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
 

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

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

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

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

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

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. 

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

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

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

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

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
 
 

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
 
 

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
 
 

Also there is a difference between glaze stains and body stains.

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 

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

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

Maybe not the brightest bulb?

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

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. 

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

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
 

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.