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glazenerd

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    St. Louis, Mo.
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    Crystalline glaze chemistry. Porcelain, Stoneware, Fritware, 04 Colored Porcelain clay research & formulation.
    Ceramics Monthly Articles: Jan. 2018 Cation Exchange (plasticity), April 2018 SSA Clay Formulation, May 2018 Bloating and Coring.
    Feb. 2019 Ceramics Monthly- Clay Body Shopping Guide
    March 2019 Ceramics Monthly - Porcelain 201
    June 2019 Ceramics Monthly Clay Restoration
    Sept. 2019 Clay Memory
    Oct. 2019 Firing Programs

    Email: optix52@aol.com

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  1. Dave Fairly familiar with Ohio clay. Most everything you find will be iron pyrite (iron disulfide). Most have 6-8% iron content total. The big issue, most terra cotta bodies have higher levels of sulfides: lots of coal fields in Ohio. Your dark cube is one indication of that, the other is your slip peeling. If you do bisque fires on a regular basis: make a 1” x 6” x 1/2” thick test bar with this clay. Throw it in with your regular bisq at medium, or higher speed: if that is your normal cycle. Snap it in half when it comes out: look for dark coloration. That will tell you what direction you need to go. Nerd
  2. Goatrider: In the first pic, very small cracking on the rim. That indicates you need some ball clay: start at 10% OM4 or equal. The green color is from calcium usually, other minerals will present that color. From the color of the second pic: 6-8% iron content. Final blob pic: lack of alumina is the biggest factor of pyroplasticity (melt). Kaolin is 37% alumina: whatever kaolin you have on hand. Wild clay 75%, ball clay (OM4) 10%, kaolin 15%. Will get you started, but will have to check melt before you dive any deeper. Nerd
  3. Looking closely, I can see pinholes that are clear. I can see others that are black. I will vote coffee. Tom
  4. I think the last time I bought commercial clay was 10 plus years ago? I have blended a lot of wild clay, but that is really outside this topic. I blend two porcelains primarily: the first is for tile work, the other for crystalline glaze. The tile body centers on laying flat without me having to flip, weight, and baby sit it in the process. The porcelain for crystalline centers around a chemistry that does not hinder or interfere with crystal growth. They both met basic formulation standards; cone 6, absorption hovers around 1/2%. Not a fan of sodium based body fluxes for many reasons. Side note: I use the same basic tile body for throwing with minor adjustments. I purposely blend it to be a touch on the short side to allow for water absorption. The only red clay I blend comes from hematite bearing wild clay; which keeps a deep red color at cone 6. Tom
  5. Babs Zinc is processed two ways: the French process and the American process. The American process is just simply “roasting” (we call it calcining) that removes impurities. It is denser because it typically 80% pure, with 20% of various minerals commonly associated with zinc. The French process involves vaporing zinc at higher temps, resulting in a purity above 99%. It is finer, with less density. The zinc you are looking for is called Maximo 910 in the States, unless it has been sold to another company yet again. Tom
  6. Morgan: If the Frost is combined with other clays as part of your total reclaim: then no specific protocols are necessary. If you plan on using Frost specifically, then you do need to follow specific protocols. Yes, Frost reclaim would be more prone to cracking. Frost has 30-32% sodium based flux which in part plays a role in its rapid drying/cracking. Bentone MA is highly processed hectorite, with magnesium removed. To give you an idea of how fine BentoneMA is: OM4 runs 0.55 microns, Taylor 0.28 microns, and BentoneMA, below 0.15 microns. This means it does not take much water or throwing to wash it out. If you plan on reclaiming Frost long term, then it becomes an issue. Tom
  7. Morgan: Frost is blended a bit different than most clay bodies. It has a high COE value, because it has high flux levels. Secondly, Frost uses a speciality plasticizer called BentoneMA. If you plan on using Frost, and want to maintain the translucency: then it matters. One off use, does not matter. Tom
  8. RF back for an add. I understand the dislike of sodium based fluxes in a clay body: not a fan of them myself. However, some flux needs to be present for several reasons. I would recommend potassium in the 10-15% range. I also assume your goal of “less than 0.50” absorption is meant to meet TCA specs for vitreous wall tile? I can have the long detailed discussion about why fluxes, but for now you get the short form. Tom
  9. RF You want soft slabs for a particular forming process I assume? Yet, you also have to understand that “soft” clay equals higher water content, which adds to the warping issue. The more water in the clay; the more dried shrinkage, and increased risk of warping. As to your other listed issues: 1. Kaolin and fire clay have very low plasticity: but they have some. If you work centers around slab building/forming: then adding molochite 325 would be advisable. Molochite is a grog, and 325 is used when you are trying to maintain low absorption bodies. Unlike kaolin, molochite adds no plasticity, and actually will reduce plastic properties if used in high %. Start at 5% in small test batches. Molochite 325 is used often in slab/tile bodies to control warping. Do not fall into the trap of: if a little works good, then more will work better.” Adding too much will actually cause cracking when drying. 2. 10% silica additions in stoneware/red body is the industry norm. If you use a glaze calculator: aim for a 5:1 SiAL ratio (5 parts silica to 1 part alumina)Like all things clay: too much silica can lead to cristobalite formation. Cristobalite will dunt your pieces during the cool down, usually around 400-450F range. 3. Ball clay is the primary plasticizer in most all clay recipes. Slab/tile bodies need just enough to make it malleable, and too much adds to warping/shrinking issues. Secondly, not all ball clays are the same. 8% Taylor, 15% C&C, and 25% OM4: will all impart roughly the same level of plasticity. Ball clays absorb water, as their plasticity levels increase, and as their % in the recipe increases: likewise total water content increases. Tom
  10. 1st. understand that hand mixed clay, or even clay mixed in a Soldner will take a week to ten days before plasticity is achieved. 2nd Adding kaolin or Hawthorne will lower plasticity, not increase it: both of these have very low plastic values. The original recipe is 50% Redart, 15% Goldart, 15% Om4, and 10% silica. OM4 is a medium plasticity clay, and it is common to see recipes with 25% OM4 for that reason. C&C ball clay is about 40% more plastic than OM4; and is readily available and cheap. Use the original recipe, and simply change out OM4 for C&C. I dry mix all the ingredients before mixing. Both Redart and Goldart have higher sulfide content, which produces your brown clay at cone 5/6. The final issue would be “soft slabs”? Slabs for tile, or slabs for hand building? High plasticity in clay bodies intended for tile can come back and bite you in the form of warping when drying, and excessive shrinking. Tom
  11. About all I fired was tiles. I a 6.5CF square top loader, and a 15.5CF front loader. The easiest method is to calculate your current tile size (in setters?), to come up with the width x length x height. You mentioned upright? ( see below). My top loader has 3” brick, with 1” insulation: extremely energy efficient. Front loaders are the easiest ergonomically, but have more hot and cold spots. When the temp gets into the 1800F plus range: static pressure likes to push the bottom of the door open ever so slightly, which allows cool air into the bottom front of the chamber. Easy to rectify with insulation. The upper rear of the chamber tends to fire hot, which I remedied by drilling a hole from the top; into the rear corner of the chamber. If you fire speciality glaze that is fickle to temperature variations; then a kiln vent would be advisable for a front loader. If the pic I posted resembles what you are doing: then it a simple matter of calculating how many setters wide, by how many setters deep, by how many setters tall: to arrive at a chamber size. Allow 2” of free space around the edges, and calculate space required for shelves and legs. Tom
  12. Was going through old files, when I came across the papers Marcia Selsor wrote on crystalline glaze back in the 1970’s. I know she does obvara and raku now: so that bought up a question. Q: Did your passion about a certain form, glaze, or technique change, and what caused that change? Tom
  13. Questions; heavy vegetation around the site it was collected? Does it feel sticky or tacky? Was it collected from a water shed area? Any dark/black material visible? Strong mold/bacteria odor? Calcium (lime) is a strong natural deflocculant. However, suspect more going on than that. Tom
  14. Sorry for the long step away: had eye surgery and just got released. C&C ball clay is 35-40% more plastic than OM4. So if the recipe calls for 25% OM4, then 15% C&C will have close to the same plasticity levels. You can just substitute the remaining 10% with clay. Tom
  15. Talc has high magnesium, up to 20%: which can alter color. Ball clays need to be under 2.00% total iron and magnesium to keep the color on the white side. C&C ball clay would be a choice. Aluminum content dictates refractory limits. Tom
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