Wild Clay Processing

[Min]

Starting a pinned thread on testing and working with wild clay as there seems to be a fair bit of interest in this.

Few links discussing this below, please feel free to add info and experiences using wild or found clay.

https://digitalfire.com/article/how+to+find+and+test+your+own+native+clays

https://www.leelachakravarti.com/blog/wild-clay-top-tips-for-making-pottery

https://ceramicartsnetwork.org/ceramics-monthly/ceramics-monthly-article/wild-clay-and-glaze (this one is behind a paywall but you can see 3 articles a month for free)

 

[Kelly in AK]

The article by Leela is nicely written, a great place to start if you've never done this before.

A couple pictures from an expedition to Sheep Mountain, about two hours from Anchorage. A massive landslide the day before deposited a thick layer of clay right by the roadside. It was happy timing, we had planned a long hike up the mountain to get to a bed of kaolin that's documented there. It was very plastic, threw like a dream, and didn't even shrink that much. Unfortunately this clay was nowhere close to pure kaolin. At cone 03 it softened and expanded into a puffy marshmallow like consistency. The bowl in the back held a sample at cone 6. Bubbly foam.

[Callie Beller Diesel]

Am making a post that links to discussions about wild clay, for easy reference purposes.

 

 

[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

 

[Kelly in AK]

Wonderful breakdown of a slice of earth, as dug. I very much appreciate comparing the photo to the analysis.

So those white looking bits in the brown ball clay layer, are those shale? 

[glazenerd]

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

[glazenerd]

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 

[glazenerd]

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

 

 

Edited July 14, 2023 by glazenerd
Added link

[Kelly in AK]

My clay body, local Anchorage clay, has its faults that I’ve been working to mitigate over the years: Narrowing in on particle size distribution, exploring additives, playing with firing schedules. I’ve made a lot of progress, but there’s a variable I haven’t taken head on yet. That is optimum firing temperatures.

Right now my best results are 98% clay, 2% Veegum. That makes it play like pottery clay. A little extra attention to attachments and bottoms prevent predictable shrinkage cracks. For both forming and firing stability I tried Lincoln 60 at 20% which helped in the forming part, but only changed the firing range imperceptibly.

 I’d like to get a full cone of useable range in the kiln, right now it seems a hair’s breadth between 04 and 03 will produce vitreous pots. A regular 04 gives me 5% absorption, a solid 03 and things begin to slump, higher still and they dunt and bloat. Between those two are pots with1% or less absorption. Tough as nails, and ring like a bell.

Would adding kaolin, a nice alumina rich clay, be a reasonable tack to explore?

[glazenerd]

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

[Kelly in AK]

Thank you so much Tom, as always, I value your insight and advice.

My clay produces excellent terra sig, in good volume compared to other clays I’ve made it with. That, in my mind, correlates with a significant amount of ultra fine particles, though I know there are other factors involved in how deflocculated clay settles out  

“Cones” made from the clay tests is brilliant. 

[glazenerd]

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

[Kelly in AK]

That rich red! Looks like it’s on fire. Beautiful. 

[GoatRider]

I'm starting to evaluate the wild clay in my yard in Maple Grove, MN. I've floated off the organic material, and sieved it with a paint strainer bag. It bisqued up nicely, but melted into bubbly baby-poo green at cone 6.

I'm not expecting to make great art with this, just making a statement about how crappy my garden soil is, and get an intuitive understanding of how clay is processed and tested.

My next step is to try it at cone 1 to see if I can use it as earthenware, and maybe try it as a slip over a high-fire clay body.

I have 2 pieces that I'm going to raku fire on Thursday, weather permitting. If it can take bisque it can take raku.

[Dave Earley]

I found clay in my back yard while leveling the ground for an above ground pool.  Here is a sample I fired to cone 10 and some bowls (unfired) that I threw. Clay in bowls put through an 80 mesh screen.

[Dave Earley]

Here are the bowls fired to electric cone 10 with a semitransparent blue glaze

[Min]

1 hour ago, Dave Earley said:

Here are the bowls fired to electric cone 10 with a semitransparent blue glaze

Nice! Are you thinking of running some absorption, shrinkage and warping tests on it? One time experiment or do you have enough of it to use for more pots?

[Kelly in AK]

This is cool. You can have your own clay mine, with pool! You’ll hardly have to leave the house. 

[glazenerd]

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

[Dave Earley]

I have a whole two acre lot with a stream that has cut through it.  There are no cracks in either bowl , they ring nicely. I dipped the rim of the left hand bowl in an ash glaze.  the brown cube was my melt test fired on a cookie to cone 10.

Edited December 11, 2023 by Dave Earley

[Dave Earley]

Here is  a view of the clay vein in my yard, and a lidded casserole fired to cone ten with white and rutile slip under a mild chrome glaze.  

[Pluton]

Somewhat south of your location in Strongsville, OH, between Peninsula and Streetsboro, our surface clay goes down 80 to100 feet, and is I believe part of the Hiram Till deposit from the last glacial retreat.  It looks to be same as what you have in your yard.

I have been using this successfully for several years, mostly wood-fired to cone 10, but read on...

Un-amended, and fired in oxidation to bisque temperature it gives an attractive orange terracotta and a deep brick red at cone 6, which is probably its sweet spot.

Like most 'found' secondary clays it has a rather narrow window for vitrification but works quite nicely wood-fired to cone 10 in a 'neutral' (i.e. alternating oxidation and mild reduction) when blended with 25% fireclay (Hawthorne Bond) to provide additional alumina.  Color is an attractive (to me) dark red/brown with a purple tinge, and it gives nice effects with ash.

In heavy reduction at cone 10 it needs considerbly more fireclay to keep it 'open' and avoid bloating, that is, to counteract the fluxing effect of the high iron content in reduction.

I dig it wet, dry it, slake it and blend to the consistency of half-and-half, strain through window screen to remove large stuff, sediment briefly , then allow to settle in buckets for several months followed by air drying to throwing consistency.  This retains all the fine silt and the finer fraction of sand which moderates the shrinkage.  It throws nicely with and wthout the fireclay addition (blended as slip), much like a grogged body.

It gives a nice range of colors in ash glaze, depending on firing conditions,  but has rather high shrinkage which could be reduced by calcining a fraction, which I have not done since I have other simpler options.