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glazenerd

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About glazenerd

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    Now & Then

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    https://www.facebook.com/Clay-Tech-199305684284517/

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  • Location
    St. Louis, Mo.
  • Interests
    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. Having been alerted to possible risks: you seem to be comfortable with proceeding. Terra Cotta seems best suited: but suspect you will have to engineer it for this specific use. In the world of ceramics: 200F is not that extreme. The more relative points would be absorption and expansion. 12-14% absorption is readily obtainable if talc and magnesium are avoided in the formulation: both would lower absorption. Spodumene would be the body flux of choice because it would lower the expansion properties considerably. Equally important: peak firing should not exceed cone 04 (1945F) to ensure highe
  2. Hi Titi: Black clay does have impurities, and most likely uses sodium flux: which adds to the pinhole issues. A slow his cycle is necessary: running at 108F (44C) an hour climb from 1250 to 1800F (1000C). Looking at your glaze program: extend your hold time on step 3 to 20 minutes. Sodium fluxes off gas vigorously at the top end of the firing cycle. Tom
  3. I gave up on bentocrap years ago. After I learned a few things about clay chemistry- I developed my own suspender. T
  4. I use 80% alumina hydrate, 20% talc, and 3% V-gum T. I fire it to cone 10. throwing?- No. Hand formed 3/4" walls. Very low expansion- very high heat tolerance. The talc adds enough magnesium to fuse the materials. A semi-quasi knock off of a cordierite body. You can research "corderite body"- it is what you need for your purposes. Tom
  5. Clay that slumps indicates the lack of alumina. Wild unprocessed clays usually do not have any appreciable amounts of natural fluxes: some rare exceptions like clays harvested along a coast line (salt migration). Slumping can also be an indication of ultra fine sub micron particle size. If you only have a mild issue: 5-10% kaolin (any type will work) will fix the issue. Kaolin has 37% alumina content and will spike cone firing values quickly. Tom
  6. Never had an instructor: then again I never intended to go down the rabbit hole this far either. I noticed early on in another forum where I lurked mostly: there would be ten different answers to one question. I understood that application and style could vary from potter to potter: but never made sense on the chemistry end of the equation. So I made the decision early on to seek out information by those with PhD behind their names. Oddly enough, found variance there as well- who knew? If I had a mentor; that would be Ron Roy: who I had the pleasure to spend three days with at NCECA in KC (201
  7. Oddly enough: I would like to spend a day unseen in a corner, watching a newbie throw their first form, open their first kiln firing, or sell their first cup. The joy and excitement is contagious. Would have loved to spend a day with W.G. Lawrence (Phd@ Alfred) and pick his brain. Same for Orton Jr, Koppatchu, Horton, and a few others who laid the foundation on which modern pottery is built.
  8. Min correction noted. Should have been more clear. I use tangerine, purples, etc. most run in the $25-30 a lb range, and require 8-10% additions to achieve color saturation.
  9. Porcelain bodies will have 22-25% alumina, and stoneware lower. Alumina content will effect color development in red bodied clays: but typically only effects lighter pigment stains. . The % of stain required in a clay body will always be higher than glaze: because the molar % is different. Encapsulated stains do not work well in clay bodies because because the amounts of calcium or zinc required to release them is also much higher than glaze. Go through the Mason stain chart and find those specifically marked as "body" stains. The red body stain will produce a bright red at 12%, and lower % wi
  10. LeeS Sorry for the second post: had to switch over to my IPad to find a picture. I mixed a simple OM4 Terra Sig recipe and added 7 drops of Darvan 7 ( Credit Marcia Selsor for technique) Then placed it in the freezer for 15 minutes to show the effects of temperature on particle size. OM4 ihas a particle distribution below 1 micron. So the effects of settling would increase as particle sizes increased. In addition, inadequate deflocculation would add to this problem even in warmer weather.
  11. LeeS Like all things clay: kaolin has a range of particle sizes. An example would be- 0.50 to 10 microns in particle size. The theorem in slip casting is called "card stacking": where particles drop out of suspension and stack like cards. The closer you get to this principle: the denser the piece will be. Cracking off the bottom is an indication that larger particles settled out; with finer particles settling last creating a weak point. You made no mention about deflocculants? While the general theme about deflocculating centers around specific gravity; more to it than that- these additio
  12. Like the elbow trick- easier than the little press form I made.
  13. Agree with clay body suggestions. The same glaze will have variations on porcelain, white stoneware, buff, or red bodied clays. Potter preference and style plays more of a role in selection of clay and glaze: your "dream" glaze and forms work better on some clays than others. I have a test tile trick that I have yet to share: good as time as any. I peel the label off a regular empty soup can: punch a hole in the other end to prevent suction: and punch out an 100 test tiles in less than an hour. Limited use: but if you are simply exploring color development or testing glazes on a variety of cla
  14. Salt: Looking at your cone pack- you hit perhaps cone 5+ or so. Clay body fluxes are still off-gassing at this cone temp. In your first picture; upper right hand is a blister with a raised rim and exposed clay in the center. Anytime you get a raised rim: that means off gassing spars in the clay have created enough pressure to raise the glaze where it pushes through. Raised rims also occur when firing over red body clays; when inorganics have not been properly burned off- not applicable in this case. Potassium body fluxes create blisters, with less population and sodium body fluxes create
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