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Posts posted by glazenerd

  1. @neilestrick,  I am not aware of the specific mesh of stain, except to say it is larger. Body stains are the largest, they are used to add color to brick. The rest are either frit or zirconium/silicate encapsulated. So if you use a common frit size: 50-80 microns; or 120 to 200 mesh in pottery speak. In the world of particles: anything over 50 microns is classed as "grain size", in lieu of particle size.

    Om4 is the popular terra sig clay of choice; which has a particle distribution of 0.50 to 20 microns. When deflocculated, siphoned, and collected: terra sig averages 0.50 to 5 microns typically. In mesh size- 2500 to 24,000 mesh. So to visualize in our world: mixing sand with golf balls. The whole reason terra sig acts like a glaze is particle size.  So yes, particle size is involved.

    however, let me float this observation: I know iron and titanium bond to larger particles and calcium, sodium, and magnesium bond to smaller particles. ( some variations, but mostly  applicable.)  which means terra sig from Om4 would lose most of its iron and titanium content; but its calcium, sodium, and magnesium content would soar. So I tend to believe that a change in clay chemistry is playing in this mix as well. So @Callie Beller Diesel Question about soluble salts is on point as well.


  2. Moh

    look for a porcelain with a shrink rate in the 10.5 to 12 range. High plasticity in porcelain = higher memory.

    unglazed once fire: try this stacking arrangement. Less drag, more pieces per square foot. I rarely loss a piece to inversion using this firing  stacking method.


    i make this piece both ways: slab rolling then adding the raised deco. I have also slip cast this as well.


    this piece starts as a 14 x 16, and ends up 12 x 14

  3. Bill: 

    People often ask what I would spend lottery winnings on? I could easily drop $500,000 on testing equipment. Only draw back: I need to actually buy a ticket. I see that midline a lot on thicker pieces. One of the reasons I use potassium in test pieces: I can track it by discoloration. Unspent potassium always leaves behind a vapor trail.

    Tyler there is a book that might actually be useful to me.

  4. 5 hours ago, Min said:

    Potters refiring stoneware, especially forms like large flat platters, still have to take the cristobalite squeeze into account and slow the cooling down during the inversion.

    Just one more of the endless variables in pottery. 

  5. 2 hours ago, neilestrick said:

    Small world!

    What I was getting at with the glass info, is whether or not thickness really affects strength all that much. Once a weakness is established, will it just run through the body like scored glass? Is it more difficult to establish that weakness in a thicker piece? In Pinnell's test, which was done by pressing on bars spanning two supports, would a thicker piece actually be stronger than a thin piece, or do they all crack through at the same degree of pressure regardless of thickness? Are thin pieces more flexible, like wood?

    I have a 2500x scope upstairs, but no USB camera on it. Amazing actually what you cannot see with the human eye. I have seen grazing lines on the interior of porcelain.  Common to see iron saturation " spots" in stoneware bodies: which means the COE values in those areas are lower.  Two things I have observed: the thicker the wall, the porosity is higher in the center than the outside. Our hands, slab rollers, etc. just do not have the compressive strength. The glass content changes from the outer to the interior. 

    I read a study done years ago by an engineer in Brazil on thermodynamics and wall thickness. The most notable citation being: in a 1/2" thick wall: it can take an additional 30 minutes for temp to equalize to the kiln atmosphere. Potters prove that when they fire: adding a hold time or slowing down the ramp on the upper end to allow spars to off gas. The further the heat has to penetrate, the longer it takes for temps to equalize. 


  6. 4 hours ago, Bill Kielb said:

    I will read through, but again I did this for a year and realized there is a whole bunch of real world firings happening right now - no significant cristobalite issues.

    Fair amount of truth to that. Our generation are the beneficiaries of the research and corrections made by them before us. Problems from cristabolite inversion are rare; but not gone. Most commercial bodies have been formulated to deal with this issue. However, like all things pottery: still some squeakers once in awhile. I see it more when older recipes are used. Operator error as well. 


  7. 3 hours ago, Bill Kielb said:

    I still would guess the glaze is not a great fit and crazing  as most likely.   could easily be both. No idea if they buy or mix their own.


    Here is a good general reference to cristabolite and glaze compression from a college in Kansas. 




  8. @Bill Kielb 

    Thought I would move my response here from the pinging plate thread. Doubt they want to hear it.

    you mentioned in that thread cristabolite would not form until cone .14.

    Actually it starts to form by 1150C ( 2100F). It increases dramatically by 1250C, 

    however, if feldspar levels fall below 7%, it increases by 31% at 1150C (2100F). For that reason, stoneware formulation states a minimum of 10% spar additions.  For the same reason Si:AL ratios can increase those levels as well. The conversion is metakaolin to spinel ( at 2050F), and from spinel to mullite. Spinel is alumina/silicate: with excess silica being ejected as it converts to mullite.  So if alumina/silica ratios start going above 4:1, then excess silica is ejected as crystallyttes: (highly divided silica). All stoneware bodies eject some: if ejected silica increases,then spars have to increase to incorporate that ejected silica into a melt. Cristabolite inversion occurs on the way down. 

    Ougland & Brindley did a comprehensive study using x-ray defraction back in the 50's. 

    2192F (1200C.). Glass 62.   Silica 21.    Mullite 19.       The 21% free silica increases COE, and increases the cristabolite.
    2372F (1300C).  Glass 66.   Silica 16.    Mullite 21

    the reason my posted firing schedules change at 2050F to a much slower ramp is to increase mullite. It also allows off gassing as spars. 



  9. Lapis:

    "Potugese >stoneware<  going to give an answer for other potters as well. The pinging sound I suspect is due to cristabolite inversion: a problem more specific to >stoneware.<  cristobalite inversion occurs as the pieces are cooling! not heating up. Potters who like to peek or crash cool around. 450F subject stoneware pieces to thermal shock at almost the exact temperature cristabolite formations are at their inversion range: beta to alpha. 

    the clay body is compromised: the glaze will soon follow. 



  10. Perhaps a real world application.

    Low fire bodies (06-04) commonly have tested COE values from 6.50 up to 9.00. Laguna EM series is a good example: EM 337 , low fire sculpture comes in at 9.50. Yes, grog plays a role: but all the silica in a low fire is no where close to being melted. They make talc additions in low fire to handle thermal expansion: and now you know why. Silica lowers COE if it is fully incorporated into the amphorous structure: if not, it can raise COE..a lot!


  11. You did not list a cone fire: which makes a difference.

    as Liam suggested : large particle clay bodies will not work. This alone would knock out most (if not all) stoneware bodies. A high plasticity porcelain is smooth; but that comes with shrinkage rates at 13-14%. At low fire temps: you should find several smooth earthenware bodies. Low fire produces a softer fired body, but should be okay for jewelry. For jewelry, working properties would be as important as fired density. Low fire bodies have higher COE values than cone 6-10: so glazes need to correspond with cone rating.

    i do not buy commercial blends, so others will have to chime in on that.  Where is Pugaboo when I need her.



  12. 2 hours ago, Magnolia Mud Research said:


    "Undissolved quartz was present in Glaze B.  The quartz caused an increase in the thermal expansion of the glaze.  When the glaze was fired to a higher temperature and all of the quartz was dissolved, the thermal expansion of the glaze decreased.  Therefore, it is not always true to say that increasing the amount of silica in a glaze will lower its thermal expansion.  This is only true if there is no undissolved quatz (sic) present in the glaze" [Benson, Jennifer L. "Effects of Glaze Variables on the Mechanical Strength of Whitewares." (2015). pg. 67] 




    Lawrence addresses this issue in his book: Material Science for the Potter.

    free silica reacts much differently than amphorous glass. Most clays have up to 1/3 free silica, some higher, some lower. The flaw in glaze calculators: no variables for this issue. Sodium based glass (soda glass) is softer to begin with. Lots of variables on the original question: clay wise- mullite development will give the most strength.


  13. Interesting read Min, TY. Although he is referencing MOR values alone: and cone 04 produced the highest values. Two things missing in that observation: 60% red art delivers a fair amount of iron disulfide. I wonder how much the iron reduced, and accelerated the density of that perticular body?  I realize PSI was the topic, but absorption is just as important.  Linda discussed cristabolite inversion: a topic rarely addressed in oven ware. One of my favorites was the research done by Ougland & Brindley of The British Ceramic Society. They used x-ray defraction to quanitify their studies. The amount of glass, mullite, and free silica in a porcelain body at C6 and C10. 

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

    glass content is good, but mullite is the tough stuff that gives a body strength. Mullite can be manipulated by the Si:AL ratio to some degree.


  14. Mark:

    nothing like the voice of experience.

    the third parameter is material access. The various clays sold routinely in the pottery trade represents less than 10% of the clays actually available. I buy directly from Old Hickory mines in KY: of the nearly 30 varieties they mine, only two are sold in this trade.  Manufacturers have custom mixes blended at the mine, or custom slurries shipped in on rail cars. All the science and chemistry has been done before they unload product. When you stop and think about it: potters have an extensive database rolling around in their heads at any given time. Material science, chemistry, production, equipment use and repair, marketing, book keeping, and shipping. Impressive actually.


  15. Hitch:

    the term you need to research is MOR (modulus of rupture) as LT pointed out, particle size is a major player in green and fired strength. For instance in green strength: a 0.75 micron ball clay hovers around 400 psi. A 0.30 micron ball clay can approach nearly 800 psi. Particle size distribution is a common theory in clay formulation: but packing density increases psi.  Large particle/ mesh weakens a body because of the porosity. Glass/mullite is stronger, increasing the content allows for thinner walls. Here is a place to start

    https://books.google.com/books?id=vtzkmgkvYj8C&amp;pg=PA10&amp;lpg=PA10&amp;dq=modulus+of+rupture+for+clay+bodies&amp;source=bl&amp;ots=SWcZDpO-rL&amp;sig=ACfU3U2UxN8hNvngWGeLi5hFKByZ9RvZ3A&amp;hl=en&amp;sa=X&amp;ved=2ahUKEwi40cTx2JbgAhUE94MKHRwYCiUQ6AEwEnoECAkQAQ#v=onepage&amp;q=modulus of rupture for clay bodies&amp;f=false

  16. Citing the work of W.G. Lawrence, A.F. Norton, and D.D. Buttons ( all Alfred engineers)

    the question was rehydration: the application is molecular moisture: meaning H2O has chemically bonded to the crystal lattice of kaolin. As DW pointed out above- nice job!  As DW noted: once calcined, it takes time (lots), temperature will accelerate, as will pressure.  In reading the works of those three listed: the only reference made to rehydration from direct water contact was "minimal" over an extended period. No definition of "minimal" or "extended" was given. Although in reading other abstracts: 5% rehydration occurred. Note however that is 5% of the 12-14% of original molecular moisture lost in calcination.

    so why is heat and pressure required to accelerate rehydration?   Below is a quick diagram I made of a kaolin particle encapsulated by a molecule of water. Kaolin is a 1:1 clay particle: one side is alumina and the other side is silica. It has no inner platelets that will absorb water like ball clay does. Ball clay is a 2:1 particle. Kaolin will only hold water on its surface, and will not absorb water unless heat and pressure is applied. The resistance to absorption is part of the "stretched membrane" theory introduced by Norton in 1948. A molecule of kaolin is fractionally larger than a molecule of water: so the water molecule is slightly stretched as it encapsulates the kaolin. Applied, this means the water molecule is under increased surface tension which effects how the clay/ water film reacts.

    image.jpg.80de0b251b459fd390c376e2e3a04632.jpgthe two inner circles are a kaolin particle ( alumina and silica) The outer ring to the inner jagged ring is a water molecule. The space labeled "ionic disorder" is the area between the kaolin and water. This area is where negative or neutral charges from the kaolin react with water: which is a dipole. (Long winded discussion on dipole here) This disordered ionic area creates a chemical barrier which retards moisture absorption. Again, kaolin being a 1:1 particle naturally resists absorption.

    edit addition: sorry got called away.

    temperature accelerates rehydration because temperature increase is a direct measurement of atomic motion. The "disordered area" becomes more organized  in relation to positive and negative arrangement of atoms. 

    Pressure becomes relative because increased pressure closes the disordered space between kaolin and water.



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