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About glazenerd

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    Clay Research

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    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

    Email: optix52@aol.com

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  1. Educational Post Firing Schedule Variables There are several key issues that effect the final firing schedule selected. 1. Functional or Non-functional use. 2. Wall thickness: thrown or hand built up to 3/8". Structural starting at 1/2" up. Sculptural with varying thickness/ parts. 3. High iron/ carbon bodies vs. white body. 4. Single fire vs. bisq. For the bulk of most firings; functional or non-functional and single fire vs. bisq fire comprise most firings. The additional variable is how thick are these pieces? Several universities across the world have done studies using X-ray defraction to measure heat work in gradient kilns. The general consensus being that it can take up to 30 minutes for the atmospheric temperature to reach the core of the clay body in the 3/8 to 1/2" thick range. In order for clay to fully mature, this variable has to be included in the firing schedule for functional wares. Absorption rates increase, COE values can change and firing defects such as pin holing, blistering, and shivering can be attributed to firing schedules. Sodium (Nep Sy) is the flux of choice in the USA and Canada; and is commonly used in other parts of the world. It is a cheap body flux but it does create issues. Sodium begins to melt at 2044F, and potassium at 2012F, as the clay is converting from spinel to mullite at 2050F. In application; at the same time sodium begins to off gas vigorously, the porosity of the clay is beginning to close up. Extending the time climbing to peak temperature allows the feldspars to completely off gas; thereby resolving pin hole issues while maturing the clay. Selecting a preset ramp speed or programming your own depends upon the clay body, piece size, weight, and foot ring contact. In addition, starting at single fire or from bisq also decides ramp speed. Pieces with wall thickness above 3/8", heavy pieces above 7lbs, or pieces with large shelf contact such as platters need slower ramp speeds to allow for even heat distribution. Slower speeds during the quartz inversion range is also advisable for large format pieces Quartz inversion occurs at 573C (1064F) when quartz changes from alpha to beta phase. Silica (quartz) actually expands at this temperature: part of an exothermic reaction. Just prior to this phase change and to just above this temperature: molecular moisture is being driven out of the body resulting in overall shrinkage. These two processes are occurring relatively at the same time: overall shrinkage from the loss of molecular moisture, while silica is expanding during inversion. If pieces are heavy enough, have weight, or have large shelf contact such as platters: cracking can occur. The remedy for this issue is programming a 100F per hour climb from 1000 to 1100F before resuming higher ramp speeds. You can actually increase firing speed to 180 to 270F an hour if firing porcelain or white stoneware. The overall size and weight of the piece may still justify a slow ramp cycle once you pass the inversion temperature range. Wadding, sand, or alumina may be placed under large/heavy pieces to facilitate movement during the firing cycle. Dark and red bodied stoneware produce buff, terra cotta, and brown bodies that potters love. While they produce warm toasty colors, those colors come from iron disulfide. (Pyrite) in addition, lignite coal particles are common contaminants. Both sources of sulfides require special firing cycles to prevent blistering, bloating, and carbon coring. Inorganic carbons burn off from 1250 to 1750F, and require heavy oxidation during this temperature range. Rather single firing or bisq firing: programming a slow cycle of 108F an hour (slow speed) from 1250 to 1750f an hour while oxidizing the kiln is required. If single firing; you are simply programming a bisq fire, while incorporating the final ramps to peak temperature. If firing large, heavy, or large foot ring pieces: then adding a quartz inversion cycle is required. If firing dark or red bodied stoneware; then programming a slow ramp (108F) from 1250 to 1750F while oxidizing the kiln is required to avoid blistering, bloating, and coring. Once you reach 1800F in a single fire, then you can increase ramp speed to 180 to 270F until you hit 2050F. At this temp, speed is then reduced to 108 to 125F an hour to allow escaping spars to escape before the clay body vitrifies. University studies from around the world all report an endothermic reaction at 2050F as observed by X-ray defraction. It is a key reaction temperature in the firing cycle; when the porosity of the body begins to close rapidly. Most clay bodies in the USA and Canada use Nep Sy (sodium) as a body flux. At 2044F, sodium becomes reactive and off gasses vigorously; which appears as pin holes in the glaze. Rather single firing or starting from bisq; slow ramping from 2050F to peak hold allows the extra time for off gassing spars to dissipate. Recommended ramp cycle from 2050 to 2232F is 108-125F an hour. A commonly used peak temperature is 2190F with an extended hold: use the recommended ramp cycle for this program firing. This slow ramp cycle towards peak range also has the added benefit of extending element life. Tom
  2. Sneaky? I thought is was rather overt myself. Besides, it is viewership round up month. premium porcelain is 50% grolleg, 25% silica, and 25% Nep Sy. + 2% macaloid (Bentone ma) this is the fix it mix. If you do not have grolleg, then use EPK. EPK will diminish translucency a bit- your call. how much fix it mix you add is directly proportional to the amount of water you use throwing, which equates to how much fines you lose. So you have to make that judgment. For a gallon full of dried reclaim 1/8-1/4 cup of fix it mix will work. Most of the members post pictures of their work in the gallery. I am limited where I can post mine.
  3. Dearest Oldlady: coleman porcelain actually originated from experimental bodies used at Alfred. This article covers clay recycling, and Coleman in perticular. Premium cone 10 porcelain is 50% grolleg porcelain, 25% silica, 25% Nep Sy, and 2% Macaloid. (BentoneMA) instructions are in the article https://ceramicartsnetwork.org/ceramics-monthly/ceramic-supplies/ceramic-raw-materials/techno-file-clay-restoration/
  4. A recent thread on culture made me think about periods of art history. Being an arm chair historian, I often go through the history of potters before us: Adelaide Robineau's Scarab Vase comes to mind. Historians put art (including pottery) into periods: Medievel, Renaissance, Baroque, and Neo-Classical for instance. (Google "periods in art history"- you will find them) Each period in recent history lasted 100-200 years: the current period began in 1800. So if periods last 200 years, that would mean beginning in 2000, a new period should have begun.....but it has not! Qotw : what name would you ascribe to the current period of art history that began in 2000?
  5. Controlling the rate of drying has been on my agenda for a few years now. Couple of years back I was mixing a wetting agent into porcelain in an attempt to control it: those who got samples know how badly that flopped. so I have been working on a spritz made mostly of organic/food safe materials. To date, I have extended drying time by roughly 30%. When I get up to 50% or so, I will unveil it. However, I have no laboratory rats to test toxicity on, so PM me if you want to volunteer to drink a cup or two. ( that was Westerner joke by the way). T
  6. Been following this topic, but I know just enough about Japenese/Chinese/Korean culture to say something wrong. One of the many times I wish Baymore was around: these cultures were his speciality. Being a Westerner, I will add a twist to it. Modern technology has managed to put most of the worlds population onto a single stage/platform. The assimilation of culture that once took centuries, now only requires a mouse click. Marco Polo bought back porcelain pieces from China: and Europe spent 500 years trying to replicate them. Any potter can click Sung/Ming dynasty and start throwing in an attempt to duplicate it. The unfortunate side effect of instant knowledge, is the loss of culture and application. The old saying goes "imitation is the sincerest form of flattery." Should the blog writer be offended or flattered that others are trying to imitate her culture? Obviously some have hijacked strictly for ill advised sales promotions. Yet others love the work, love the culture, and are sincerely trying to imitate it. Personally I think she should educate rather than reprove. It is a futile effort to chastise those who have no respect to begin with. thats my 5 cents worth. The extra 3 cents if for shipping and handling. Tom
  7. Lee: let me give you an example of being me. YOU form your favorite stoneware clay and notice how pliable it is, how it takes texture, how fast it dries, and how plastic it is. i form the same clay and: 80/10/10, with 8.2CEC, potassium body flux, with 30% plastic materials. You have much more fun than I do But TY.
  8. Lee: your option 2 will work. Powder down your reclaim: you only need 500 grams. Make 4-100 grams test samples. Bar 1 add 5% Nep Sy. Bar 2 Add 10% Bar 3 15% and Bar 4 20% all four are flat tiles. Fire them flat on a kiln washed shelf to avoid plucking. Then run absorption test on each one- be sure to note on each bar the Nep Sy addition. Whatever level produces 2% or under absorption level-use that to fix the rest. You only need to do this test once: assuming all your reclaim has had the same prior results. From this point, weigh out dry reclaim in bulk: do not need to powder it down. EX. 10,000 grams of dried reclaim x 10% Nep Sy = 1000 gram flux addition. From this point, slurry it down as usual. Or pug.. Sounds like operator error at the factory! Although I hear more and more instances of clay misbehaving. Not impossible they dug through a vein at the mine with much lower sodium content- Custer went through that in the 90's. There are some ball clays that have changed, as well as some fire clays- it happens. Clay supplier on the East coast has been having tackiness issues, drying issues. From my testing they have dug through a vein of smecite deposited in the ball clay field. Tom
  9. Mousey: search clay makers and find a porcelain body with pyrophyllte. Grog is a widely misunderstood additive.
  10. Mary: in my testing, I noticed Hudson clay is extremely sensitive to application thickness. Also noticed color shifts when I increased potash content. Soon as I get time, going to slurry down a sample and spray it on. Tom
  11. Marbled being the key word--- first run some blend tests. 3-5-7% of stain: find your color. Chris Campbell is a good resource as mentioned. step 2 (requires some estimating). Slurry the white clay into a paste ( slightly thicker than honey) with the color blend you decide upon. the fun part- slab roll your clay down to tile thickness 3/8". Use a 1 or 2 or 3" paint brush ( brush size = color vein size.) paint a strip of yellow down the length, leave a blank stripe, then your other color, blank space, then yellow, blank, then color, etc. etc. fold over evenly, repeat process, fold again, repeat process, final fold. Send through a final time under slight compression to remove air. caution- fold over slowly so you do not trap air between folds. Second caution: the more you wedge- the more the color will blend. EX. Less wedge = more color definition and marbling. More wedging = color blending and loss of definition. T
  12. Kaolin can have up to 15% natural shrinkage! most of which comes from combined molecular moisture. Specifications for natural clay are not expressed by "shrinkage" as is the case with blended clay bodies.LOI (loss on ignition) is used to express the loss of weight in comparison to the total weight. EX: LOI is 13% so for ever 100 grams of material, 13 grams will be lost when heating up to bisq temperatures. Organic materials, lignite coal, and molecular moisture are examples of materials lost on ignition. The primary reason kaolin is calcined (EPK) is control excessive shrinking of material during firing; up to bisq temps. Materials can flake off or crack when there is excessive LOI. Calcining does not effect material characteristics, nor does it lower melting temps. T
  13. Rod: from table 1 in your link: Finnish clay is 50% silica, 18% alumina, 9% iron. In addition, 9% LOI and 3% carbon (discussed later. i saw nothing about lead or cryolite content, so that answers those concerns. Those addressed, your molten pool would be caused by low alumina, further compounded by the early reduction of iron (1750F/ 1000C) in a wood firing atmosphere. The 9% LOI would indicate sulfide/ sulfate content, coupled with 3% carbon. Carbon would be a direct indication of lignite coal particles. Sulfur/sulfide/sulfate content which is high in Finnish clay is directly involved in reducing the iron (9%), which is a powerful flux in reduction. Firing in a reduction atmosphere (wood) further compounds the early reduction of iron. you need to slow down the firing cycle to 60C an hour from 600 to 1000C. I have not done a wood fire, so one of our forum members who have that expertise will have to chime in on this issue. The second issue is the low alumina content. 18%. Mark C recommended the addition of refractory materials which usually consists of 10-20% kaolin; which has 37% alumina content typically. His recommendation is correct, however I recommended 20% ball clay with 27-28% alumina content because you have a plasticity issue. I was addressing the refractory and plasticity issue with one fix. The ball clay you linked will not work, because it is likewise has low alumina. However, you could blend a combination of 70% Finnish clay, 20% of the ball clay shown in your link, and 10% kaolin. If you have a glaze calculator; use the analysis for Finnish clay shown in table 1 of your link as the data in the calculator. Try various blends of Finnish clay, ball clay, and kaolin until you achieve 21-22% alumina content by weight or a SiAL ratio of 5:1. Remember you need the ball clay for plasticity, but also the alumina in kaolin to raise the PCE of the clay. Finnish clay has 9% iron, so diluting it with ball clay and kaolin will still give you a reasonably high iron content. One of our forum members who we nick named Proud Mary started a thread in Clay & Glaze Chemistry called " Hudson River Clay". She chronicles her journey of working with a local clay with an iron content of 8.4%. You will find many similarities to your journey. Tom
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