glazenerd

Picture below has been cropped to fit format: actual clay size is 7” x 4” x 4”. Sample was saw cut and wetted to illustrate layers/ color/ and grain. Sample was flipped upside down to capture color variations. The orange/gold color on top is actually the bottom of the sample. A wild clay sample can tell you a lot just by looking at it. Obviously this is a sedimentary clay because it has three distinct layers and colors. The bottom layer is thin, and is noted by the cleavage crack at the bottom left corner. The top layer is granular and light orange/gold in color, which denotes the presence of iron disulfide (iron). This sample of iron disulfide is light in color, which means total iron content is in the 3-4% range. As the percentage of iron goes up; the color will become deeper and deeper. Naturally occurring magnetite (iron) usually presents medium to dark gray in clay color. Naturally occurring hematite will present light to medium “reddish” in color, with no goldish hue. Iron disulfide typically has a gold cast because the iron is oxidizing (rust), whereas hematite is not subject to this natural process. The middle layer is dark brown ball clay which typically indicates the presence of inorganic sulfides from lignite coal particles. Lighter brown color means less inorganic material, and darker brown means more. The exception to that rule is organic particles (humus). If your sample was taken from a heavily vegetated area; then the level of organic (humus) will be higher, which like wise will create a dark brown appearance. How can you tell if its organic or inorganic? First, the collection site: open fields or valleys will have less organic material, and heavily vegetated areas will have more. Secondly, a very simple test: take a small 1/4 cup powdered clay sample and add a a bit of water at a time until it forms a pliable ball. Does not have to be all nice and neat; just pliable. If it is sticky or gummy feeling; organics. If you can roll it between your hands without it sticking or smearing; it is inorganic. Yes, there are exceptions were a sample can have both inorganic and organic materials. Besides the obvious large particles of shale; did you notice the smaller nodules? There seems to be a heavy population of them in this sample; which means the middle layer has a higher percentage of 20-60 mesh particles. Bad thing? No, it can be used for non-functional, large format pieces. If you are going to make cups and bowls, then these larger particles have to come out. Wet processing will allow the large particles to settle out quickly, or dry processing will require a 60-80 mesh screen. This sample was found in an open eroded ditch in a field, so the color is most likely from inorganic sulfide. The presence of these sulfides also indicates a coal seam is nearby: which I happen to know is correct because of the numerous coal mines located locally back in the late 1800’s. This knowledge also helps determine the plasticity of ball clay located next to coal seams; typically they are more plastic. The bottom (thin layer) is free from large particles, and because this is a sedimentary sample; also means it is finer and more plastic. As with all clay sediments; larger particles drop out first, and smaller particles drop out last. Remember, this sample was photographed upside down to capture color variance. So the thin layer on the bottom, is actually the top of the sample. Can you field test plasticity? yes. Make a 1/4 cup of the middle layer, and a 1/4 cup of the bottom layer to start. If you have a scale, you can accurately measure what you add to each sample to create a pliable ball. If no scale; add 1 teaspoon, add a second, and once it begins to form a ball, then add 1/2 teaspoon until it becomes a pliable ball that does not crumble, nor overly wet and sticky. Low plasticity clay requires less water to form a pliable ball, and a high plasticity clay requires more water for the same. Exact? no- but will give you some general sense and direction. Tom

Pic below is unglazed “iron bering” clay fired to cone 6. (unglazed) Im= Imco burgundy. New = Newman Red. Hem= hematite bearing wild clay. RA= Red Art. Mag = magnetite bearing wild clay.

Baetheus: All clay are aluminosilicates (alumina/silica) in various percentages; which gives indication to plasticity. Feldspars and silica are inert; and are non plastic. Clay formulation basics: Porcelain is 50% kaolin, 25% silica, 25% feldspar (Cone 10), and 2% BentoneMA creates a premium body. 35% kaolin, 15% ball clay (plasticizer), 25% silica, and 25%feldspar (Cone 10) is a basic high white porcelain. 50% kaolin, 20% silica, 30% feldpsar, and 2% BentoneMA is a premium cone 6 body. Less feldspar is needed at cone 10 because of the extra heat work created firing to that temp. More feldspar is added at cone 6, because less heat is done. Porcelain relies on glass development to create vitreous wares. Silica + flux = glass. Silica + flux + alumina = stronger glass. Porcelain will always have lower absorption rates due to its high glass content. Stoneware relies on density more than glass to lower absorption. Feldspar additions typically run 10% at cone 10, and up to 15% at cone 6. Unlike porcelain, flux additions in stoneware are to prevent cristobalite formation, more so than glass development. Yes, some glass does develop when fluxes absorb silica; but its primary function is to lower cristobalite formation.Cristobalite forms when excess silica is present: that fact is what limits silica additions in stoneware to a maximum of 10%. By nature, ball clays have much more silica in them compared to kaolin; so silica additions are much lower than porcelain. Cristobalite in stoneware is a primary cause of dunting in the low end of the cooling cycle, when pieces are microwaved or hot fluids are poured in. So stoneware body formulation 101: limit silica additions. Stoneware formulation body basics: 80% total clay, 10% silica, 10% feldspar (cone 10) 75% total clay, 10% silica, 15% feldspar at cone 6. Just like porcelain: more heat work at cone 10, and less heat work at cone 6. The “80% total clay” means any combination of clays cannot exceed 80% of the total recipe at cone 10, and cannot exceed 75% at cone 6. 25% Hawthorne 35, 30% Imco 400, and25% OM4 ball clay = 80% of the recipe. You can have 2-3-4-5 different clays; just do not exceed 80/75% of formula. The other formulation criteria is PSD (particle size distribution). The greater the % of large particle clay, the greater the medium and fine particle clay additions required to fill the voids creates by large particles. (porosity) In my testing; a maximum of 17% Hawthorne 35 (large particle) can be used, and then blended with medium and fine particles to keep absorption around 2%. Once I passed that 17% mark, absorption started climbing incrementally. If doing non-functional; wares; not a factor per se. Doing functional ware; big factor. Years back I loaded 100+ stoneware/porcelain recipes into Glazemaster software. I did so to determine a basis of “typical” values for both. To get you started: Porcelain SiAL ratio 4:1. Stoneware: SiAL ratio 5:1. Remember, these are target values, not etched in stone. You will be under or over a bit every time. If you get way over or under; rethink your formula. Tom

When reviewing clay specs: the alumina content will predict plasticity. The lower the alumina content; the more plastic the clay will be. Example: kaolin is 37% alumina, and nearly non-plastic. Larger grain (micron) ball clays have 27-31% alumina, and typically rated as medium plasticity. Fine particle (sub-micron) ball clays with high plasticity run 24-27% alumina. As alumina levels drop, plasticity increases: bentonite (very high plastiity) has 20% alumina. BentoneMA (even more plastic than bentonite) is highly processed hectorite with less than 2% alumina. Particle size also effects plasticity: fire clays have a percentage of large (20-80) mesh particles that lower plasticity; even though it has lower alumina. High plasticity ball clays are under 1 micron particle size; or sub-micron particles. Example: Kaolin run 2-20 microns typically; which also plays into its non-plastic rating. OM4 ball clay runs 0.67 microns and is medium plasticity. CMC ball clay is just below 0.50 microns and is rated high plasticity. I use Taylor ball clay (not available commercially) that is 0.27 microns and extremely plastic. When formulating: plasticity ratings matter. For example: a common formulation basis is 25% kaolin, 25% ball clay, 25% silica, and 25% feldspar (cone 10) 25% OM4 ball clay will create workable plasticity. If you changed that 25% to CMC ball clay: the body would absorb water rapidly and collapse quickly on the wheel because CMC is much more plastic than OM4. Let me express this another way: 8% of my Taylor ball clay will produce more plasticity than 25% of OM4. So you have to understand that parameter when formulating. Remember: high plasticity equals high water absorption. Randomly switching ball clays in equal additions (25% OM4 verses 25% CMC) will turn a plastic clay into a “fat” clay quickly. Alumina will predict plasticity in most all cases. Exceptions: as mentioned, larger particle sizes will lower plasticity even when alumina is lower. 2. Higher calcium content will increase plasticity when alumina content is higher because calcium creates isomorphic substitution (don’t ask) at a higher rate than sodium or potassium. Fireclays have higher inorganic sulfide levels which equate to higher LOI at 1750F. Inorganic sulfides = lignite coal particles. Tom

Besides the obvious issue with firing large slabs flat (dunting), the other issue is uneven heat. The side exposed to ambient kiln temperature can vary to the temperature under the slab in direct contact with the shelf; further adding to the warp issue. I have fire porcelain slabs up to 30” square by bisq firing them on edge. Use tile setters as shown, or prop them up using other wares. Tom

Kelly: apparently I posted my Split LOI on this forum back in 2019. I would post a link, but do not have a clue on how to do that. It will give you more information without expense. Tom

Kelly Can tell you a fair amount just from the description. chocolately color is magnetite; which also presents dark grey/ with blue/green (calcium) in the wild. Pyroplasticity in the lower range 04- 1 indicates low alumina, not high flux content. From my testing; alumina is in the 16% range, which also means the silica is elevated into the 70+% range. Potters automatically assume pyroplasticity in clay is caused by high flux content due to their familiarity with glaze. Glaze is in direct contact with ambient kiln temp, while that same temp can take 30 plus minutes to penetrate a clay wall- hence the hold time often used in firings. Most glazes incorporate 200-325 mesh material, while clay often has 40-120 mesh materials. Testing alumina is fairly simple: just add 20% kaolin (37% alumina) to your wild clay and fire it again to the known slump temp. You will know in a heartbeat if its alumina issues. Years ago, i developed a split LOI test that I sent to Tony Hansen and Ron Roy. From Tony’s email, I assume at some point he took it for a test drive. If you want to give it a shot; I will send a link to my private clay page. Tom

How in depth do you want to go Kelly? Specifics, General? etc. Tom

Well said. Brick making relies on malleability, not plasticity. Seen a few million bricks in my 50 year carpenter career, and pending the clay mine; chunks up to 3/8” is not uncommon. Potters avoid black coring, whereas the brick industry utilizes it to produce low absorption. Brick is fired in the 06-04 range mostly, at a fairly high speed with the intent to produce black glass (coring). It all comes down to the original poster willing to do that much work. Just so you know; 7 standard size bricks equal 1 square foot. Tom

To start, make a small tile by hand (nothing fancy) and let it dry> did it crack while drying? Next fire it to cone 06? what color? did it crack? The first step in wild clay processing is to see if its worth working with. Get past the simple test. Next, dry process 1 pound by pulverizing chunks into powder. Work it through a strainer ( window screen, kitchen strainer, etc) and repeat the above process to check how it dries and how it fires. >>outside with a mask<< How much sand? After you dry process one pound; put a 1/4 cup of dry clay in 3/4 cup of water in a clear glass container, and stir/shake well.. The sand will settle out very quickly. The thicker the sediment layer on the bottom equals the amount of sand present. 15-20% sand is acceptable, alot of brick makers add and for malleability. Commonly called temper in the wild clay circles. Run these simple tests and get back to me once you have. Note: worked with two potteries in India that used subtropical laterite clay. After the monsoon season; laterite can be found in smaller streams up to 4-5 feet thick. They simply dig it out and put it into brick molds. Sub-tropical laterite is the most common clay in India with over 40% combined alumina/.iron content. It dries so hard that it is often not fired before using as a brick. A property known as cementing occurs that makes unfired brick almost as hard as fired brick in the States. Tom

Min: The joys of wild clay processing and testing. Process of checking off boxes of possible issues, and then coming back with the best fix. In this case, your suggestion will most likely be correct. Spent the last decade telling potters not to put vinegar in their clay; so I get to eat my own words. One of the oddity cases where you have to check off the “high alkalinity” box first. Tom

Adding vinegar is an experiment to check for high alkalinity from salt. As I noted, if the Ph rises above 9.8, then a property known as cementing occurs. Cementing is extreme flocculation. If this experiment does not change any of the working properties; then you go back to looking at common issues such as silt or particle size. T

TY Min. Several years back I read a series of abstracts from D.D.Buttons (Alfred U) about the effects of acidity/alkalinity on cation exchange. After petrichor stated it was collected ocean side; an observation Button’s made came to mind. Alkalinity creates negative particle charge, which is the basis of plasticity/deflocculation. However, once alkalinity passes 9.8PH, then the reaction reverses; and cementing (hard clumping) begins to occur. Petrichor mentioned “chunks” missing when trimmed. Another indication is rapid drying and cracking: also indications of very high alkalinity. Sodium silicate for instance is above 13PH. Very little creates a strong particle charge. Any potter who has done salt firing knows how caustic salt is. The most basic experiment to check for high alkalinity would be to add 1 teaspoon of vinegar to 1/2 pound of clay. Wedge, and try throwing. The reaction will be almost immediate if it is indeed salt. After you check it; add another teaspoon until the clay reaches a point where it is not chunking when you trim it. You can add enough dry clay to keep the moisture content in check. Typically; a deep gray color in clay indicates a magnetite iron source. From samples I have seen, or have first hand knowledge of lab testing; silver color usually indicates magnesium. Tom

The working properties you describe suggest you have found wild kaolin, not earthenware. How about a very fast experiment: mix 1/3 of your commercial stoneware with 2/3 wild clay. Let it sit 4-5 days, and work with it again. If indeed it is kaolin; it is “fiddly” because kaolin is a larger particle clay with very little naturally occurring plasticity. Another indication it is kaolin: did it dry in about half the time as your stoneware? Tom

All information is sourced from PhD’s (Ceramic Engineering) that I collected over the years.

Most all the studies were related to the brick industry. We use the same red body (mostly) as that industry. Was looking at F.H. Norton Phd>earlier; same references. T

Orton — auto-correct once again.

Still looking for Orion work; lost a lot of info when my old laptop took a dirt nap. Brownell did similar studies as well.

Edward Orion, Jr. (yes, the cone guy) did the early studies on inorganic burnout back in 1906-1910 period. In reading his abstract: he cites inorganic burnout between 1250 to 1750F. T

Rob: You have magnetite (iron) bearing clay; somewhere in the 7-9% total iron content range. Classic terra cotta at cone 04, and chocolate brown at 6+. You will get red in thin layers, but if you get it too thick- orange/tan to brown as it gets thicker. Must be slow moving waters for that distribution of very fine particles; bit unusual. Also surprised there is no plasticity; Ord humus (organics) commonly found in lake/river/stream collection areas. NY State also has an unique variety of smecites; which is also highly plastic. From everything I have seen and read; 1-2 micron particle sizes The note of interest to me: “stays suspended” and “no plasticity.” Those two do not fit clay chemistry with one exception: high calcium content. Calcium will keep fine particles suspended when “common plasticizers” are absent. The melted blob at cone 6+ indicates total alumina content is 15% or less. If you want to work with it, I will post some fixes. If not, enjoy the wild clay adventure. Tom

You are dealing with several issues: 1. Pyroplasticity (melted blob) is caused by low alumina levels in your wild clay; in this case under 15% total alumina. 2. Your clay is iron disulfide: other iron bearing clays do not produce black coring unless sulfide contamination occurs (very rare) Iron disulfide (iron pyrite) will produce sulfur monoxide when fired too quickly; this gas is a powerful flux which produces black glass- commonly called black coring in the pottery biz. Slow bisq speed is 108F per hour; which is the correct speed in this case. 3. The cracking pattern in part is caused by the black coring; which is subject to severe COE expansion/contraction issues. The cracking pattern is also caused in part by the lack of sub-micron (small) particle distribution. Iron bearing clays typically tend to be larger in particle size. Small particle (sub micron) clays add plasticity, but also create adhesion in this case. There are fixes, but it sounds like you are moving on anyway.

Wood ash is not a good clay addition; caustic levels of fluxes.

Matthew: where you collected gives indication of mesh size. Most clay collected along water banks are obviously sedimentary, but pending how fast the water flows; usually find larger particle size. If you find a location along the creek/river where the current is slow; then you will most likely find smaller particle size clay. Did you test the clay without any additions before making the decision to add bentonite? Bentonite can be used, but it is not the best solution to create plasticity for many reasons. If you are after smooth; then theoretically you must increase small particle percentages (under 2 microns). Temper by wild clay definition is any large particle (20-40 mesh) added to create malleability, in lieu of plasticizers. You are actually talking about particle size distribution (PSD); a clay theorem that formulates a body based on %’s of large, medium, and small particles sizes. You are starting at 80 mesh from the description you posted. At this point, adding high percentages of fine mesh mullite, molochite, or kyanite would inhibit plasticity. All there of these are used to increase cone value, increase strength via mullite %, and or reduce warping when drying or firing. I do not know your firing method, or your peak cone value; so adding silica or feldspar would be a wild guess at this point. You can start with 60% wild clay, 20% Imco400 or Kentucky glaze#1, and 20% OM4 ball clay. In addition to smoothing out your clay; both additives will increase your dry time if you are doing large format pieces. Tom

Neil: Do not have a pic. I am talking about the thermocouple wires from the back of the block, entering through the kiln wall. Immediately behind the block.

This problem seems oddly familiar to me. I had 2 Paragon kilns that the ever fatal error codes; and I likewise replaced thermocouple and elements of one of the two. After replacing both an one: the beloved error code again. So I took the thermocouple wiring block apart again and restrung the thermocouple wires again- another TC fail. Took it apart again; this time while I was mounting the terminal block back on the kiln; I noticed the thermocouple wires compressed, and touched each other. I took it apart again, this time I put some insulating fiber between the thermocouple wires and gently held them apart while mounting the terminal block back on the kiln wall…. No more TC error after that. Tom