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Posts posted by glazenerd

  1. Actually Mark, your email convinced me to make one more post. The reason I am disconnecting is at the bottom. I cut and pasted this from the Crystalline Glaze forum- to where I just posted it under my user name there.


    Actually Gordon, I have been reading about the effects of PH on ionic exchange; in part the theory behind a terra Sig. Norton (M.I.T.) and Lawrence & Buttons (Alfred) did the original studies on slip starting in 1948-1974. Norton actually started the whole sodium silicate/terra sig when he was studying the effects of PH on the water hull. He wrote the "stretched membrane" theory in 1948: the basis of modern slip chemistry. Norton describes the measured effects of sodium silicate in solution: resulting in a particle range from 0.14 to 0.57 microns..or in pottery terms 30,000 mesh.

    If you are a big terra sig person: first buy Taylor ball clay from a Old Hickory Clay Co. In Kentucky. OM4 has a median particle size of 0.67, and Taylor 0.31. Which means you will get over twice the payload for your efforts. With freight, Taylor will cost you 0.50 cents a lb- but they sell 50lb bags only.

    Second is an ionic charge trick I will teach you. Lawrence and a Buttons also wrote on the effects of temperature on ionic charge. At 60F, the ionic charge that suspends those fine particles potters want in terra sig drops by 1/3, which directly effects the amount produced. However, at 140F, the ionic charge is at its peak: which means the quality of fine particles held in suspension is much higher- resulting in more yield.

    The trick: heat the water to 135-140F ( but no higher), add your sodium silicate first- then your Taylor ( or OM4). In 1-2 hours when it cools back to room temp 75 or so. Siphon off the goodies as usual. You now have more sig in two hours than in 24 hours.

    So remember folks- you heard it here first. As other potters hack the info from this site and add it to theirs and claim a great discovery- you heard it hear first.

    Enjoy Gordon
    tom. - was going to make a CM article out of this... But hey. Several recent threads on image theft, cultural theft: but theft of intellectual property is never mentioned.

  2. LT: there are 100 plus references to journals, thesis, books, and other resources on the effects of temp on clay bodies. 

    https://books.google.com/books?id=pQpCDCPqlS4C&pg=PA58&lpg=PA58&dq=W.H.+Sutton;+factors+influencing+the+strength+of+clay+bodies&source=bl&ots=AfkxuypAxo&sig=ACfU3U2W_Zh8NTluxpwvcnIN4zVOWBV3vw&hl=en&sa=X&ved=2ahUKEwjB-_rwj7jiAhUPnq0KHaUpC_YQ6AEwAXoECAQQAQ#v=onepage&q=W.H. Sutton%3B factors influencing the strength of clay bodies&f=false

    you need to hire a research assistant: I am a little busy.

    as I have told you in PM before- W.G. Lawrence  " Ceramic Science for the Potter." 

    F.H. Norton " Fine Ceramics, Technology and Application".  And the book referenced above are the best encapsulated information resources. 

    For the record: I have journal references lying all over the place in my various threads. Feel free to go find them. Long past tired of having post proof every time I make technical posts.


  3. Bill: results from Ougland & Brindley study on heat work.

    2192F (1200C) minimum further development of the clay body after this point. Minor decreases in absorption, along with minor increases in glass content. See chart below. Typical cone six ramp hold temperature for maturity.

    -----------------           ---------------------

    2192F (1200C.). Glass 62.   Silica 21.    Mullite 19
    2372F (1300C).  Glass 66.   Silica 16.    Mullite 21

    (Ougland & Brindley)


    Off topic,  Ron Roy emailed me: he is doing a work shop nearby in June. Looking forward to seeing my friend. 



  4. If anyone ran measured effects of heat work, it would be Orton Sr.  He wrote several abstracts for American Ceramic Society, I will nose around and see what I can find. The other source would be Ougland and Brindley from the British Ceramic Society: "Effects of a High Temperature on Kaolinite"  I read that abstract, and quoted some of it in my threads. It has since been pulled down, sadly. 

    I have been pricing gradient kilns: and potters gasp at the price of an electric. Keep waiting for a good used one to come along.  From my observation pending kiln size: there can be up to 40F difference in a large chamber. Years ago I started mixing my crystalline glazes via PH meter. I raise the PH in cold spots, and lower it in cold spots: works fairly well. 

    I would put conduction up to 2000F, and radiation there after. 

    Edit add: If Edison did not have Telsa: his inventions would have been few.  IMOIMO


  5. Sorry Bill, I broke my two cup rule. Never answer questions until I finish the second cup.

    As I recall: one study was done in Brazil , one in India for their Government, and one in Germany?  All three used gradient kilns with 6-10 chambers and 10-15C variation between chambers. The one in India was testing laterite, and reported an exothermic reaction at 2050F. The study in San Paulo? Actually used bars in various thickness 1/4 to 1/2 in a multi chamber gradient kiln: that studied produced the time of heat absorption and release at 2050F: conduction being the focus as I recall. The one out of Germany was studying local materials, and reported the reaction at 2050F. So 2050F spinel to mullite temp has been confirmed numerous times.

    U of I (Champaign/Urbana) has numerous studies up on their Ceramic Technology site. I posted a link in one of my ramblings somewhere : stoneware study thread I think. They used X-ray diffraction to analyze heat work and the phase changes in potassium and sodium. At 2190F, sodium and potassium are spent- no longer visible. I cannot confirm this: but I suspect this is where the commonly used 2190F peak with hold firing cycle came from.

    Orton Sr. Did extensive studies back in 1909-1919 range(?) noted in my Nerds Firing Schedule thread. He proposed the 108F ramp speed for several reasons: primarily to burn off inorganic carbons, secondly for heat work purposes. As you well know, cones are based on Segers work, but Orton did the initial testing on calibrated heat work.

    did I answer them that time, or do I need a third cup?  Check my Stoneware study thread, Nerds Firing Schedule thread, and possibly my Porcelain thread. I have links to studies floating all around. I do know some studies are no longer accessible: Wiley Library has been buying them up and archiving them. 


  6. Bill:

    Clay goes through three endothermic (absorbs heat) phases, and one exothermic (releases heat) phase.  

    200-250F  atmospheric moisture is driven off. Potters already know what happens when they blow through this ramp too fast. The reason most controller programs have a hold at this temperature.

    573C/ 1064F quartz inversion.  Pottery books explain this as alpha quartz converting to beta quartz. The technical aspect: at this temperature kaolin becomes metakaolin. Metakaolin is a fancy word for: all molecular water has been driven off. What is missing from the explanation is: silica (quartz) expands at this temperature: the reason bisq is so absorptive. However, molecular water begins to be driven off at 1030F.. So you have two processes going on at inversion: silica is expanding, and kaolin is shrinking because molecular moisture is being driven off. Porcelain is typically 50% kaolin and kaolin can have up to 15% molecular moisture.  The high silica content of porcelain is expanding, at the same time the high molecular moisture content of kaolin is shrinking: which explains why porcelain bodies are much more susceptible to dunting at inversion.

    At 950C/1745F metakaolin begins to convert to spinel. Spinel  is composed of alumina and silica: with excess silica being ejected as crystallyttes. The spinel conversion is what gives bisq its mass: sodium and potassium begin to melt above 2000F. The safety police get bent out of joint over many things pottery related: but in my opinion bisq is the most dangerous respiratory stage of pottery. As mentioned before: at 1730 metakaolin converts to spinel. When spinel forms, it ejects excess silica in the form crystallyttes. That white powder on bisq is almost pure crystalline silica in the 30,000 mesh range. Which is why I wash bisq straight out of the kiln; actually I converted to single fire a few years back so I would not have to deal with it. Let the glaze absorb it. 

    At 2050F is when spinel begins to convert to mullite: the only exothermic change clay goes through. Like quartz inversion, several things are going on at one time. Potassium begins to melt at 2012F, sodium begins to melt at 2044F, and spinel begins to convert to mullite at 2050F.  Sodium and potassium convert from solids to gas as they melt, which are the source of pinholes and blisters. As mentioned earlier: it can take up to 30 minute for the atmospheric temperature to penetrate a 1/2" wall. When you fast ramp in this upper temp range: your glaze might melt, but the core of the clay body is still immature. This equates to higher pinhole problems, and higher absorption rates. The slower you ramp above 2050F to cone 6; the more mullite develop occurs, and spars off gas better.

    This all reverts back to clay chemistry: porcelain bodies are held to a 4:1 SiAL ratio to: 1. Achieve maximum mullite development. 2. Minimize the amount of crystallytes produced. We only see the crystallyttes on the surface after bisq, the core is filled with them as well. Again back to clay chemistry: spar addition are there for glass content development, but also there to absorb crystallytes into that melt. If a clay body is not mixed according to certain parameters: you can fire it till he'll freezes over: it will still weep or absorb water.

    the final note is above 2230F. If the flux levels are off, and silica is too high, and alumina too low: crystallytes will not be absorbed into the melt. Free crystallytes then convert to cristabolite instead of mullite. You will know if that happens around 450F (cooling) and strange pinging noises are coming from your kiln during cristabolite inversion. As porcelain is more prone to quartz inversion due to its kaolin and silica content: stoneware is more prone to cristabolite inversion due to its much lower flux content. Porcelain typically has 25-30% flux content! and stoneware only 10-14%.


  7. 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 (cone 6 ), 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. 





  8. 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.


  9. 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


  10. 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? 

  11. 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).


  12. 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.


  13. 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.

  14. 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.



  15. 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.


  16. 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.


  17. 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.


  18. Liam:

    i agree, Mark has an inspiring success story in the pottery biz. After 45 years in carpentry, I am aware of the time, energy, and sacrifices he made to build it. Pottery is unique; full time, part time, weekenders, full time hobby, to monthly dabble. My only intention starting out was to "play" with crystalline glaze, until I accidentally fell down this very deep rabbit hole. Rabbit holes are also unique, but also common in pottery. To answer the QotW: I do not want to make any pottery plans: because it is the only area of my life that is not. I find the unplanned happy accidents of discovery a relief from a lifetime of calculations.


  19. 23 hours ago, liambesaw said:

    There will be plenty of time to retire when you're dead.  

    I spent the last 45 years telling myself that, not sure I agree these days.

    like most here, my life was the typical series of following schedules to earn a living. Planning, plotting, and projecting the path ahead. I find myself curious about not going to bed in order to get up at regulated time. I am curious to know what it is like not to meet deadlines, keep appointments, and plan for months ahead. Not sure I even know what "living for the day" means. I have held many titles over the last 45 years, most of which have little meaning: I find "couch tater" appealing lately.


  20. 16 hours ago, roderick said:

    It's grey and fires to a dark red brown grey color usually indicates magnetite iron source.around 1100C It started to boil and melt) earthenware clay 13-15% alumina, with 7-8% iron, low silica  45-50%?  Melting and boiling at 1100C /. 2012F?  Firing wood means carbon, which does not play well if high sulfide content in the clay. The iron/sulfide combination in a reduction atmosphere would lead to early glass formation at 1750-1800F ( 1000C) 

    however, I have other concerns about this early melt. I checked Finlands' mineralogical survey; lead is commonly found. Lead melts early and is fluid. Second concern: cryolite is also found. Google it. Melt at low temperatures and also creates fluidity and will boil at higher temps. Sodium and potassium melt at 2012 & 2044F respectively (1100C) range. So you are melted and boiling at 1100C, which means potassium and sodium are not responsible, nor is calcium. These are concerns, but most likely early glass formation from the iron.

     It is not sticky at all, although in the bucket it looks like it should be as there is a fair bit of surface water on the clay. sesquioxides like to hold water on the clay surface- one way to tell if your clay has them. Cracking is another way.

    I don't have much experience with raw clay but I guess this needs an addition of a plastifier like Bentonite absolutely not or Ball Clay or something? you need a ball clay with 27-29% alumina content. A CEC ranging between 7-8, or a WOPL of 33-34 at roughly 20% addition. In street terms, a medium plasticity ball clay: 80% raw clay, 20% ball clay. 

    So a couple of questions... 

    Should the clay be dried and powdered first? yes.Or can I just work out the water content and wedge the appropriate amount of plastifier into it? no


    Thanks in advance




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