Jump to content


  • Content Count

  • Joined

  • Last visited

Everything posted by glazenerd

  1. Benz: i did well in chemistry and decided to go to junior college to pursue chemistry. I opted for jr. College because I was not certain I wanted to pursue chemistry. I walked out after two weeks realizing I did not want to spend my life inside a building. Being a farm boy, I prefer to be outside. Most of our schools and junior colleges dropped construction trades nearly two decades ago- and it is showing up now. T
  2. Nancy: i wrote an article on " cation exchange" in the January 2018 edition of Ceramics Monthly. (CEC) cation exchange capacity is a numerical value assigned to clays that predict the level of plasticity they can impart. A high plasticity ball clay can have a CEC of 9.00 or so, and bentonites can be 50-150. Plasticity is created by a negative ionic charge in the clay water film. Pyrite is FeS2. Or iron- disulfide. The chemically bond sulfur, along with sulfides from lignite coal particles can cause coring, bloating, or blistering in clay bodies. You can look on my page: think I wrote about in the May issue last year. Hematite and magnatite do reduce in a kiln. Iron is considered a flux in clay bodies. Clays form the same way, regardless of geography. They are influenced by the same natural mechanisms. There is some deviations if parent minerals are isolated to specific places. Clays in sub-tropical climates are also much different because of the acidic climate in which they form. All vegetation has potassium, calcium, and sodium: as it decays those are washed into surrounding soils and through clay beds: part of the reason sedimentary clays tend to be so plastic. at one point I use to post lengthy sections on clay chemistry, stopped doing that. Then I started posting synopsis, but that generally requires me to explain why. Tom
  3. Unrelated to pottery, but the Homebuilders just did a survey in 2017.. For every ten people retiring from the trades, only three are getting in. Our local skilled labor pool is down by almost half. Labor prices have gone up 50% in the last two years! and I suspect they will double in another year or so. The youngest person in the trades that I know is 35. A friend of mine in HVAC just ran a help wanted ad: starting pay $20 hour, no experience, plus health ins- zero applicants. Another year, if that long I will watch from the sidelines. pottery wise: the only bisq paint your own shop in my area closed in the mid 80's
  4. LT the way I phrased my response was misleading, given your response. I was only referencing that higher levels of calcium and potassium are commonly found in sedimentary clays. Calcium was referenced in relation to cation exchange: any clay with a CEC above 11 is classified as a "swelling" clay. Calcium is more beneficial to plasticity in clay bodies because it does not stretch the water membrane: even though calcium has lower CEC values. Swelling being defined as absorbing water into the platelets causing a change in particle size. Not to be confused with the gelatinous nature caused by the large covalent sodium ion stretching/ distorting the water membrane. Nerd
  5. I suspect that the fine clay particles are part of the cause of the river clay cracking on drying, along with a reasonable probability that these fine clay particles are in the 2:1 Smectite mineral group which will expand and/or shrink as water moves in and out of its flexible crystal structure. Tom should have further insights. correct LT. with one add...calcium. The weight comes from water absorption: bentonitites can absorb 15 times their weight in water. Which also plays a role in rapid dehydration. Not aware of the topography or vegetation in that region: but where heavy vegetation is: potassium, calcium, and sodium is sure to follow. the orange sandy clay is high in iron, sand, alumina and larger particle. At 60% you get the fired color. the white clay is fine particle, higher cation exchange (plasticity), higher in natural fluxes. At 25% it will help with plasticity, and supply part of the particle size distribution. (PSD) the alluvial river clay should supply sub micron particles, and plasticity. 15% T
  6. Low alumina causes the issues. In clay, low alumina contributes to slumping and reduced strength. Pieces tend to be moe fragile, there is also the effects on green strength. In glaze, alumina is increased for strength; primarily to reduce cutlery marks. Think of alumina as the skeleton, on which everything else hangs. It is the "bones" in clay and glaze. The orange sandy clay has the iron and alumina: but little natural fluxes. The "whitish" has the fluxes and is fine grained: but a little alumina. The river clay most likely has a lot of humus, but should be high in potassium, sodium, and calcium. So I would look at a native clay mix of 60% sandy clay, 25% whitish clay and 15% river clay to start. the engobe mixture will be drier because of the added kaolin and frit. You can add 5% of your whitish clay to help with drying issues. Given you location, I doubt you have ready access to bentonite or macaloid. Tom
  7. You can run an absorption test on an unglazed test tile. I am sure Tony Hanson has the protocols floating around on his site somewhere. High absorption = no vitrification. No vitrification = high expansion (COE)
  8. @njabeid You claimed your post was long and boring: after reading this one you will retract that. your engobe recipe falls into a hybrid of sorts. Engobes by application is a clay body slurry applied over the primary body. Engobes can have up to 50% clay, while many glazes contain up to 20% clay. At 35% , at best you are at the starting point of an engobe. After analyzing your recipe: alumina is 11.74% molar- which is low. In application, it causes weakness and mechanical issues in a clay body. Secondly, boron (borax) is not used as a flux in clay bodies because it is a secondary glass former, but it actually interferes with typical soda glass formations if levels get too high. I am sure the original recipe included it to ensure a lower temp melt. I also believe this engobe was formulated to match a stoneware body at cone 5-6. Red bodied clay in the States either comes with higher clay% with lithia fluxes to create lower COE values (4.50), or lower clay% with higher flux levels that vitrify at cone 5-6. When you fire a cone 5-6 body to lower cone values (1170C/ 2066F) it results in higher COE values because vitrification does not occur. All of that said: I would estimate the final COE of your clay in the 7.50-8.00 range. I have an extensive list of tested clay bodies supplied by a friend: from that I am assessing your body higher because I doubt any lithia is present. Your engobe calculates to 5.49 COE, and your clay body I estimated at 7.50, to 8.00. I think this is where the typical COE grazing pattern is coming from. Then you have the low alumina issue of 11.74% molar. Finally, 10% zirco lowers the COE by 0.50%. 45% kaolin (calcined), no talc, 20% frit. 3110, 10% Nep Sy, 10% silica, 5% borax, 10% zirco = 6.32 COE. Alumina 15.61 SiAL 4.61 ** you should not fire this much above your current peak because of the frit content. Test it first! It would also serve you to run shrinkage test bars on your clay so you can keep track of changes as you dig or blend. Tom
  9. Not too long, nor boring: all info that I need. Judging from the wet to dry- dry to fired: your iron source is hematite instead of iron disulfide (pyrite). Pyrite would not fire that dark at lower temperatures. Your original clay has 2-2.5% iron and the new has 5 or so % iron content. ( estimated). I will run an estimated body to see what kind of COE the clay would have vs. the engobe. Apparently you have limited access to clay. T
  10. @njabeid Could you please post your current engobe recipe? If you have them; perhaps a picture of the old and new clay-unglazed. Tom
  11. but the engobe obviously doesn't fit the clays (two different mixes). Exactly: I think there are several problems going on together. I found it interesting that the engobe is fracturing in a very distinct crazing pattern commonly associated with COE issues. Although other samples suggests delamination of the engobe, which could be poor application, oil/ dirt on the surface, but also bond failure. Except for one piece they made it through the bisq fire- that one piece would be oil/ dirt or application issues. You asked why the engobe would change? How long has it sat since you first mixed it. Flux (spars) have soluble salts that migrate out and form granules- seen any evidence of that? back to clay ( my area) low fire clay bodies do not vitrify in the traditional sense; which means the metakaolin (clay) is in an expanded state. With that comes COE values ranging from 7.00 up to 9.50. The engobe recipes shown have significant talc additions: which is used to lower thermal expansion, and thereby lower COE. Summed up: your engobe is shrinking at a much higher rate than the clay. Which is where I think the COE graze pattern, the bond failure, and the cracks are coming from. From what I understood: previous engobe issues had been corrected. One thing I would do because high iron in the clay is involved: is crack a piece open and look for black (glass) coring. If there are enough sulfides in the clay, this could easily occur. Doubt it is occurring, but needs to be ruled out. Nerd
  12. "Rough sandy clay" is most likely just sand. However, I am seeing issues that make me question that. I see a deeper color; which means more iron in the new clay. "Rough" can also come from higher alumina and. Iron content. Your "blowouts" suggests the natural fluxes in the old clay were higher than in the new: clay/ interface bond failure. I see pinholes in one sample: also a new issue. The easiest way to help solve this issue is to run your glaze on a calculator, and check the flux levels, and more importantly the COE. If the glaze fits, then dial in the engobe to the flux levels and COE of the glaze. It will not be exact, but head in that direction.I would also calcine all the kaolin/clay used in the engobe. Tom
  13. Donna: Sounds like it ran just fine: you melted just past cone 7. There should be a section in your manual about TC (thermocouple) offset. All new kilns with controllers need calibrated: nothing unusual. Tom
  14. Harriet: I did not have the time yesterday to analyze your recipe; this morning I did. ( all values in molar %) cone 4 22% OM4, 20% Tenn.1, 58% Nep Sy. (molar). KnaO 8.97. Alum. 18.67. Si 70.49. COE (est.) 7.57 cone 4 mod. 22% OM4, 20% Tenn1, 40% Nep Sy, 18% EPK. KnaO 6.87, Alum. 21.19, Si 70.05 COE (est.) 6.85 cone 6 standard porcelain recipe. KnaO 3.94, Alum. 20.41 Si. 74.08. COE (est.) 6.00 ---------- Direct Anaylysis cone 4 KnaO 8.97. Cone 4 mod. KnaO 6.87. Cone 6 KnaO 3.94 ( molar) Alum. Cone 4 18.67. Cone 4 mod. 21.19. Cone 6 20.41. Edit note: the original recipe was all ball clay, which would put it in the stoneware or earthenware catergory. Molar levels for cone. 6 stoneware runs from 2.89 to 3.29% molar. In which case this recipe has nearly three times the KnaO levels of cone 6 stoneware. I had mentioned in another recent thread that the lack of alum levels play a larger role in slumping rather than flux levels. Your original recipe had larger pieces slumping, and smaller ones in tact. It is a good case study for alum vs. flux levels. The reason I posted in molar was to give a direct comparison to KnaO: your recipe was 8.97, and typical cone 6 was 3.94. Your recipe has over double the KnaO level of cone 6, although you are firing to cone 4. The alum levels were 18.67, and yet it still almost with stood over double the flux levels. Tom
  15. Waiting for the next experiment.
  16. Babs: I have used 10% rutile in crystalline; got into an orange color. Not sure how it would react to potassium.
  17. Well, Mark is right- shy on alumina. Om4 and Tenn 1 are fined ultra fine grain, which is also playing a role. At cone 6, 30% Nep Sy is more than enough to get vitrification. Om4 23%, Tenn 1 22% , EPK or equal 18% Nep Sy 37% . are you adding a defloc before casting?
  18. How much of the recipe is flux? Potassium, calcium, sodium, etc? Should be listed as Nep Sy, G-200, Kona, etc. or just post the recipe? T
  19. @Harriet Welcome to the forums. Plenty of folks to help solve the issue; but info and pics help a lot. You already stated "vitrified at cone 4". So are we to assume this slip is rated for cone 4? What type of slip? Porcelain, earthenware, etc? What do you mean by large? Premixed or your own recipe? Room temp at the time of casting? Does it state a range of cone firings? 04 to 4.. For example. We need info to help assess this issue. Tom
  20. From the report: Lewis County clay has 31-42% alumina, so variations would not surprise me. Usually clay gets more plastic as you move down a hill: the potassium, calcium, and potassium from decaying plant life mixes with ground water and deposits in lower levels. Some deposits are sedimentary, others deposits from when icebergs came across: Ohio, Kentucky, and Tennessee are prime examples. Alumina levels come from the parent material: kaolin often picks up alumina from bauxite or mica. Usually high plasticity ball clays are found next to coal seams: but that also comes at the expense of added lignite coal particles and sulfides. That said, in NY hectorite is formed in the high elevations of the mountains from decayed plant life. I received a sample last year: perhaps one of the most plastic and unique materials I have seen. Not impossible for that same scenario in your location. There are many rather cheap ball clays that can be purchased and added to your native clay to correct plastic properties. You have the unique opportunity to work with hematite (iron), which produces colors much richer than the common iron source in most pottery clays. You should be able to produce buff, terra cotta, and browns with little effort. Although your terra cotta color will be deeper than the classic color. All part of the learning curve; you will get there. Enjoy the journey! Tom
  21. @Beggs n Achin There is no single book that details clay chemistry in specifics that I am aware of. There are natural clays that are ready to fire straight out of the ground. They have the right flux levels, alumina levels, and silica levels. To give you a quick review on alumina vs. cone fire. Low fire clays typically only have 15-16% alumina by weight. Compared to the typical ball clays used in pottery which have 26-30% alumina. Kaolin (porcelain) typically runs 37% alumina, with some minor deviations. A low fire body with 50% red art (15% alumina), 15% talc (no alumina), and 15% silica (no alumina) and 20% ball clay (27%) alumina. Of the entire recipe, there is only 13% alumina. There is a common belief among potters that low fire bodies slump at higher cone fire due to flux levels, that is actually a misrepresentation. Clay typically has a cone value of 32, so like silica and alumina: it takes a lot of flux to get it fluid enough to slump. From the low fire premise we come to your native clay. You are firing a native clay with 36 to 40% alumina, 4.32% hematite iron. Unknown, but the natural flux content is minimal given its host mineral is shale. So I added talc, which has high magnesium which melts just above 1500F. I also added frit which begins to melt at 1475F. So if you do not have the heat to fuse this highly refractory native clay, then I will add materials that do melt. @Benzine I have certainly had my share of flops over the years; part of the learning curve. T
  22. I once fire crystalline glaze all the time. I use a very simple ramp: 160F an hour to peak, with 10 minute hold. Yes it takes longer, but I do not have to worry about inversion temps, easier on the elements, and draws less power. I make my own (very pure) porcelain: so I do not have to worry about organics or iron reduction.i make my own glaze suspender as well: which gives me the needed clay source in the glaze. Single fire glazes do better with 5% clay additions: EPK will work just fine. T
  23. Beggs: your local clay runs from 31 to 42% alumina: I used the median value of 36%. Lewis County clay was specifically tested at 4.32% iron- which is sourced from hematite. Most pottery clay (red or dark) has 5-8% iron (disulfide). The good news about hematite is it will produce deeper iron colors even though there is less of it. However, once you get into the 2100F range; the orange/ red color will darken until it becomes brown. friend of mine works at a brick mill: I had the pleasure of walking through their 300 foot long tunnel kiln when it was under repairs. Without getting into a lengthy explanation about the differences in brick and stoneware: brick is fired in reduction and is dependent upon sulfides reducing the iron to produce density. Pottery from the 40-50's were actually made from brick recipes; our modern stoneware has its roots in brick/ clay recipes. The only reason I recommended frit was due to the fact you do not have the means to produce enough heat. Frit begins to melt at 1475F, and it will provide you with pseudo vitrification. If you can get into the 1850-2000F range in the future, then I can recommend other (cheap) additions to get the job done. Silica melts at 3150F without flux additions, yet it is melted everyday at much lower temperatures. Alumina incorporates into a melt at much lower temps as well: it's all about the fluxes. Yes, a cheap hand held pyrometer would serve you well; it will give you some sense of consistency in firing using your current method. I would also recommend some welders gloves if you are getting your hands that close to an open heat source. remember it's just clay: and like any glaze it can be manipulated to produce results. T
  24. Lewis County is primarily shale, with some mica. Iron is from hematite 4.32%. Alumina averages 36%, so it is a sesquoxide. Water of Plasticity (WOPL) 29.3%. Dry Shrinkage 4.2% The green cast is coming from shale, and this is a low plasticity clay. mix 50% of your local clay, 15% talc, 15% OM4 ball clay, 20% frit ( pick one) this should produce a workable body. The only reason frit is added is to keep it from crumbling. This is not a workable body on its own- sorry. After you mix it, you will have to let it sit 7-10 days before plasticity develops. Tom
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.