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Kevin B.

Native Clay, Odd Smell...

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Hi All, long time infrequent visitor to CAD, first post - sorry it's a bit of a long one.

 

So, through a long series of events, I'm coming back to ceramics after several years of not touching any clay.  As an Anthropology and Archaeology student in college (with an art history minor) I learned to make pottery with experimental archaeology - basically recreating local examples of native pottery using the methods and materials that were used centuries ago to make earthenware.

 

However, what we learned on was a somewhat locally produced modern processed clay body with lots of sand mixed in to withstand bonfire and pit firings.  Don't get me wrong, I still use that clay body and experiment with it still since it is totally reliable in a pit firing, and even with all the grit added it's a pleasure to work with, but I've always thought of using clay harvested with my own two hands.  As chance would have it, I have a relative who lives in town and has local clay deposits popping up all around the spring fed pond on her property, and she recently gave me the OK to collect clay.  What's more, her house is literally less than a mile from an area with a known archaeological ceramic history.  (I've actually worked with the head archaeologist overseeing the current dig in the area, lovely woman with a wealth of information on our local native peoples and their material culture. - small world, right?)

 

So this past week I collected some rough clay from the pond, made a thin slip and ran it through a 1/8" screen to remove the small rocks, gravel and sand.  What I was left with was a clay that dang near matches what has been found archaeologically - gritty, silty, and predominantly large clay particles resulting in a mildly plastic, sticky, grey clay body that almost entirely settled out of suspension and left a layer of clear water in under 24 hours.  In other words, good thing I wasn't planning on making terra sig from it, I'd have to dig a ton just to get a couple pounds...  However, it suits my purposes perfectly since I'm going to me using it for hand-built recreation native pots and possibly some traditional raku-style chawan if it proves plastic enough.

 

Here's where it gets interesting... Remember I mentioned an odor in the title, right? While I was processing the clay I noticed a mild aroma.  At first I couldn't put my finger on it, but then I realized the closest thing I could describe it as was the same leathery, smokey smell you get from a German rauchbier that's been made with beechwood smoked malt.  Now, I was going to have the clay tested anyway to see if it's foodsafe, but this odor has me wondering if there's any point to going ahead if there's already some known chemical or toxin in the clay resulting in this aroma.  Or maybe it's because the clay is fresh and has a lot of organic material to rot out, I honestly don't know.

 

Thing is, I've poured through all my ceramic books and searched the net to see if this smokey smell has ever been documented, but I haven't found anything where anyone describes this phenomena.  So I guess that's my story and question all rolled into one - anyone know what would cause clay to have a leathery/beechwood smoke odor?

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it could just be pond scum...maybe some old burnt wood.

Marcia

 

Thanks for the response, it did have a pond scum/fishy/algae smell before I washed/diluted it but it dissipated soon afterwards (made sure to not get a whole lot of scum when I dug it).  Burnt wood might be the answer, but how long does that smell stick around in the soil?  Family hasn't had a bonfire on that property in 20+ years, and before that it's been probably close to 200 years since the farmers in the area originally cleared the land for grazing.

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Just a wild guess. Recovery from a scummy pond makes me wonder if the conditions were anaerobic.

IIRC this tends to result in the presence of hydrogen sulphide. Residual memories of long-ago chemistry

classes suggest that the smell in low concentrations can be a lot more subtle that simply "bad eggs". 

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Make a test tile and fire it (put it in a known clay container or dish in case it melts).  See if the smell goes away after firing.

 

Also, allow some of the clay to dry, then crush it into powder and reconstitute to see if that makes a difference. 

 

Agree with Marcia -- you can take the clay out of the pond, but you can't take the pond out of the clay.  Could be the residual of an organic release under the pond that is slowly seeping up  through the clay -- including an ancient forest fire or other event. 

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Farmers burn brush and tree stumps when they clear field.Anyway, I would use a finer mesh like fiber glass window screen for sieving the clay. Make a slurry first and pour thru the sieve.Clay can really stink depending on what has been living in the pond. Once you fire it, I doubt there would be any problem of hazards. I think it is an organic smell which should burn off. ..Unless there has been a toxic waste dump in the area.

 

Marcia

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In your case I would not worry about the smell (unless it makes you physically uncomfortable), although PeterH may be on the right track. In any case, it is highly likely this smell will burn out in the firing. It will for sure burn out if it is organic in origin.

 

What you really want to know is if this clay is usable. My advice below to a very similar post recently. Some of this may be useful, although the focus below is on stoneware not earthenware. However, the real question is whether or not to continue to test this clay, and this is a procedure for going about that.

 

Pitfiring and raku are great for decorative work, but unlikely to be consistent with the concept of "foodsafe" IMO, although I am happy to be corrected by resident experts in these specialty areas..

 

1/8 inch is not really very fine, and I am betting you still have a lot of sand in your clay. Try sieving to 80 mesh and even 120 mesh and see what is left on the screen. I bet it looks like dirty beach sand. What passes these sieves will yield more clay-like material, and if it is really clay and not silt (which is larger than clay particles) than you should be able to produce some terra sig with it with proper processing...

 

As with all native materials, how useful this material beyond the testing phase will ultimately depend on how much processing is necessary to get into workable form.

 

 

Posted 26 June 2016 - 07:10 PM

I assume you have access to as much of this clay as you want. If not, forget about it. Unless the material is so important to you (eg sentimental value) that you are willing to go through all the work just to be able to use it a few times.

 

If you work at stoneware temps, first step is to fire a chunk to stoneware in a small bowl. Do not bother with other temperatures since you likely do not work at those temperatures anyway. Do not sieve or clean the clay except to get rid of large rocks, roots or sticks and other obvious debris (in any case not more than 12 mesh). Just fire it mostly raw.

 

If it turns into a bubbly molten mass in the firing, put it on a back shelf (carefully labeled) and forget about it until you have NOTHiNG else to do and want to kill a lot of time for maybe something you can use.

 

If the chunk remains intact then you have something to work with. It is either just right (a 1 in 100 event) or (more likely) somewhat refractory, but either way you have a path forward to find this out.

 

Next step would be to clean the clay to a level you are likely to be able to (and want to) replicate if you start to use it in some considerable volume. No use sieving to 100 mesh to run tests if you are unlikely to do that for every bit of it going forward when you actually start to use it. The chemistry and behaviour of the clay will almost certainly be different in these two cases.

 

Then make test bars which you will want to weigh wet, bone-dry, bisqued and high-fired to get an idea of Loss on Ignition. You will also want to measure shrinkage by measuring the bar (or some markings on it) at all stages along the way. Finally, if you want the body to be usable for functional ware, you will need to boil it for a couple of hours, then let it sit in the water for a day, and then reweigh. This will give you an estimate of porosity, or how much water the fired clay absorbs.

 

As you are going through this process, gather this information for some commercial clay bodies that you actually use (from the spec sheets from the manufacturer) and compare.

 

At the end of this, you will know if your clay is

 

a) the holy grail: stoneware out of the ground. If you get this, pop the champagne corks.

 

or B) a refractory clay that will need some additions to be usable in the rest of your ceramic activities. But that is a whole nother chapter.

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curt:

In your case I would not worry about the smell (unless it makes you physically uncomfortable), although PeterH may be on the right track. In any case, it is highly likely this smell will burn out in the firing. It will for sure burn out if it is organic in origin.

 

What you really want to know is if this clay is usable.

 

... lots of good advice

 

I agree totally that the smell (and its cause) are very unlikely to affect the physical properties of the fired clay.

Toxicity in another issue, but organics and sulphur compounds are likely to burn out.

 

However I am concerned/intrigued by this -- far from bullet-proof -- chain of logic:

- smelly clay from a pool

- anaerobic in nature

- sulphides present

- lower than normal pH (more acidic)

- clay more flocculated than normal

- plasticity of clay reduced

- usability of clay reduced

 

Purely top-of-the-head thoughts from somebody who hasn't been there, but just maybe food for thought.

 

If it is possible I would try to collect some clay from a dryer environment (just a test sample) and check for:

- smell

- settling properties

- plasticity of a treated sample

If the properties are different from your smokey clay, then maybe the choice of location is significant.

[Probably only worth doing the 3rd if the 1st two are different?]

 

PS I initially "sort of assumed" that ageing an anaerobic clay would remove any problem. I now get the

impression that the sulphides (with bacterial assistance) oxidise to sulphates, and things become even

more acidic. [cf acid sulphate soils]

 

PPS last and by no means least

curt: and if it is really clay and not silt (which is larger than clay particles) than you should be able to produce some terra sig with it with proper processing...

If you have a deflocculant about, this sounds like a good thing to try, at least as far as seeing if a significant "colloidal" layer forms.

Obviously if the sample is acidic you may need more deflocculant than normal.

- If this doesn't work, can your sample really be similar to that used for native pots?

- If it does work, maybe your current sample gives an over-flocculated suspension?

 

 

 

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Smoky odors imply a partially oxidized organic substance, not a sulfide or mercaptan.  Many of the partially oxidized organic materials are of the 2-2 di-methyl-chicken-wire structures accompanying decomposing woody materials from leaves, stems, and such that collect in a pond.  Your description of the settling of the slip you made is similar to that I make from my pond.  Use it. 

Mean while: Think perfume, not sewage. 

 

And when you fire it, remember what Jim Croce said in 1972 in "You Don't Mess Around with Jim" and stand up wind.

LT

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Concentrate on the physical properties, not the smell.

If the effort was worth levigating the clay thru a wire mesh and lots of trash was removed, find another clay source. If you make clay beads and can crush the unfired beads and fired beads between the index finger and thumb, find another clay source.

 

What are your goals with this natural clay?

 

What tribe of Indians was in and around Connecticut b/n 1725 thru 1750?

 

See ya,

Alabama

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Wow! Haven't been able to jump on here in a few days and I am just floored at the response and the information you guys/gals shared.  Thank you!  First off, I should probably clarify/answer a few things...

 

 

In your case I would not worry about the smell (unless it makes you physically uncomfortable), although PeterH may be on the right track. In any case, it is highly likely this smell will burn out in the firing. It will for sure burn out if it is organic in origin.

 

Seems to be the case - after a few days of oxidation in the bucket the smell has almost dissipated, it now just smells like organic heavy mud.

 

 

 

1/8 inch is not really very fine, and I am betting you still have a lot of sand in your clay. Try sieving to 80 mesh and even 120 mesh and see what is left on the screen. I bet it looks like dirty beach sand.

 

Correction, I thought I was using 1/8", I was actually using 1/16".  Leaving the silt and small grit in is not an oversight in my case, it is deliberate. Remember you're talking to someone who started out in experimental archaeology.  I'm aware of mesh sizes, but in doing primitive and native style pottery "mesh" is a bunch of modern western mumbo-jumbo. ;)  But seriously, thanks for your post, if I ever decide to really process the clay I'm going to follow your advice to the letter.

 

 

Purely top-of-the-head thoughts from somebody who hasn't been there, but just maybe food for thought.

 

If it is possible I would try to collect some clay from a dryer environment (just a test sample) and check for:

- smell

- settling properties

- plasticity of a treated sample

If the properties are different from your smokey clay, then maybe the choice of location is significant.

[Probably only worth doing the 3rd if the 1st two are different?]

 

Where I live it's all dry forested rocky hill tops and soggy wetland/river valleys - I'd be amazed if I found clay where it's dry.  The most local clay producer near me (Sheffield Clay, in Sheffield MA) takes their stuff from what was an ancient riverbed.  Finding clay near water or where there was water seems to be the norm in my area.

 

 

If you have a deflocculant about, this sounds like a good thing to try, at least as far as seeing if a significant "colloidal" layer forms.

Obviously if the sample is acidic you may need more deflocculant than normal.

- If this doesn't work, can your sample really be similar to that used for native pots?

- If it does work, maybe your current sample gives an over-flocculated suspension?

 

How native clays were processed in my neck of the woods was - clay deposits would be dug and dried, smashed/ground and then sifted through a woven basket to get out the larger stuff before re-wetting and shaping - this left a lot of silt and grit in the clay which acted as temper to help the pots withstand the temperature fluctuations in firing.  It's very rough stuff, but it did its job.  I've seen examples of early to middle woodland (approx 3000 yrs old) native pottery taken from the ground close to where I found this clay, and all I can say is the composition looks almost exactly the same for the two.  Whether it's the same clay or not - well, that requires some chemical testing for which I currently do not have the funds. (Nor do I think the museum who has the sherds in their collection would want to give one up to a hobbyist who is just curious about their composition.)

 

 

Smoky odors imply a partially oxidized organic substance, not a sulfide or mercaptan.  Many of the partially oxidized organic materials are of the 2-2 di-methyl-chicken-wire structures accompanying decomposing woody materials from leaves, stems, and such that collect in a pond.  Your description of the settling of the slip you made is similar to that I make from my pond.  Use it. 

Mean while: Think perfume, not sewage. 

 

And when you fire it, remember what Jim Croce said in 1972 in "You Don't Mess Around with Jim" and stand up wind.

LT

 

Will do!

 

 

What tribe of Indians was in and around Connecticut b/n 1725 thru 1750?

 

Lots! And there were even more before that time frame. Although here in the northwest corner where I live it probably would have been Schaghticoke, Potatuck, or one of the smaller local tribes.  In fact, the lake that's a 1/4 mile from where I dug the clay is named after Chief Waramaug who was the sachem of the Potatuck from 1725-1735.  The later woodland period pottery of this area has quite a bit of "Iroquois" influence because they traded with the Mohawk to the North West in NY.  Before that is a much simpler style of chord marked and incised earthenware.

 

 

Whew! Now that that's done here's what's been happening over the past couple days...

 

After letting it settle for a few days I carefully poured the water off the top of the clay, and was able to get all but an inch.  This I mixed back in to make a thick slip, which I then poured in to a dryer setup I've used before for clay - basically a 2'x2' frame made of 1x4 resting on top of plywood into which I put 2 layers of heavy cotton duck to wick away moisture.  Interesting to note - when I went to mix it back into suspension to pour out, what I thought was a solid settled layer was not a solid compact layer at all, it was still in suspension somewhat.  Felt like I was putting my hands through syrup, not soil.  Now all I have to do is wait for it to dry to the point I can use it - should be a few days.  I'll keep you guys updated on its progress.  Pics to come soon - having trouble uploading.  Off to the FAQ section I go!

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I think you should read "Traditional Pottery of Papua New Guinea" by Margaret Tuckson 2 1/2 times and Pottery for the Archaeologist by Anna Sheppard, 2 1/2 times

To shed light on clay preparation.

 

There is primitive pottery and modern pottery! To combine one with the other makes it nearly impossible.

 

See ya,

Alabamà

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Years ago I once read that freshly dug clay had no smell or had a faint smell of dirt!

 

But once wedged and placed inside a bag, and aged 72 hours, it would smell like dirt. I didn't believe it until I tried it!!! I think the jest of the article was to say, once the clay smells like dirt, it's OK to use.

 

What book it's in, I have no idea.

Alabama

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