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About glazenerd

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    Clay Research

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    St. Louis, Mo.
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    Crystalline glaze chemistry. Been stuck in clay chemistry and formulation. Writing formula limits for porcelain, stoneware, and low fire bodies. Developed new cone 04 colored porcelain body, cone 6 porcelain, and cone 10 bodies for public use.. 60yrs old.

    Email: optix52@aol.com

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  1. glazenerd

    Cone 6 Firing Schedule- Nerds

    Cone 6-10 Firing Schedule for dark or red bodied clay bodies. "Originally published in May 2018 issue of Ceramics Monthly, pages (68-69). http://www.ceramicsmonthly.org . Copyright, The American Ceramic Society. Reprinted with permission." These bodies are typically high in iron; but all include higher levels of carbons such as sulfur and lignite coal particles. White stoneware bodies are excluded. Firing defects include bloating, carbon coring, and blistering. There is a division among researchers dating back to the mid 1920’s on the exact mechanism that causing these firing defects. Roughly half concluded that CO gases escaping from carbonaceous materials reduced the iron. The other half concluded that the sulfur (sulfides) from lignite coal and/or from iron sulfide reduced the iron. Due to modern milling techniques and my own experiments in this area: I tend to lean to the sulfide part of this equation. Either way, bloating, coring and blistering is caused by the early reduction of iron: causing it to act as a vigorous flux which in turn begins to form glass as early as 1850F. This is nearly 200F degrees earlier than normally found in a firing cycle. The early formation of glass from reduction of iron creates an impermeable barrier which traps off gassing feldspars, and traps any remaining carbons. Pyrite (iron disulfide) is the most common source of iron in raw clays. Iron is what gives stoneware that warm toasty color potters love, and the red/brown colors. However, the sulfur that is chemically bonded to the iron is problem child. Heat too fast, and starve the oxygen in the kiln: and bloating, coring, and blistering will be the result. Reduction can occur in an electric kiln that is heavily packed, sealed, and heated quickly. *Edward Orton “The Role Played by Iron in the Burning of Clays”. Amer. Cer. Soc. 7:112 A.G. Bole/F.G. Jackson “The Oxidation of Ceramic Wares during Firing. Amer. Cer. Soc. 7:183 Kramer/Fritz “The Role of Oxidation in Porcelain and Ball Clay” Amer. Cer. Soc. 12:13 While these researchers reported their findings mostly in the 1920’s: Brownell, West, and Lawrence found similar results in the late 1950’s and into the mid 1970’s. From this research the following temperatures were reported as carbon burnout phases: Jackson reported that carbons began burning off at 800F, and accelerated to 1760F. which others confirmed. Orton believed sulfurs were completely removed between 750-1110F. Others concluded from 750 to 1700F, all carbons were burnt off. The consensus among all the researchers was that high levels of oxidation were required to completely remove all carbons. In the research done nearly thirty years later: the consensus was carbon removal occurred mostly between 1200-1750F. It was Orton who concluded that ferrous sulfide began vitrification as early as 1800F; which resulted in carbon trapping, and trapping of off gassing spars. F.G Jackson performed that most extensive laboratory research of the effects of sulfur, including decomposition and the evolution of reactions on other materials. Two primary results came from all of these studies. 1. The kiln must be heavily oxidized, especially from 1200-1750F. Prop the lid open slightly, remove bunges if need be. 2. The ramping temperature must be slowed down. The higher the iron level and carbon content: the slower the ramp needs to climb. (discussion to follow). Orton originally recommended a rate climb of 75F an hour for high iron/carbon clay bodies: which was later adjusted to 108F an hour. Others suggested a rate climb of 125F an hour in the 1200-1750F burn out zone. Only testing will conclude which rate is correct for the clay body you use. However, as a rule of thumb: the higher the iron content- the lower the rate of climb. Observations and conclusions I have made from testing: 1. If carbon coring is occurring: oxygen needs to be increased and ramping rates lowered by up to 60 degrees an hour. Carbon coring indicates that heavy reduction of iron is occurring around 1800F and high levels of inorganic carbons are present. 2. Bloating occurs when iron is only partially reduced: creating patches of glass instead of the hard shell associated with coring. This means some off gassing spars are being trapped, while some is escaping. This indicates a partial starvation of oxygen and iron being reduced in the 1900-2000F range: just prior to off-gassing spars. Ramp speeds need to be lowered by 20-30 degrees, and pulling a bunge or propping the lid to add slightly more oxygen. 3. Blistering (large craters) indicates that minor reduction in iron is occurring. A complete barrier has not formed, but the clay surface has become extremely dense causing escaping spars to push through under more pressure. Slowing down the ramp cycle by 10-20 degrees an hour during the burn out temperatures should resolve this issue. Pulling a bunge if the kiln is heavily loaded is also recommended. I cannot give you an exact firing schedule because red bodied and dark bodied stoneware has such a wide variance of iron and carbon levels. I can tell you that ramping between 108-125F an hour from 1200 to 1750F, while supplying lots of oxygen inside the chamber will resolve most all of these issues. If they continue after these recommendations: you have a heavily contaminated clay body that is probably best just to discontinue its use. It is also advisable that you do not start any type of reduction until 2050F, when a normal clay body begins to seal up. “Carbons” are most often used in an inclusive/broad sense when addressed in most pottery books. Organic carbons such as peat, twigs, bark, and other decayed plant matter burn off with little effort and no effect. Inorganic carbons such as lignite coal, iron pyrite, or other forms of sulfides are the primary contributors to coring, bloating, and craters. Tom Edit Note: permission is granted for educators to print and distribute this information as part of their instructional criteria.
  2. glazenerd


    Having never read the books or seen the movies: only commercials.
  3. glazenerd


    Most interesting. I see a bit of translucence, a bit of bone china, and the effects of Parian. Think you have dialed into a technique that works. I can certainly see market appeal.
  4. glazenerd

    Confusing Glaze Result

    Joseph: The recent complaints I have seen about clay being too wet, or not acting or firing like before; is not really manufacturer related. Like the big Custer change back in 1999; some of the raw clay mines are older; and are digging into new areas. Some of the old tried and true ball clays have enough compositional changes, that in turn effect water retention, color, etc. I would suspect clay makers are working through the changes; some will require a fair amount of time to resolve. i would recommend you stay away from any porcelain labeled as " translucent." Given the glazes you are using and the application rates: they would cause you some aggravating pin hole issues. Remember, higher translucency = higher flux levels! which = higher off gassing of spars, which = higher incidence of pin holes. Just as you are finding out that dark and red stoneware bodies can have variations in natural iron and magnesium levels; which effect glaze color. So again, raw clay materials are changing as they dig through the strata. There is an old tried and true ball clay that has been used in Terra Sig for years. I recently received an updated analysis showing higher carbon, sulfur; and larger particle size distributions. I tested the WOPL index last year: it now holds ten percent more water than it did five years ago. It is widely used in stoneware as well, which would translate to "wet" clay. It also means Terra Sig will not settle out like before, and the SG will be different. the higher CEC and WOPL changes the physical properties of the end product. * notice I intentionally left out producers and product names. Tom
  5. glazenerd

    Confusing Glaze Result

    Joseph: These are all stoneware bodies with various levels of iron in each. All are cone six, all with clear glaze: so the color is coming from the clay blends only. Obviously, the dark brown on the lower right has the highest content, and the blush color the lowest. The deeper blush has iron, with natural magnesium. The yellow/ straw is the middle of the road iron levels. I have a grey floating around somewhere. So yes, the possibility of lower iron in the raw clay is a possibility. I think LT's suggestion will help narrow down the cause.
  6. To obtain translucency; two primary changes in clay chemistry has to occur. The first being high purity grades of kaolin are required.The second, at cone six flux levels have to increase by 15-20%. Giving the conditions you are describing; I will go with 15-20% increase of nepheline syenite. Why? Because sodium in an aqueous solution will cause a hydrophobic reaction: meaning it has the tendency to repel water. In your case: rapid drying = cracking. Pending the sodium level, there may be a slight exothermic reaction as well. Anyone who has poured a plaster mold has felt this exothermic reaction when as it sets up: the mold gets warm. Wait 30-60 minutes after you pour: is the mold slightly warmer? If so, simple proof of high sodium content. If this holds to be true: then you need to pull the form much earlier. Only experimenting with the time will tell you when. High levels of sodium also means higher PH levels. The higher the PH, the faster it will dry. Higher PH will also effect plasticity, which will also cause cracking. You can try this simple (possible) fix: add 3 drops of vinegar to one cup of slip: stir then pour. Check the form for improvements. If cracks are less, but still present: add 5 drops of vinegar.in essence, you are Titrating the PH level back into the 8.35 range; where porcelains should be. Lastly, if Nep Sy is the flux of choice (usually is because it is the cheapest flux available) then you will have other issues arise in the near future. Nep Sy has 14-20% soluble salts that will build up on your molds in a short time. This build up will degrade your mold much faster than the norm. if you have other suppliers available to you, mark's advice of finding a new slip is a good idea. All depends on how much time you want to spend dialing this new slip in.
  7. glazenerd


    I have a crystalline recipe that uses cobalt and tin; plus a dash of rare earth oxide. The end result is similar to yours.
  8. glazenerd


    The pastel(s) are unique and appealing. The bone white of the porcelain certainly adds to the effects. Sometimes experimenting pays off.
  9. glazenerd


    Obviously cobalt, but adding tin perhaps?
  10. glazenerd


    Nice color development. On porcelain I assume?
  11. glazenerd


    Very nice piece AH, well done.
  12. glazenerd


  13. glazenerd

    PQotW: Week 41

    1-3 the rest, no idea. Although I have been reading posts on kiln building. ( reason I knew #1 )
  14. glazenerd

    Why not underfire clay

    Cone 5-10 bodies are primarily fluxed with potassium/ sodium spars; which at cone 04 only fuse materials together. These body types are actually expanding, becoming more porous until they hit 2050F, when they begin the early process of vitrification. These body types have absorption rates in the double digits if only fired to 04-06. So they will fail over a period of time if used for functional purposes. Plenty of threads on fired pieces absorbing water causing mold, foul tastes, and odors. Low fire bodies use different fluxes; such as talc ( magnesium) or boron. These have lower melt temps and do increase density, although still not for functional use. The more applicable effect is changing the COE of the clay to be more in line with low fire glazes. Nerd

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