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Posts posted by glazenerd

  1. Organics: most likely Ord or Muck variety humus. Both carry very high CEC values (plasticity), as both absorb and hold moisture. Not uncommon for these materials to take several days longer to dry compared to commercial clay. You cab burn off Organics by heating up to 1150F. Organics may be the only plastic material in your wild clay: burning them off might have adverse effects. Testing is the only way to know. There is a Facebook group- "Wild Clay Club" devoted exclusively to processing/firing locally sourced clays. Not many threads about it on this site.



  2. On 9/27/2019 at 6:56 PM, Babs said:

    And something else a lime green  maybe magnesium??

    Think the crystal forming can make what is sometimes a satin/ shiny blue glaze turn matt and also the blue will " drop " out of the glaze leaving it amber shiny and splurges of matt  blue...interesting if not wanted. Nice greys but....

    Wish Nerd would chip in..


    Been running "reaction" tests since last spring on colored porcelains. If the kaolin has higher titanium; there will be A color shirt; but those are fairly subtle. Iron bearing stains react excessively to zinc lazes. Zirconium based stains are the most stable. % of stain matters; but brown/gray result is typically a reaction. "Ody" stains are the most reliable and predictable.  When I get a chance, I will snap a picture of some reactions in yellow, purple, and brown.  Not been overly active lately- many posts get past me.



  3. Depends if the grog is being used to increase structural support: sculptural for instance. If used for appearance; then the parameters change. Mullite and molochite are commonly used in red or dark body for "speckling; or manganese/iron in white body or porcelain. The human eyes can see 50 mesh; so 20-50 mesh is used. 1% of 60 mesh manganese for example will produce 400 "speckles" per square inch. Stoneware bodies will handle 0.25 to 1.00% additions with nominal effects on absorption. If your clay body produces a consistent cream when you throw; there should be plenty of fines to encapsulate grog additions. Larger grog particles will produce visual effects; but are also tougher on the hands. I would start at 0.50% addition in a small test batch. Mullite, molochite, Manganese, and magnetite iron are all non-absorptive materials.

  4. Hey dank:

    Iron based stains work poorly as body stains. "Seal Brown" is similarly based iron brown stain that mixes dark, but fires very light with a heavy taupe cast. "Woodland" is more expensive but requires far less stain:1-2% for lighter brown/ deep tan. The bottom right hand corner is Woodland. At just 5% it fires a deep brown. Of equal importance is the clear glaze you select. Any glaze containing zinc will produce a matte/ microcrystalline finish. Boron based clears produce a clear/Hugh gloss finish that illustrate colors well. Matte clears will change color as well.




  5. Chloe:

    been thinking about your unique goal. You will need 1lb of OM4 ball clay (or equal), 1/4 lb of silica, and 1/4 lb Nep Sy, Custer, minspar, mahavir or flux readily available to you.  Mix it at 80% OM4, and 10% silica and flux. If you do not have a scale then: 1 cup OM4 ball clay, and 1/8 cup each of silica and flux. Mix it throughly dry and place it in a bowl by your wheel. (Keep a tablespoon in it.)

    After you finish throwing; with clay cream still on your hands: spoon one tablespoon on one hand and rub your hands until coated with the mix. Now go back and let the piece run in your hands; allowing it to pick up the dry mix. Then collapse the piece and cone the lump several times to throughly mix it in. Remove and wrap in plastic. This lump can be reused in 5-7 days. Remove, wedge, place on wheel; cone a few times- off to the races. Enjoy!  Modified to fit your protocol. Exact chemistry? No.. but close enough for your needs.


  6. Retha: Welcome to the forums. 

    If you are starting from dry powder premis: do one 500 gram test batch with 50 grams of ball clay (10%), and a second 500 gram test batch with 75 grams (15%) ball clay. If you have moist clay; then calculate dry weight by subtracting 20% for water. 1000 grams moist minus 20% water (200 grams) = 800 grams dry weight x 10% ball clay = 80 grams added to moist clay. Ball clay comes in low, medium, and high plasticity; so your experience with local materials will have to guide you. White stoneware has low iron and magnesium; which will also guide your ball clay selection. When hand mixing or slaking premix clay; it will take 5-7 days for plasticity to develop; 3 days if using a deairing pugger. 

    Bentonite can be used as a plasticizer; more commonly used in porcelain. 3-5% addition would be a good starting point. Important that you throughly blend in bentonite before adding water. Bentonite does not take kindly to being wedged into moist clay; I would avoid it. 500 gram test batches are more than adequate to test plasticity. Do not overdo ball clay additions because they do absorb water; excess can cause slumping or folding when thrown- in addition to higher shrinkage rates if handbuilding or doing slabs. 



    On 5/3/2020 at 2:56 PM, pkenin@comcast.net said:

    IN the middle of 100 glaze crystalline tests. No problem with shades of green, shades of blue. Looking for tips on two things. 

    #1 Oxidized red ground or crystals without the use of cadmium ( toxicity) Red is developed in reduction. Typically, 3-4% copper colorant is fired in oxidation (green) and then reduced during the cooling ramp by heavy reduction at 1350-1650F range. There are some who "strike" fire: reheating the piece to 1350-1650 in a reduction atmosphere. You can also grow crystals on a red bodied porcelain. 

    #2 Yellows  straw yellow is made with low % of red iron ox (0.75 to 1%) There are rare earth lanthanides that will produce a medium yellow at cone 10- but pricey and a bit hard to control.

    #3 Creation of distinction of ground colors and crystal colors  Zinc is used to control population. (Field to crystal ratio)  The base crystalline recipe is: 50% frit 3110, 25% zinc, and 25% silica. The 25% zinc is adjusted up or down: increasing produces more crystals, and decreasing creates more open field. Run a test at 24% and see how the results appeal to you. ( artistic preference decides population) if 24% is still a touch heavy, reduce to 23.75%, or raise to 24.24% for slightly more. Many zinc varieties out there; some more reactive than others. Which is why you see such a wide variance in recipes: 23.25 up to 30% zinc. 


    Thanks all   Perry Kenin 

    More info in the link below. On the first page-4th post zinc additions are discussed. There is a pic that shows 24-25-26% zinc additions.


  8. 4 hours ago, jrgpots said:


    A few years ago I ground up some mica, placed it in a crucible and set it on fire.  The sulfur dioxide burned out, leaving black iron oxide.  It was a cool experiment.   Anyway,  I'm curious how much can you add to glaze or clay body before you have a problem?   

    Commercial iron bearing clays run 5 to 8.4% iron content; less after being blended. Your experiment illustrates what causes carbon coring. Starve the oxygen during burnout and sulfur dioxide gas reduces the iron and causes carbon coring (black coring.) Supply plenty of oxygen during burn out phase; and it off gases as carbon trioxide with no adverse effects.

    4 hours ago, Babs said:

    It had silver flakes through it. Really attractive. It was like the stuff found sheeted in granite sometimes. What wouls that do if added to clay?

    The large opaque, clear, or silver crystals found in granite is crystalline silica. In the States; certain  granite is ground to make silica.

  9. Being from the West coast and given the price of the clay and results. Most likely IMCO Burgundy clay which fires dark brown at cone 5. Add 3% +/- black stain and you have black stoneware. I would recommend bisq firing on slow ramp to 1800F (cone 06) If it is indeed what I suspect: IMCO is an iron disulfide clay with a fair amount of inorganic purifies. A slow bisq will burn off the impurities; which in turn should minimize the pinhole issues. Avoid any glaze with zinc added: zinc reacts strongly to iron disulfide.  If you have a picture of the bottom unglazed area of your piece: it would with identifying the clay. (Problem)

    Colored porcelain (black) fires ultra clear.



  10. On 4/29/2020 at 12:17 AM, liambesaw said:

    This is great @glazenerd, is it safe to assume during a long bisque for FeS clays, we should vent or unplug a bung or two for extra oxygen?  I'm thinking that in addition to any carbons swooping up oxygen during combustion, that also the sulfides in the clay also need oxygen to convert to sulfur dioxide and gas out?

    I've had sulfide issues like you've shown, on a previously used (lizella) body, with the special slow bisque, when packed tight, stacked, with the peeps in.  I've now switch to 2 peeps out til 1800 and have had a lot better results.  But I also haven't gone back to that specific clay body.

    Thanks for the write up


    Liam: not addressed, but a good point. Several other  universal kiln principles still apply.. A dense bisq load as you pointed out would benefit from a pulled bunge. Red body clay is often used for very large platters and yard planters. Potter posted a pic of a 56lb bowl awhile back; with 3/4" walls. A case were sand should be under the foot for movement; and the ramp rate slowed to 80F an hour or so. An extended hold at 1800F would also be advisable. Several studies show it takes ambient kiln temperature up to 30 minutes to penetrate a 1/2" clay wall. 


  11. Color development can also be achieved by changing your firing speed. Most controllers have slow, medium, and fast preset firing speeds. By simply changing from fast to medium firing speeds; additional color development can be achieved. Red bodies containing iron disulfide should be fired as recommended to bisq temperatures 1800F/ 1000C. Glaze firing can then be adjusted to fast or medium for color development in the glaze fire.


  12. Firing Red Bodies with iron disulfide.


    Red bodies that source iron disulfide, and or red bodies that have inorganic sulfides require changes in the firing cycle to prevent blistering, bloating, and carbon coring. (Also called black coring) Hematite and magnetite have oxygen binders that creates the crystal lattice. Iron for example is FeO Fe= iron. O = oxygen. Firing clay bodies that source these two iron varieties, cause few issues. Most red clay bodies in the USA and other places source iron as iron disulfide. (FeS) Fe= iron S= sulfur/ide. When firing red body iron disulfide special attention to the firing schedule is critical. Disulfide is an inorganic binder that off gases between 1250F (655C) to 1750F (955C). For this singular reason all red bodies needs to be bisq fired to 1800F (1000C) minimum.
    Edward Orton (the cone guy) did the early research work on iron bearing clays from 1904 to early 1920's. He wrote numerous ACER journals on the kiln firing techniques required to properly fire and burn off the sulfides in red clay bodies. The slow ramp cycle on Orton controllers is based on his research. In lieu of the slow bisq ramp: a programmed ramp from 1250F to 1750F at 108F an hour climb is required to successfully burn out the sulfide content. Pending the size of your piece (over 10 lbs. or over 3/8" walls) you may need to slow down to 80F an hour between 1250F to 1750F. If you single fire red bodied clays: simply choose slow glaze fire or program a firing segment from 1250F (655C) to 1750F (955C) at 108F an hour climb.

    An additional test was performed to check the reaction of the three iron sources to other metalloids. Copper carbonate was added to a zinc free clear glaze, then a zinc free clear glaze: then finally 3% zinc was added to the clear base glaze. Copper carbonate had strong reactions to all iron varieties in comparison to the porcelain control tile. Zinc free clear glaze performed well, with the exception of the magnetite (M) sample.  When zinc was added at. 3%: there was a color shift on all samples; with crystallization occurring on the IM sample. Magnetite had the strongest reaction to zinc with a wide color shift and blistering occur.
    Ledger: IM- IMCO Burgundy. N- Newman Red  H- hematite  RA- Red Art M-magnetite.
    When using clear glazes on red bodied clay bodies; there are often complaints about matting. In this close up (3rd pic) of a zinc free (left) and zinc added (right) crystallization of the iron  has occurred; which is often mistaken for matting. Iron crystals refract light differently, giving the appearance of matting. Where the clay and glaze meet is commonly known as the clay- glaze interface: enough of the iron has been pulled out of the clay body into the glaze to cause iron crystals to form. In addition: red clay bodies that incorporate iron disulfide as the iron source can cause matting if improperly fired. 
    Close up of zinc free and zinc added clear glazes and the reaction to iron varities.

  14. Any red body clay sources iron to obtain its primary wet color. Iron disulfide with additional sulfides from lignite coal particles can have dramatic effects on colored glazes. The picture below shows yellow, red, green, and turquoise stains mixed with a zinc free clear glaze. The Mason Stain color number is noted on the sides. The pure porcelain tile to the left illustrates the dramatic color shift iron causes. The magnetite and Red Art samples cause enough reaction to render stains unusable. Porcelain on far left was used as a color control reference.
    Ledger: IM- IMCO Burgundy. N- Newman Red  H- hematite  RA- Red Art M-magnetite.

    The starting point for red bodied clays is the color when moist. The most commonly used clays in North America are IMCO Burgundy, Newman Red, and Red Art,  in addition magnetite, and hematite are locally sourced and or used in other countries. Iron bearing clays can also be grey (Canada), or have a distinct green hue ( Southern USA).  Green hue typically indicates that higher levels of calcium and magnesium are present. Grey can be  higher magnesium; but most often the color comes from sulfide contaminants (lignite coal particles). Red Art also has a high sulfide content which darkens the red hue.  Clays with high sulfide content require different firing schedules than hematite or magnetite bearing clays. Notice IM and RA have reddish brown and reddish/ grey color when wet: both discoloration indicate clay impurities that include lignite coal particles.
    Ledger: IM- IMCO Burgundy. N- Newman Red  H- hematite  RA- Red Art M-magnetite.

    Notes: all test samples shown are blended at 73% clay, 12% C&C ball clay, and 15% mahavir potash as a baseline:  fired in oxidation.
    Red body is a generic term used to describe iron bearing clays that have a red hue. There are other iron bearing clays that can present as green, grey, and black; these are exceptions, not the rule for commercial bodies. The three iron sources found in natural clay are hematite, magnetite, and iron disulfide. Iron disulfide is the common iron source in the USA and Canada; however hematite and magnetite are included because they are sourced in other countries. Hematite and magnetite are also harvested by local potters in North America. Typical iron bearing clays average between 5.0 to 8.4% of iron by weight.
    The tile graph below shows unglazed red bodied clays, with a locally sourced magnetite ( dark gray) sample from NY. (TY Mary)  All five fire to a traditional Tera Cotta at cone 04; although there is some variance in color depth. At cone 3, enough heat is present to cause some reduction resulting in color shifts. At cone 6; iron disulfide typically turns a deep brown; magnetite is nearly black; while hematite maintains a Terra Cotta color pending alumina levels, or a deep reddish brown. The note below identifies the labels for each clay; as well as their iron source.
    Additional Test Note: IM (IMCO Burgundy), and RA (Red Art) contains iron disulfide as the primary iron source. Newman Red (N) is a blended material that incorporates primarily hematite. H (hematite) is a locally sourced material that is not commercially available. Magnetite (M) is also locally sourced and not available commercially.large.IMG_0243.JPG.33841623f57a473a06e4f216efe8eb16.JPG

  17. Meghann:

    Mined clays can go through subtle changes as they dig the deposits; but unlikely to cause radical differences. Stoneware bodies have been changing over from potassium to sodium fluxes: which off gasses a bit more. The more likely culprit is kiln pack density. The denser and more tightly loaded (kiln pack) your kiln is: the greater the likelihood fuming will occur. A glaze with high levels of cobalt blue for instance will stain (fume) adjoining pieces. Have you changed  how you load your kiln? Denser, closer together?

  18. Meghann:

    Min and Neil pointed out fuming from glaze and clay material- to which I agree. You clay body obtains its color (lt. brown) from iron disulfide. The iron disulfide and the fluxes in your glaze produces a gas as they melt- that gas is the source of "fuming" as Amin pointed out. You are not over firing by any evidence I can see. 

    I do have two questions: you mentioned your white stoneware does this as well. Does this occur when you fire only white stoneware, or is it mixed in with this body? Second: you said this has not occurred before- is either the clay or glaze a new batch?  Can I assume that you have fired this clay with this glaze with no prior issue?  Is this the first time you have left some clay exposed when glazing? Do you have a kiln vent? Was it running?



  19. Hulk- good teaching moment.

    The blister formation in the first picture illustrates a large-raised- volcanic shaped rim. When blisters with "hoods" form: this is not only from red bodied clay- but also from additional sulfides. The additional sulfides are primarily from lignite coals particles: suspect this is a body made from Red Art or equal. The other indication is barren spots in the center of these blisters. 

    Nep Sy produces small, finer bubbles with no hood.

    Potassium produces larger bubbles, but less numerous.

    Iron disulfide (red body clay) will produce blisters (large) , and when excess secondary sulfides are present- hood will form. 

    Nep Sy gasses with less pressure than potassium; but iron disulfide (sulfides out-gasses under more pressure creating blisters- not bubbles.


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