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glazenerd

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  1. Dave Fairly familiar with Ohio clay. Most everything you find will be iron pyrite (iron disulfide). Most have 6-8% iron content total. The big issue, most terra cotta bodies have higher levels of sulfides: lots of coal fields in Ohio. Your dark cube is one indication of that, the other is your slip peeling. If you do bisque fires on a regular basis: make a 1” x 6” x 1/2” thick test bar with this clay. Throw it in with your regular bisq at medium, or higher speed: if that is your normal cycle. Snap it in half when it comes out: look for dark coloration. That will tell you what direction you need to go. Nerd
  2. Goatrider: In the first pic, very small cracking on the rim. That indicates you need some ball clay: start at 10% OM4 or equal. The green color is from calcium usually, other minerals will present that color. From the color of the second pic: 6-8% iron content. Final blob pic: lack of alumina is the biggest factor of pyroplasticity (melt). Kaolin is 37% alumina: whatever kaolin you have on hand. Wild clay 75%, ball clay (OM4) 10%, kaolin 15%. Will get you started, but will have to check melt before you dive any deeper. Nerd
  3. Looking closely, I can see pinholes that are clear. I can see others that are black. I will vote coffee. Tom
  4. I think the last time I bought commercial clay was 10 plus years ago? I have blended a lot of wild clay, but that is really outside this topic. I blend two porcelains primarily: the first is for tile work, the other for crystalline glaze. The tile body centers on laying flat without me having to flip, weight, and baby sit it in the process. The porcelain for crystalline centers around a chemistry that does not hinder or interfere with crystal growth. They both met basic formulation standards; cone 6, absorption hovers around 1/2%. Not a fan of sodium based body fluxes for many reasons. Side note: I use the same basic tile body for throwing with minor adjustments. I purposely blend it to be a touch on the short side to allow for water absorption. The only red clay I blend comes from hematite bearing wild clay; which keeps a deep red color at cone 6. Tom
  5. Babs Zinc is processed two ways: the French process and the American process. The American process is just simply “roasting” (we call it calcining) that removes impurities. It is denser because it typically 80% pure, with 20% of various minerals commonly associated with zinc. The French process involves vaporing zinc at higher temps, resulting in a purity above 99%. It is finer, with less density. The zinc you are looking for is called Maximo 910 in the States, unless it has been sold to another company yet again. Tom
  6. Morgan: If the Frost is combined with other clays as part of your total reclaim: then no specific protocols are necessary. If you plan on using Frost specifically, then you do need to follow specific protocols. Yes, Frost reclaim would be more prone to cracking. Frost has 30-32% sodium based flux which in part plays a role in its rapid drying/cracking. Bentone MA is highly processed hectorite, with magnesium removed. To give you an idea of how fine BentoneMA is: OM4 runs 0.55 microns, Taylor 0.28 microns, and BentoneMA, below 0.15 microns. This means it does not take much water or throwing to wash it out. If you plan on reclaiming Frost long term, then it becomes an issue. Tom
  7. Morgan: Frost is blended a bit different than most clay bodies. It has a high COE value, because it has high flux levels. Secondly, Frost uses a speciality plasticizer called BentoneMA. If you plan on using Frost, and want to maintain the translucency: then it matters. One off use, does not matter. Tom
  8. RF back for an add. I understand the dislike of sodium based fluxes in a clay body: not a fan of them myself. However, some flux needs to be present for several reasons. I would recommend potassium in the 10-15% range. I also assume your goal of “less than 0.50” absorption is meant to meet TCA specs for vitreous wall tile? I can have the long detailed discussion about why fluxes, but for now you get the short form. Tom
  9. RF You want soft slabs for a particular forming process I assume? Yet, you also have to understand that “soft” clay equals higher water content, which adds to the warping issue. The more water in the clay; the more dried shrinkage, and increased risk of warping. As to your other listed issues: 1. Kaolin and fire clay have very low plasticity: but they have some. If you work centers around slab building/forming: then adding molochite 325 would be advisable. Molochite is a grog, and 325 is used when you are trying to maintain low absorption bodies. Unlike kaolin, molochite adds no plasticity, and actually will reduce plastic properties if used in high %. Start at 5% in small test batches. Molochite 325 is used often in slab/tile bodies to control warping. Do not fall into the trap of: if a little works good, then more will work better.” Adding too much will actually cause cracking when drying. 2. 10% silica additions in stoneware/red body is the industry norm. If you use a glaze calculator: aim for a 5:1 SiAL ratio (5 parts silica to 1 part alumina)Like all things clay: too much silica can lead to cristobalite formation. Cristobalite will dunt your pieces during the cool down, usually around 400-450F range. 3. Ball clay is the primary plasticizer in most all clay recipes. Slab/tile bodies need just enough to make it malleable, and too much adds to warping/shrinking issues. Secondly, not all ball clays are the same. 8% Taylor, 15% C&C, and 25% OM4: will all impart roughly the same level of plasticity. Ball clays absorb water, as their plasticity levels increase, and as their % in the recipe increases: likewise total water content increases. Tom
  10. 1st. understand that hand mixed clay, or even clay mixed in a Soldner will take a week to ten days before plasticity is achieved. 2nd Adding kaolin or Hawthorne will lower plasticity, not increase it: both of these have very low plastic values. The original recipe is 50% Redart, 15% Goldart, 15% Om4, and 10% silica. OM4 is a medium plasticity clay, and it is common to see recipes with 25% OM4 for that reason. C&C ball clay is about 40% more plastic than OM4; and is readily available and cheap. Use the original recipe, and simply change out OM4 for C&C. I dry mix all the ingredients before mixing. Both Redart and Goldart have higher sulfide content, which produces your brown clay at cone 5/6. The final issue would be “soft slabs”? Slabs for tile, or slabs for hand building? High plasticity in clay bodies intended for tile can come back and bite you in the form of warping when drying, and excessive shrinking. Tom
  11. About all I fired was tiles. I a 6.5CF square top loader, and a 15.5CF front loader. The easiest method is to calculate your current tile size (in setters?), to come up with the width x length x height. You mentioned upright? ( see below). My top loader has 3” brick, with 1” insulation: extremely energy efficient. Front loaders are the easiest ergonomically, but have more hot and cold spots. When the temp gets into the 1800F plus range: static pressure likes to push the bottom of the door open ever so slightly, which allows cool air into the bottom front of the chamber. Easy to rectify with insulation. The upper rear of the chamber tends to fire hot, which I remedied by drilling a hole from the top; into the rear corner of the chamber. If you fire speciality glaze that is fickle to temperature variations; then a kiln vent would be advisable for a front loader. If the pic I posted resembles what you are doing: then it a simple matter of calculating how many setters wide, by how many setters deep, by how many setters tall: to arrive at a chamber size. Allow 2” of free space around the edges, and calculate space required for shelves and legs. Tom
  12. Was going through old files, when I came across the papers Marcia Selsor wrote on crystalline glaze back in the 1970’s. I know she does obvara and raku now: so that bought up a question. Q: Did your passion about a certain form, glaze, or technique change, and what caused that change? Tom
  13. Questions; heavy vegetation around the site it was collected? Does it feel sticky or tacky? Was it collected from a water shed area? Any dark/black material visible? Strong mold/bacteria odor? Calcium (lime) is a strong natural deflocculant. However, suspect more going on than that. Tom
  14. Sorry for the long step away: had eye surgery and just got released. C&C ball clay is 35-40% more plastic than OM4. So if the recipe calls for 25% OM4, then 15% C&C will have close to the same plasticity levels. You can just substitute the remaining 10% with clay. Tom
  15. Talc has high magnesium, up to 20%: which can alter color. Ball clays need to be under 2.00% total iron and magnesium to keep the color on the white side. C&C ball clay would be a choice. Aluminum content dictates refractory limits. Tom
  16. Kaolin is a 1:1 particle, which means it only holds water on its surface. Ball clay is a 2:1 particle, and pending the CEC (cation exchange) will absorb water into its inner platelets. Naturally occurring calcium plays a large role in CEC: EPK has 0.18%, and #6 Tile has 0.30%. May seem like nothing, but its enough to create better suspension. Particle size also plays a role in suspension. Tom
  17. Just when I think I have seen it all; most of it anyway. I fired the above shown sample to cone 04, and was surprised when I opened the kiln. The new sample is on the left, and a standard terra cotta on the right. From the deep color (reddish/orange), possibility that this sample might have limonite in it. Limonite ( natural yellow ochre), which would account for the deep color hue. Wild clay can get wild, even for those that are familiar with it. This sample will certainly go to the lab for analysis. Not cracked it open yet to check for black coring, but I suspect there is. I dipped both in water for a quick absorption test. The terra cotta drank it up, and the limonite? sample did not. Tom
  18. 1. Water hull and stretched membrane were both theorems introduced by F.H. Norton, Phd. The water hull in simple terms meaning a single molecule of water is nearly equal to sub-micron clay particles, and a single calcium particle. Without paragraphs of science; the theorem states that sub-micron clay particles, and calcium particles are more effective at creating plasticity because they are of equal size. Sodium and magnesium are much larger; and the water molecule has to “stretch” ( stretched membrane) to encapsulate either. That gelantious effect created when you put bentonite in clay or glaze is in part a result of “stretching” the water hull. To further illustrate is Darvan. Powerful suspension agent, but few know that Darvan works in part by neutralizing sodium and magnesium ions. This allows remaining calcium to work more potently, in addition to the strong negative ionic charge created by high alkalinity. In discussing wollanite: then you get into alternate silica structures. Polymorphs I believe is the correct term. Pure silica has a high COE 12-14, while melted silica has a low COE 4.50 or so. So silica that has undergone thermal changes due to volcanic, or heat/pressure form a different crystalline lattice- thereby lowering it natural COE. For that reason, wollastonite reduces shrinkage, and adds to plasticity due to the calcium content. Personally, I add 2% whiting. 2. Plasticity develops over a 5-7day period unless it is ran through a de-airing pugger. Plasticity levels climb over the next several weeks. Plasticity is never an immediate reaction. A big mistake potters make is increasing ball clay/plasticizers until it is plastic upon mixing. However, in 5-7 days they have a bag of silly putty. Your projected plasticity in a week or so govens ball clay additions; not what it feels like when you mix it. Also note that not all ball clays impart the same level of plasticity. Kentucky 5 for example is on the lower end of plasticity; yet it is still deemed plastic. OM4 is a medium plasticity ball clay, and FHC (Foundry Hills Creme) is high plastic. You have to calculate those levels when adapting a clay recipe. A common clay recipe might call for 25% OM4, and if you replace it with 25% FHC: you will have silly putty. If it calls for 25% OM4, and you replace it with 25% Kentucky 5; it will mature to the short side of plasticity. Ball clay specs will often give CEC (cation exchange values. OM4 has a CEC around 5.8 (last I looked), and FHC is above 9. (from memory). The higher the CEC climbs, the more plastic it is. A recipe calling for 25% OM4, can be replaced with 15% FHC to achieve the same plasticity level. Formulation rule #2; the higher the plasticity, the more water the clay will absorb. That is also translated; the higher the CEC value, the more water it will absorb. You do not want a tile body to absorb or hold any more water than necessary to form: because excess water = higher shrinkage. Throwing bodies need enough water absorption to make it malleable, but absorbing higher amounts of water will cause it to slump or fold on the wheel. The exception is hand forming, moreso pieces that require several days, or multiple steps to create. In this case, the additional water absorption will delay drying, and make the clay more suitable for carving or detailing. 3. In your earlier post, you listed the mineral composition of the clays you like. Did you notice the alumina hovering around 25%? 24-30% is common for ball clay and fire clays, although fire clays often drop below that. Kaolin typically runs around 37%. If you formulate using clay(s) with lower alumina and higher silica; then you will have higher SiAL ratios; 5:1 is normal for stoneware. Porcelain uses kaolin, which is higher in alumina, and lower in silica: so the SiAL for porcelain bodies hover around 4:1. The additional formulation criteria also comes from these natural SiAl levels. Stoneware bodies naturally have lower alumina and higher silica: so the silica addtions are limited to 10% for that reason. Porcelain bodies (kaolin) have higher alumina and lower silica: so silica addtions run 20-25% for that same reason. Bonus note: Porcelian relies heavily upon glass/mullite development to create nearly zero absorption. This is the primary reason flux additions run in the 25-30% range. Stoneware bodies focus on PSD (particle size distribution) to keep absorption under 3% (functional ware).Stoneware recipes can have up to 5 different individual clays; which plays a role in particle distribution, but also in working properties, and final fired color. Stoneware being more dependent upon PSD, also means flux additions ae kept to 10-15% total. Flux in stoneware does develop some glass, and does lower absorption some. However, flux addtions serve a more central target of preventing cristobalite formation. Tom
  19. Kelly: I will assume your 03/04 cone range equates to 1950-2000F range I have seen wild clay samples turn to a puddled blob at cone 06. From what you report; alumina is in the 14-15% range. Yet, I also suspect you have a fair distribution of ultra fine particles. The U of I (Champaign/Urbana) did some extensive studies on particle size = melt temp many decades ago. Although the lack of alumina is the primary cause of pyroplastic deformation, particle size can add to that issue. I would run a line batch: 10%, 20% ad 30% kaolin additions. Almost all kaolin has 37% alumina, and ball clays run 24-30%. I make test cones which mimic typical Orton cones. I just make a 2” square, and carve a cone when it gets leather hard. I do not get overly particular about it: just some resemblance to a cone. The angle, and the narrowing tip is the most important. Can put it anywhere in the kiln on a waster slab. I do find it interesting that your clay is so reactive in just a single cone variance. I do not get overly involved with processing new samples. Get the obvious sticks and pebbles out of it, make a cone; fire it up. I want to know the properties before I begin processing. Like the post above, I will just place one of the small slabs/pieces in the kiln and fire it. The one thing about wild clay: you have to have a starting point that gives you some basis/direction to steer the processing. Tom
  20. Epson salts are basically magnesium sulfate. 20% +/- magnesium; which is the clay world is a body flux. The sulfates will burn out. Given the amounts used; nearly zero effect. At most, the magnesium will change a high white body to an off white body; and that would require larger additions than what is being discussed. Tom
  21. The first step in identifying what type of wild clay you have collected is color. The sample(s) below indicate the presence of iron (pyrite) simply by the orange/deep orange color created when iron oxidizes in nature. This is an odd sample because the piece on the left has visible sedimentation lines, that vary between a 1/16 and an 1/8th of an inch. In addition, the left sample has a solid brown color on top and the bottom; which is rather slimy to the touch. The thin lines obviously indicate a sedimentary deposit, and the slimy coating, which differs in color is silt. In further evidence of being silt; it washes away rather quickly under tap water; while the remaining clay is undisturbed. There are also gray/white areas on the top of the large sample on the right; which is commonly referred to grey gumbo clay in this area. Yes, it is clay; but its composition and structure makes it unsuitable for pottery. In this case however, the percentage is low; so whatever is left after washing, will remain. I will add 2-3% more silica to compensate for the instability of the gumbo clay. What else do I know about this clay before I begin processing? I collected it in a heavily wooded drainage area; so I know organics are present. Although not plainly visible; there are visible black lines between the sedimentary layers, further evidence of organics. I also collected it wet, and after just two days; the pungent aroma of bacteria is already present. Yet, I also need to determine if the black color is only from organics. I also know that there are large coal seams nearby. In direct sunlight, I can see that the overall hue is darker than a normal iron pyrite color. From experience, this dark hue is most likely from lignite coal particles, and not organics. After I process it a bit; I will run a Split LOI test: if the lower test firing results in higher LOI numbers, then I know it is all from organics. If the higher test firing results in higher LOI numbers, then I know it is lignite coal particles. You only want to clean out sticks, twigs, rocks, and sand before you run a Split LOI test. The large sample on the right is also revealing. It was taken just below the sedimentary sample on the left and bottom. This sample has no real distinct sedimentary lines, and the color is nearly uniform. Judging from the color; iron content is in the 4-6% range. How do I know that? On the left lower corner of the sample on the right side, is a 1/2” spot of deep orange/red color. That spot is nearly pure iron pyrite clay; which runs in the 8+% range. I know because of the color, and similar samples ran at labs. Natural iron pyrite clay will be lighter as the iron content declines, and darker orange/red as iron levels increase. My educated guess, that overall iron content will come in at the 5% range. I will take a knife, and cut a 3” square by 1/2 thick sample and throw it into a test kiln unprocessed to check fired color. It is advisable to use a waster slab, just in case; but you need to check if you even like the fired color before processing very much. I have several 1/2” thick samples which I broke in half just using my hands. The higher the iron and alumina content in a sample, the more pressure it takes to snap it in half. In this case, it took a bit of pressure to snap; so I an estimating 5% iron and 24% alumina. Yet understand that is based on snapping many samples, and many of those sample being lab tested for mineral content. There is one more oddity in the right sample; did you see it? Directly across from that 1/2” dark orange spot, is a nearly black line. That is a solid line of coal; and that also means that the dark overall hue, and the dark sedimentary lines are most likely lignite coal, and not organics. Your sample will tell you a lot before you even process; you just need to learn what it is saying. Lignite coal particles means inorganic sulfides: which translates to blistering, bloating, or even black coring if not fired correctly. So now my clay sample just told me how to fire it. I will program a rate climb of 108F an hour from 1250F to 1800F to ensure all inorganic sulfides have been burnt off. Usually, a slow bisq firing program will accomplish the same thing. Take a real close look at your samples; they will tell you many things. Tom
  22. Anything organic will burn off, and at most leave ashen residue. Paints are pigments, which is not the same as stains. As mentioned, you can use metallic oxides: cobalt, copper, etc. A possible cheaper way (I stress possible) is to find powdered glazes that are dark blue, dark green, etc: some contain metallic oxides, and others stains. Ask your local dealer. Do not get too carried away pending how high you fire: powdered glazes also contain high levels of fluxes. Clay/slips can tolerate flux additions, but do not overdo it. Stains work best with porcelain (white firing), or other high white firing bodies. Iron bearing clays will change the intended color. Tom
  23. Another way to help you identify your wild clay. Different types of clay has different weights and mass for a variety of reasons Here is a list of known clay that you can compare against. Filling and smoothing off just like you would flour: 1/2 cup of fire clay 88 grams (1/2 cup = 120ml) 1/2 cup of ball clay 57 grams 1/2 cup of kaolin 32 grams Use a scale or kitchen scale. Use cleaned processed wild clay (dry) to start. (no sand) Weights can be used to give you an indication of what you have. Your result will vary some. Tom
  24. Vik: One of those cases where the tricks of the wild clay collectors comes in handy. Pour the whole bucket through a pillow case; tie a loose knot on the end, and hang up outside and let it firm up. With current outside temp, check it morning and evening until it “feels” right. Then you can pug it up. Tom
  25. Kelly: In this case, yes it is shale. This field ditch drops off into a ravine about another 1/2 mile up. There are shale outcroppings in that ravine/ditch. Tom
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