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Everything posted by glazenerd

  1. Melanie: this article in CM will give you the basics on clay bodies. Direct link: https://ceramicartsnetwork.org/ceramics-monthly/ceramic-supplies/ceramic-raw-materials/techno-file-clay-body-shopping/
  2. B-mix (white stoneware) has 10-25% kaolin content- hence the "white." B-mix usually blends 50-60 mesh fire clay in lower %, which still allows for some body but without feeling any tooth when throwing. In addition, higher % of fine particle ball clay, with higher % flux= lower absorption values. But that also means higher COE values. (stoneware only application) you can use absorption data to help pinpoint COE values if none are given. To lower absorption: the body requires less large particle, more fine particle, with higher flux% in order to lower absorption: which means higher COE. conversely larger particle with less flux = higher absorption! with lower COE. ( generalization). Tom
  3. Spiffy: porcelain will always have a different feel than stoneware due to structure of kaolin clay. premium porcelain typically use premium plasticizers: macaloid or gums. Premium porcelain bodies have purer kaolin grades, premium plasticizers, and higher flux levels in order to achieve higher degrees of translucency. They typically have higher COE values for those reasons: Frost for example is 6.99 COE. Frost uses a gum ( not v gum) that creates that tacky feeling. Premium porcelain are off white to light buff in color. standard porcelain uses ball clay as a plasticizer: they can be tan to light grey in color. They fire to a high white, but little to no translucency. They are more user friendly starting out. COE bounce from 5.25 to 5.75 pending the amount of ball clay and flux. Buy according to cone rating. EX. Fire cone 6 to 6. Do not fire Cone 10 to cone 6. Porcelain is dependent upon flux levels to produce the glassy matrix that results in nearly zero absorption if fired correctly. Tom
  4. Tom: it is already broken down- just sat it in there to check space.
  5. Hi Moon- welcome to the forum. some pottery supply houses sell "china sand" which will work. Plain river sand will also work. River sand is also used by brick layers, also used in pool filters. If the edge is wide enough to support the weight, then yes. the other issue is quartz inversion which occurs at 1064F (563C) when firing large pieces, heavy pieces, or pieces with a lot of shelf contact: slow the firing down to 100F an hour from 1000 to 1100F. Do a forum search as "quartz inversion", I recall that thread from a few years back. Tom
  6. Leopold: large particle mesh does not suspend/disperse well in slip: so throwing would be a better option. If you are applying this slip over a clay body; while constantly stirring to keep them momentarily suspended, that would be possible. Mica changes from different deposits: so I cannot say with certainty that higher mesh size 50-80 would help, but theoretically I would think it would help. Mica is usually opaque, so I am curious about the source material. If the black color comes from iron, manganese: you will get your desired color. If it black from carbon, it will burn off at higher temps. I would have to assume this perticular mica source has been used for speckles before. Tom
  7. 1. Water content of the clay..the obvious variable. 2. % of ball clay or other plasticizers. These hold water longer than kaolin, dry slower. 3. Sodium vs othe fluxes: sodium accelerates drying..hydrophobic material.
  8. Low fire bodies typically run between 7-9.50COE, unless talc has been added to lower that number. Vitrification does not begin until 2050F. A cone 6-10 porcelain body fired lower would have a COE of 10 or above: the higher the silica %, the higher the expansion when under fired. correct, the COE of pit fire would be high regardless of body formulation. Again, the higher the silica%, the higher the expansion. The reason pyrophyllte is used in raku, or other "thermal shock" bodies; much more tolerant of rapid changes in heat. silica starts expanding at 1064F, (563C)
  9. Neil: got the porosity issue , did not know if there was a preferred COE.. Apparently not.
  10. Neil: if flame and oven ware are done in the 4-4.50COE range- what range do you think would work? T
  11. Bill: I have a ten page article on sanitary ware I am trying to find. Read it about 5-6 years ago: although the article was mainly based in the reduction of failures on the assembly line. They did get into firing temps however: as I recall it was around cone 1-2....But it has been too long. Next time I get up to the warehouse, going to cut up a few tank lids and take them on a test drive. Denice: back in the 40-60's, absorption test were done in autoclaves with 100 psi of 350F steam injected. There are numerous journals from those days on atmospheric moisture causing delayed crazing. They autoclaved glazed pieces as well, in an effort to pinpoint that problem. Tom
  12. LB: think we are surrounded by nay- Sayers!!!! Now I will have to fire one this summer.
  13. Leopold i can offer you an educated guess. Mica forms in sheet layers. ( schist) 20 mesh is large enough to have many; the heat caused expansion in those layers: and the outer layers popped off. I assume you already know most mica has 13% potassium, so I will venture to say you were hoping they would bleed? Tom
  14. Erin: your piece from class is on a high iron stoneware body. Copper reacts to iron, color shifts. The glaze run also indicates higher cone fire. Your new piece is on white stoneware or porcelain ( low-no iron), so that alone will change color. The glaze does not strike me as being mature: need more heat. I know DH fires a lot of SCM, so I would follow his firing schedule. Your pinholes are from potassium by the way: sodium creates much smaller pinholes. Tom note: EPK would be the better kaolin: much finer grain than #6- better melt.
  15. Well Pres: I talk to clay. I ask it questions. The really bad news: it usually answers. ( and I nod my head listening).
  16. By the way- a notice stating: "do not microwave or use in oven" would also be advisable.
  17. @manders "But isn't quite as glassy" from your comment, along with 1% absorption: sounds like they used ball clay as the plasticizer. Titanium content dulls out the shiny- translucent properties associated with porcelain; but the 1% absorption indicates high glass content. I will assume that absorption value came from your own testing? Several things you can do to ensure success: If you follow the common firing to 2190F with a hold- replace that with a peak of 2230F with a short hold. Secondly, if you have a programmable kiln: from 2050F up to 2230F, ramp at 125F an hour to this peak. The extra heat work will produce a tighter body- and should lower absorption. A cone 6 porcelain body will tolerate heat: you could also fire to a light cone 7. As Bill pointed out: glazing inside and leaving the outside unglazed causes an unequal stress from the glaze contracting. Getting the COE of your glaze as close as possible to the COE of the clay will be the big factor dealing with this issue. The freezer/ hot water test Min suggested will tell you if you got this right. if you have access to Laguna Frost porcelain (c5), this might be a better clay option for several reasons. (You can fire it to a light c6). It has premium plasticizers, along with higher body flux content that will produce a much higher degree of translucency. This translates to higher glass content, with zero absorption at c6. The added glass will make the unglazed areas "tactile", to use your description. The added flux will also increase the COE of the clay, making glaze fit easier. Frost has a COE of 6.99- much easier to match. The clay you are using is probably around 5.65. COE ( educated guess from your description) testing is imperative for functional use. Tom
  18. Benz: I have the opposite problem. I see some of the beautiful forms others make and wish I had the skill level to achieve it...but alas I do not. I long ago accepted the artistic gene skipped over me. Tom
  19. Nancy: some info about clay- firing. at 1000C; both the clay and engobe would be in an expanded state. Let me explain, at 563C molecular moisture has pretty much been driven out of clay. Kaolin can have 10-14% molecular moisture: so it obviously reacts more to calcining. Ball clays, fire clays, chlorites, can have as little as 2% moisture and upwards of 6%. Bentonites, hectorites, and smecites are unique- I will skip them. So moisture is driven out by 563C, but at this exact temperature silica converts from alpha to beta phase: and it actually expands. Both things are going on at quartz inversion (563C) silica continues to expand up to 2050F, once you cross this mark: then it begins to shrink as vitrification starts. If you go back to your test bar shrinkage rates: you gave them at bisq 1000C and glaze 1120C. The differences are a direct reflection of the expanded state at 1000C, and as the body is beginning to contract at 1120C. The pops at 1000C are most likely dirt, or oil marks: and the engobe simply let go. There are other conditions that can cause that: but you are using frit as the primary flux ( pseudo), so they are unlikely. Again referring back to your test bars: the difference between bisq and glaze values. You then have to convert those values: your clay at glaze is X% and the engobe is Y%. The cracks in the original pieces is a direct result of the shrinkage values of the clay and the engobe. (COE). a third variable is at play here. You said your original clay blend and engobe worked for six months then changed. The white clay is alluvial: and should be consistent. The sandy clay should also be consistent. The river clay however is sedimentary: and is subject to change. The river clay is the unknown factor. Any location could be subject to changes in fluxes, silica, iron, and alumina. The fact that your new blend is 7% shrinkage, also tells me there will be a slight change in COE. Alumina, silica, and iron levels predict COE and shrinkage values: flux levels will also cause variations. tom
  20. 7% shrinkage would be a typical ball clay value. That tells me a lot actually. Let me go back through the thread and see what the shrink rates were on prior blends. t
  21. Nancy: sintering the kaolin only removes molecular moisture: it does not change any properties beside shrinkage. You added the frit, so it "should" bond at 1120C. Should have already bonded.
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