Chemical analysis of clay

[Kelly in AK]

I’ve been using my local clay for a few years, both for pots and as a glaze ingredient. Done lots of testing to figure out how to play well with it. Something I always thought would be nice was to have a chemical analysis of it, like those available for most commercial clay minerals and glaze ingredients. I don’t think I really need one, and the more I use the stuff, the less it seems necessary. But still, the thought doesn’t go away: It would be nice to know more about what’s in this clay. I’d love to see that table of percentages of oxides for my own clay that I do for the other clay minerals I use. 

Does anyone out there have advice for getting a chemical analysis done on clay, where to start, or even thoughts on how useful that data would be?

ps- I’ve exhausted my leads on published research here. Made a few calls to boot. There has been massive testing of our clay but mostly for engineering/seismic research. If an existing chemical analysis is out there, it’s tucked away somewhere I can’t find it.

[Hulk]

Glazenerd (a frequent contributor to the Forum) may have some suggestions.

Edited June 3, 2023 by Hulk
Glazenerd is a handle, like "Hulk"

[Min]

Have a look at this post, @GreyBird had an analysis done on her found clay from along the Hudson River. Page 1 of that thread has the data she recieved, on page 2 she posted where she got it done,  https://www.minerallabs.com/. At the time it was $500- for the three tests she had done.

Edited June 3, 2023 by Min

[Kelly in AK]

Thank you Madeline, that was a very useful thread! 

[Min]

Glad it helped!

[glazenerd]

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

 

Tom

[Kelly in AK]

I’d like a table of percentages of various oxides in the clay, like you would use if you were plugging in a material in a glaze calculation program. So, it’s generally specific (Attempting humor because I’m not sure how to answer). Not too deep.

 I have some data generated on my own, like shrinkage, absorption, and plasticity tests. Adding vinegar to it changes the character a little, so that tells me something about the ph. It’s thixotropic, but if I’m careful to pick clay with less silt it’s much less pronounced as a property. Tricky, because it’s all the same color. I haven’t done a l.o.i. test yet, but that seems like something I could manage. When it come to things like “How much potassium is in this? How much iron?” I just have to guess based on what I observe coupled with bits gleaned from published research on the clay.

Predominantly chlorite and illite is what I get from reading till my eyes glaze over. It’s known geologically as “Bootlegger Cove formation clay,” or BCF clay. There’s no variety in types of clay in the area, just this one, but it is abundant. The variety seems to be in the proportions of silt to sand to clay in various deposits. It’s iron rich, but not like that beautiful hematite clay. This starts gray-blue-green and winds up orange-red-brown, chocolatey when heated a little too high. The clay plastic enough for pottery vitrifies at cone 03 (<1% absorption), and slumps within half a cone higher. At cone 04 it’s dense and strong, but porous, around 5-7% absorption. A narrow range! I’d kind of like to know what the predominant flux is that’s making it vitrify so low.

Clearly I’ve started. Down the rabbit hole. I don’t want to go too much deeper. Just a little bit. A little bit further. 

[glazenerd]

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 

Edited June 4, 2023 by glazenerd
lack of coffee before typing

[Kelly in AK]

Wonderful information Tom! That gives me plenty to chew on. And you called it spot on, I thought there must be some active melter in there, low alumina never crossed my mind. 

Thank you for taking time to share this. I always enjoy reading what you have to say on the forum, glad you’re a contributor.

[glazenerd]

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 in AK]

Excellent, I’ve been meaning to send you a message asking about that. I’ll dig it up! 
 

edit- found it

Edited June 5, 2023 by Kelly in AK

[PeterH]

57 minutes ago, glazenerd said:

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

I did a search for "Split LOI" in "Content tiles and body" covering "everywhere" and only got hits for two of your postings in this thread.

... and another for glazenerd postings in 2019  mentioning LOI (it insisted on a some text to match) again without success.

... a search for anything by PeterH posted in 2019 and containing "the" also failed.

... looks like none of my posts before 2022 are there.

@Hulk How long are posts kept? Are there any non-active backups about?

 

BINGO a google search for "Split LOI" test found

So either my searches were NBG or the search function doesn't search all  postings.

Edited June 5, 2023 by PeterH

[Hulk]

Good question Peter.
Google returns a link to an archived post, it's here in the Forum.
I'm seeing there are archived threads going back to 2010 or so.

I'm not sure why the Forum's search function isn't getting it - it may be that the search function only looks at open threads?
...that's what I'm finding, looks like the Forum's search function does not look at "archived" threads - threads that are closed to further comment.

a) having picked at random a thread that is closed to more comments, selected what may be a unique string, no hits, the Forum search does not find it; enter the same string in Google (quoted), plus CeramicArtsDaily, bingo, found it, top hit.

b) from there, picking at random an older thread that is not closed to more comments, select what may be a unique string, search, easy find, the Forum's search finds it.

[Kelly in AK]

Yeah, I tried searching the forum first with “split loi” and it wasn’t productive. Next I looked at glazenerd’s profile to see if I could scroll back through posts and find it that way, but the “activity” log didn’t go back that far. Did a similar search in a search engine (that incidentally, isn’t Google) and Tom’s post was somewhere near the top. All in all, took about a minute. 

Search functions inside websites, forums, social media sites, etc. are often not as effective as a well constructed phrase in a search engine. It’s good to know this.