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Everything posted by glazenerd

  1. By the way: this is a classic case where the lack of formula limits for clays comes into play.
  2. glazenerd

    Wide Range firing clay bodies

    This explanation from EU suppliers actually makes sense. I know kaolin and ball clays for the most part have a cone 32 rating without fluxes. So formulating to vitrify at roughly a high 5, low six makes sense: as I also know it will easily handle several cone above that. Thin wall, large angular pieces might have issues: but the majority of work will tolerate it. I formulate and use a cone six, but have fired it to cone nine: so I get it. I think I need to clarify however: when I originally posted about broad firing ranges: that was in reference to cone 04 to cone 10. Formulating a cone six and firing to cone 10 I get: the rest I do not. On the other spectrum: formulating for cone 10-12 and expecting vitrification at cone six: sets my alarm off as well. Thank you for inquiring from your local sources: a question I have long pondered. i think it should also be noted that kaolin and ball clay from Europe tend to have higher spar levels, and often finer grains. Hematitie is also the source of iron in natural clays: so some variance in formulation standards should be expected. All of which would play some minor roles in vitrification temps. t
  3. glazenerd

    Wide Range firing clay bodies

    To some degree I can speak for clay makers in the US: I know a few of them. I spent some time with a West coast clay maker at NCECA 2016 in KC. In particular we were discussing the evolution of Coleman porcelain: which by most measures is the industry standard on this side of the pond. So I had to ask the $64 question? Why do some of the US clay makers still use a wide firing range in their specs? Answer: supply and demand- some demand 30 cents per pound clay, so we supply it. yes Sputty, my nerdiness does not deal well with non specific applications or information: nor does it deal well with Wikipedia answers. I am interested in the responses from the clay makers you contacted. My question was sincere; I have genuinely been curious why I have seen so many posts stating a broad firing range? There are many US makers engaged in the same practice. Yet I know from firing crystalline glazes that pottery does have its oddities. Run a crystalline glaze on a calculator, and the 35/1 SiAl ratio does not work- but yet it does. Slips are not on my research agenda yet, but I have used engobes for many years. There has been a long history of misapplication: slips being defined as engobes, or vice versa. Then again, there has never really been a defined distinction. Clay has 20-22% water content; increase it to 40% and you have a slip. So an engobe would land somewhere in the middle: which still places it above the plastic limits and into the liquid limits. Then again, everything I just stated could be dissected and dismissed by other definitions of use. Which is why 98% of potters just buy it all premade. In regards to clay, from my own research and testing: there is a defined molar percentage that produces a vitrified body above cone 3. Below cone 3, clay is just various degrees of tightly fused. T
  4. glazenerd

    Engobe Questions

    Deuces: the better way to look at engobes is effect: what effect are you after? Engobes are used as a coating that completely mask the clay body under them. Some examples would be a porcelain engobe over a stoneware body: often used in crystalline glazes. Perhaps a porcelain over a dark bodies stoneware so glazes are clearer. Often times potters use one primary clay body, and use engobes to change things up without having to stock multiple clay bodies. engobes are used for carving; a stained engobe over porcelain: then carved through for effect. White engobe (Zircopax or titanium) over a dark stoneware: then carved. At cone 04: white earthenware over terra cotta, or vice versa, carved, layered, etc. The easiest way to use an engobe is to just mix it from your current clay. Add body stains, colorant oxides, etc. Porcelain takes stains, produces better whites than stoneware. Second option would be to select a stoneware and porcelain bodies with nearly equal COE values. EX: stoneware COE at 5.65 and porcelain at 5.75. Use one for the body and one as the engobe: buy quantities accordingly. Then you do not have to deal with recipes at all. Engobes are more about coating than anything else. once you get a few miles down the road with it: experiment! The fun stuff.if you want to do some intricate detailing that requires time. Dry the engobe clay completely, powder it down ( outside with a mask) then add 2% bentonite, blend dry, and add water slowly until you hit the creamy paste state. 2% bentonite will buy you extended carving time before it dries. if you want super white use porcelain : add up to 10% zirco. Add 2-3% feldspar to counteract the properties of the zirco. The easiest engobe is the body you are using, or using a second body in the same cone range, with a closely matching COE. Makes life much easier. t
  5. glazenerd

    Wide Range firing clay bodies

    Sputty: I have wondered for years why it was common practice in Europe to assign Cone 04 to 10 firing range on clay bodies: now I understand. Common practice becomes common acceptance.
  6. glazenerd

    Wide Range firing clay bodies

    Sputty: The sharp dividing line in pottery is: functional vs. non-functional. non- functional use is much more liberal and non- restrictive. There is much more wiggle in application. Functional work narrows down the freedoms into stricter parameters. Thus the wide variance in responses. T
  7. glazenerd


    Tile source: https://kruegerpottery.com/collections/bisque-tiles i am sure there are others.
  8. glazenerd

    Engobe Questions

    Sputty: perhaps using a paint analogy will work. 1. Terra sig..one coat: flexible, will move with the surface. Will show some of the surface under it. 2. Slip: 2 coats: will move less, but some with the surface under it. Will hide some but not all of the surface under it. 3.engobe 3 coats: will not move with the surface under it ( surface tension) will hide everything under it. Engobe is more akin to plaster over drywall. Essentially engobe is a thin coat of clay over another clay. That said: clay chemistry comes into play. shrogren (PHD) study: cristabolite formulation exponential if spar level under 10%. Ron Roy did the dilameter testing in this study. checks pulse.. I'm good...
  9. glazenerd

    Engobe Questions

    Bangs head on kiln lid. 85% clay content? - faints! Okay, my cardiac event has passed- onward!
  10. glazenerd

    Natural clay

    Mix exactly 100 grams of clay with 35 grams of water. Hand mix to a pliable ball. If it feels tacky, then you have a high plasticity clay that will handle cone 3-5 on its own. If it is dry and surface cracks form: you have a short clay that will need some help to mature and can handle higher temps. Next roll the entire lump into a thin strand, roughly 1/8th if possible. If it maintains its pliability: congrats you have a great cone 3-5 clay. If it begins to fall apart before the 1/8th bench mark: you will need to add at minimum 10% feldspar, 10% silica! and 10% ball clay. (oM4) works just fine. These additions will put you in the cone .6-8 range. potters add fine sand to clay all the time: it makes clay life interesting. The only exception would be: does the fine clay have a green/grey hue? If so, it has high levels of calcium hectorite. If so, then cone temp goes way down. Hectorite is common in CA. T
  11. Finished COE graphs, alumina to CEC comparison charts. Okay, so I lead a boring life! It is what we Nerds do.

    1. Show previous comments  1 more
    2. glazenerd


      LT: think you misread my post. The COE is one graph, and CEC/ alumina is a separate graph.

    3. Magnolia Mud Research

      Magnolia Mud Research

      Ooo Kaaay.    

      Still, such a correlation would be nice.  Measure CEC and get a COE as a bonus. 

    4. glazenerd


      Interesting thought, but sodium, potassium and calcium all exchange at different rates. I do think you could work a loose table off of spars. I have enough data tables, might run that over the winter. My next article is due out in August? I think. The one after that will make your lights go on. <teaser ad>

  12. glazenerd

    Spooze Question

    Tiny crack in porcelain; then later you state a larger crack in porcelain after bisq. 1. If on the bottom of the bowl: possible compression issue during throwing. 2. More likely blowing through the quartz inversion temp(563C) too fast during bisq fire. 3. Porcelain formulated without enough plasticizers, resulting in rapid dehydration. ( short clay) which door would you like? 1-2-3 repairing cracks should not be the norm, but rather the exception. T
  13. Hi Kaley, and welcome to the forum. several questions to help narrow down the list of "possibles" 1. Are you using a premix: if so name please? 2. Are you mixing your own recipe, if so what body flux are you using? 3. Pics of this film perhaps? 4. Whose stain are you using: and color #? 5. Have you mixed this slip before without stain? If so, did you notice a film? tom
  14. glazenerd

    The Act of Pugging

    Actually clay and glaze follow prescribed chemical reactions. The only variables are the humans mixing and firing it. Then you can add the errant nature of internet info, regurgitated wiki posts, and fiction verses fact. t
  15. LT chapter 6: nucleation and crystal growth i'm in! TY. Tom
  16. Gokul: i have been looking for this link for some time, and came across it today while reading some notes. http://lawr.ucdavis.edu/classes/ssc219/biogeo/exchca.htm#clay some definitions before you read 1:1 describes a clay particle: in particular kaolin. On a microscopic level kaolin looks like a saltine cracker: a single layer with alumina one one side and silicon on the other. This structure is the reason kaolin is mostly clean: no iron, magnesium, sulfides- etc. but it is also the reason kaolin is mostly non- plastic: it has no inner layers in which to hold water. For this same reason, it dries much more rapidly. 2:1 describes a clay particle with two outer layers of silicone; with one inner layer of alumina. On a microscopic level it looks like a sponge. Ball clay, bentonite, hectorite, and macaloid are all 2:1 clay particles. 2:1 particles do hold and absorb water: pending their structure and chemical composition: they can hold lots of water. Bentonite can hold up to 15 times it's weight in water. This absorption plays a role in plasticity, and also explains why stoneware takes longer to dry. be sure to click and view the table for a CEC and AEC. Important section for you to understand. cation exchange = cations are positive! and they are exchanged for negative anions. The more negative the charge, the greater the plasticity. anion exchange: anions are negative and are exchanged for cations resulting in a net positive particle charge. There are three points of interest to you: PH, anion exchange, and hydroxols/ oxides. Most all of your clay is acidic (PH), most all of it is classified as " kaolinitic" meaning even your ball clays have kaolin like properties, and lastly: sesquioxides. Most all of your clay has higher levels. Pay close attention to the AEC levels associated with kaolin. Strong AEC levels combined with acidity creates the problem known as cementing. Cementing means clay particles are tightly bonded, creating a cementing effect. The shearing of your fresh pug shown above attests to the AEC of your clay. Simple clay plasticity rule: CEC = deflocculation = plasticity. AEC = flocculation = colloidal cementation. both are PH dependent. terra sig works because sodium silicate averages 11 PH ( high alkalinity). Drop just a few drops of sodium silicate into a bowl of slip and watch the film spread across the surface. You are watching the effects of a negative ionic charge spreading. (CEC) vinegar works in throwing water because it repels the clay from your fingers. Vinegar ( acidic) works when joining pieces because treating the joints causes flocs ( particle accumulation) to occur. iE: sticky clay. Bentonites work because they run between 9-11 PH.( in part) Sharing some info while I have time. tom
  17. glazenerd

    How do I make black slip?

    Megan: The photo below is the black recipe I spoke of. The piece is upside down, so you can see the black color with and without glaze. Mason Stains have a chart; only a select few are marked as " body stains." U.S. pigments also has a select group marked as "body stains." The 3% red iron oxide and 5% black stain is based on dry weight. So in calculating that for slip: simply deduct 40% of the weight to reach dry recipe. EX: 10lbs of slip minus 4lbs. (40% water) = 6 lbs dry material. Slips are generally 40% water. +/- start with small batches until you get the color you are after: make notes of the additions used in each. T
  18. glazenerd

    How do I make black slip?

    Several of these colors start with small amounts of iron, including the yellow.
  19. glazenerd

    Pressure gauge reading

    No Babs, posted it to the black slip thread: it landed here.. Sorry OP oh Mods, need your help.. Move above post to black slip thread please.
  20. glazenerd

    How do I make black slip?

    There is a trick to it: start with 3-5% iron oxide to darken the slip down. Then you use much less stain- 4-5%. Iron is cheap, stain is not. Mason has two black stains: one with cobalt and one without. Forgot the numbers- sorry, but no cobalt. The cobalt will bleed a blue hue. t
  21. Pres: would you please start a new thread titled: " pictures of people taking selfies of their hands." I have to see Callie taking pictures with her nose.
  22. glazenerd

    Glaze Bilsters...

    Gokul: one thing about clay and glaze testing: you have to keep detailed notes. The recipe, application rate, firing schedule, etc. When you retest this issue: same clay, same glaze: but only change the hold time and application rate. The usual protocol is changing one thing at a time. If multiple things are changed at one time; then figuring out the source of the issue gets complicated. By changing the hold time only, you will know if the glaze boiled. Then change the application rate in a second test. If problems persist: then look at the recipe. t
  23. glazenerd

    Glaze Bilsters...

    Gokul: the color of the foot ring attests to the iron content of your than and ball clay. You sent me the spec sheets a month or so ago. You have unusual circumstances in India: it either takes months to get testing samples, order by multiple tons, or just wait. I think we discussed the clay Min suggested a couple of months ago as well: availability issue as I recall? to give you a point of reference: Western clay with 1.50-1.80% iron with 500-600 PPM sulfates/ sulfides run 9.50 LOI typically. the clay Min suggested is well below 1% iron, yet the LOI is nearly 13%. Even though it is a large particle, much cleaner ball clay: it still has much higher sulfate/ sulfide content. What makes clay formulation in India so tough. If the calcium bentonite would ever show up; that i had you order two months ago: could reformulate a whiter stoneware body. hematitie is FeO3, and has ferro- magnetic properties: even on a molecular level. Unlike iron pyrite, hematite binds sulfides to it, making it much harder to burn them off. Now how that effects glazes with these clays included, I am uncertain. Few years back I ran several glaze tests on clean porcelain : testing sulfides in the glaze. Below is a pic of 4% moly sulfide on porcelain at cone six: classic glaze defect blister. However, when iron/ sulfur in the clay body causes early vitrification: the escaping spars escape under pressure: causing a raised area around the blister: like a volcanic cone. And when off gassing spars are not under pressure, but simply have not been given time to escape: you get small pinholes and bubbles. follow the bubble pattern and it will tell you the causes. tom
  24. glazenerd

    Glaze Bilsters...

    Gokul.: the Than clay and ball clay also have 2-3 times the iron compared to Western clay. You also have higher carbon and sulfide contents. The joys of sub- tropical alluvial soil. However, as Mea pointed out, glaze application is on the heavy side. easy enough to test: cut the 40 minute hold to five minutes. Cut the application back 20% or so. Fire the same glaze for accurate comparison. the foot ring on the second picture is telling as well. See the color shift from the bottom to just under the glaze line? The clay is certainly mature.
  25. glazenerd

    Glaze Bilsters...

    By the way, break those examples open ( smash actually into small pieces). Look for darker, to black blotches, discoloration, etc. All early signs of carbon coring. t

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