached to a vacuum pump. It'll take a bit of work to set up, but it looks like quite an efficient way of doing things.
  13. John Sankey has a good article on COEs, which includes a table of the different oxide expansion factors that various authors have calculated. Some of them vary quite a bit. Another good article is on the digitalfire site. I think everyone who relies on calculated COEs should read this.
  14. I wonder if adding Epsom salts (dissolved in water) would improve the plasticity of clay contaminated with sodium silicate. I've found that this works for clay that's been deflocculated with sodium polyacrylate (aka Alcosperse aka Darvan).
  15. Tim, I think the corrosion may be due to the formation of silicon tetrafluoride. The following is a quote from Out of the Earth, Into the Fire by Mimi Obstler, in the section on fluorspar, and I'm guessing something similar happens with cryolite. "At temperatures somewher between 1652oF and 2190oF (provided silica is present, the disassociating temperature of fluorspar in the absence of silica rises to 2300oF - 2500oF), fluorspar breaks apart and releases volatile fluorine gas. The liberated gas attracts silica from the glaze and/or claybody to form silicon tetrafluoride (SiF4), which escapes from the kiln, leaving behind the newly separated calcium. ... "The consequences of using fluorspar can sometimes be injurious to the ceramic surface, the kiln interior, and even the potter. The formation of silicon tetrafluoride robs the glaze and/or claybody of needed silica. In addition, the forcible exit of silicon tetrafluoride is often marked by blisters and pinholes. ... Although there is a difference of opinion as to whether the liberation of this volatile gas injures kiln interiors (see Parmalee 1973), Ceramic Industry reports definitively that silicon tetraflouride corrodes furnace linings. Our experience confirmed the corrosive nature of silicon fluoride. A test series that added 20% - 40% of fluorspar to Sanders Celadon glaze not only corroded the kiln shelves, but also attacked the surface of adjoining pots. Even more important is the fact that tetraflouride gas is a known carcinogenic material"
  16. Moving studios at the moment. Let's hope the new one isn't as noisy as the last.

    1. glazenerd


      Hope it turns out to be the perfect artistic nest for you. I live in the country, I hear an occasional "moo."

    2. GiselleNo5


      Ohh that's awful. :( How is it even enjoyable for THEM at that point?

    3. Pieter Mostert

      Pieter Mostert

      Wish I had a studio like yours, Nerd. To be fair, my old studio wasn't always that bad, but it was bad enough for me to decide I'd had enough.

    4. Show next comments  15 more
  17. Min, the figure of 5 microns given here is for the median particle size (half the particles are smaller than this). Mesh size relates to the maximum particle size. If the maximum particle size of Imsil A-25 were 5 microns, then yes, it would be 2500 mesh, but I suspect the rep you emailed just assumed the figure you gave was the max size without checking. I don't know what the actual maximum size is, but unless the particle size distribution is very narrow, the mesh size is likely to be significantly smaller than 2500. I haven't seen anything shows it to be 325 mesh, and even if it were, it would still pose more of a risk than Sil-Co-Sil 45, where the median particle size is about 10 microns. On the other hand, if you look at the graph for Sil-Co-Sil 45, you'll see it has slightly less than 30% of its particles smaller than 5 microns, so the difference isn't massive. Maybe it's not worth the increased risk to use a silica where 50% of the particles are less than 5 microns, maybe it is.
  18. +1 for what Bruce said. Just to put things in perspective, the silica you need to be worried about is that less than 10 micrometers in diameter (see http://digitalfire.com/4sight/hazards/ceramic_hazard_quartz_crystalline_silica_toxicity_251.html). From the spec sheet for Sil-Co-Sil 45 that Tyler linked to, that's about 50% for a 325-mesh silica. I couldn't find any particle size distributions for Imsil A-25, but given that its median particle size is 5 micrometers, compared about 10 micrometers for Sil-Co-Sil 45, I would guess that the percent of Imsil A-25 less than 10 micrometers is equal to the percent of Sil-Co-Sil 45 less than 20 micrometers, i.e. 80%. Of course, this doesn't tell the whole story, since the smaller the diameter of the silica particles, the more hazardous they are, so you'd need to compare the tails of the particle size distributions if you really wanted to quantify how much more dangerous Imsil A-25 is than Sil-Co-Sil 45.
  19. Hmm, I get about 40% B2O3, 31% CaO from that analysis. This is % weight, just to be clear.
  20. OK, here's my improved version. I've attached a rectangular strip along what used to be the pouring edge, extended the partitions, and inserted wooden blocks to direct the glaze towards the openings through which it'll be poured out. I'm not sure how well this will pour, but it's better than my original version, even if it looks clunkier.
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