Wild earthenware question

[petrichor]

Hello,

I have recently started experimenting with some local earthenware i found close to my home. I made the clay into a slip and used a regular kitchen sieve to remove the largest particles. I find that the clay is harder to throw than my standard clays - no surprise - but I wonder why? It collapses pretty fast, and i can only make small objects. My main problem though, is trimming. The clay very often cracks up when i trim: the trimming tool sticks and suddenly a whole chunk of the clay is missing. also, it just cracks up. I have tried to trim the objects when they are less dry, but then the walls have no strength at all.

My questions are:

- is this typical for earthenware? usually i work with stoneware, so i don´t really know.

- should i have sifted the clay with a finer mesh? a kitchen sieve is pretty crude, but i only have a glaze sieve and nothing went through. i also was concerned that particles that i would like to keep would get stuck.

- any other theories why the clay is so fiddly?

- i have bisque fired it, but not glaze fired it yet. but if i used a too coarse sieve, can it be that the finishes objects will be of poorer quality because the clay may contain non-vitrifiable materials?

[glazenerd]

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

Tom

[Min]

Sounds like it could be Norwegian quick clay.

Moving this thread over to chem section.

[petrichor]

6 hours ago, glazenerd said:

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

Tom

Sorry, i should have written: i have found "blue clay", a marine clay very common in norway. it is blue/grey but fires red.

[petrichor]

5 hours ago, Min said:

Sounds like it could be Norwegian quick clay.

Moving this thread over to chem section.

maybe? we call it blue clay, a marine clay that is silver blue but fires red.

[Min]

It sounds like the problems you are having are from the same reasons blue / marine / quick clay causes mudslides.  From the little I understand of it salts are responsible for the liquefaction of the clay, on a lesser scale this sounds like what is happening with it being easily collapsed while throwing and then the trimming issues. I don't think it's particle size, I think it to do with cations.  Perhaps @glazenerd has a fix for this? Altering the charge on the particles maybe?

[PeterH]

Looks an interesting topic.

Quick clay
https://en.wikipedia.org/wiki/Quick_clay
... high-sodium marine clay changes is characteristics after the sodium is leached out.

Quick clay landslides: an online documentary about Rissa
https://blogs.agu.org/landslideblog/2020/06/18/rissa-ngi-video/
... showing stiff-ish "found" clay liquefying under pressure to a pourable consistency. After adding salt it it is stiff enough to hold a knife upright.

Quick though: are you looking at the properties of the as-dug clay, or has the washing involved in the filtering removed even more sodium?

PS If you can give us the location where you found the clay perhaps somebody can find it on a geological map.

[glazenerd]

TY Min. Several years back I read a series of abstracts from D.D.Buttons (Alfred U) about the effects of acidity/alkalinity on cation exchange. After petrichor stated it was collected ocean side; an observation Button’s made came to mind. Alkalinity creates negative particle charge, which is the basis of plasticity/deflocculation. However, once alkalinity passes 9.8PH, then the reaction reverses; and cementing (hard clumping) begins to occur. Petrichor mentioned “chunks” missing when trimmed. Another indication is rapid drying and cracking: also indications of very high alkalinity. Sodium silicate for instance is above 13PH. Very little creates a strong particle charge. Any potter who has done salt firing knows how caustic salt is. 

The most basic experiment to check for high alkalinity would be to add 1 teaspoon of vinegar to 1/2 pound of clay. Wedge, and try throwing. The reaction will be almost immediate if it is indeed salt. After you check it; add another teaspoon until the clay reaches a point where it is not chunking when you trim it. You can add enough dry clay to keep the moisture content in check. 

Typically; a deep gray color in clay indicates a magnetite iron source. From samples I have seen, or have first hand knowledge of lab testing; silver color usually indicates magnesium. 

Tom

[Kelly in AK]

My local clay Anchorage, Alaska, has been compared to Norwegian quick clay. Blue-gray, fires red, is thixotropic. It occurs in lenses with silt which looks identical. I’ve found that I have to be careful to not get too much silt. I sieve the stuff to about 60-80 mesh, but the silt easily passes through.

Too much silt and it’s definitely more thixotropic, has very poor green strength, and splits easily.

Even when I have processed high quality clay I still add 1-2% Veegum. 

[Min]

That's interesting Kelly that you found adding Veegum works for you. Just curious, have you ever tried adding vinegar?

[Kelly in AK]

I didn’t try vinegar, but did use 0.2% epsom salts for a while. It made a noticeable improvement, nothing like the Veegum though. 

[PeterH]

Have you thought about adding table salt? (Quick clay is formed by leaching sodium from the original marine clay.)

... although I believe that potassium chloride is now used for quick-clay landslide mitigation
https://tspace.library.utoronto.ca/bitstream/1807/124221/1/cgj-2021-0230.pdf

Edited May 13, 2023 by PeterH

[Kelly in AK]

For pottery clay I imagine salt would at least have some negative consequences in firing. I, for one, have not tried it. 

Watching fluid quick-clay suddenly become solid with the addition of salt is dramatic. The chemistry  teacher at my school has done it with our local clay. It’s a pretty complex phenomenon, the salt leaches out (over a geologic period of time) and leaves the sedimented structure like a microscopic house of cards. When that’s loaded with enough water the slightest disturbance breaks it loose and everything flows. Particle size plays a role too, the layers and proportions of clay, silt, and sand affect what areas become “sensitive.” Reading through some of the research after the 1964 Alaska earthquake suggested to me that there were areas of clay on top of, below, and adjacent to what liquefied that held together.

My experience trying various proportions of clay:silt surprised me most by how they dewatered on a plaster slab. The purest clay dewatered slowly, like ball clay. Silty clay that was smooth and moldable (but poorly plastic) dewatered in minutes. Weirdly, I’ve made pots of it that begin to dry so fast because the water is flowing through them as they sit, the ware board just sucks it out. I really have to use my imagination to try and understand it, and probably have no business trying to explain it, but I can say for certain what’s worked best for me. 

I try to pick clay as far above sea level as I can find it because I want as much soluble salt leached out as possible. It’s ultimately about getting the most clayey stuff though.

The more clay is in the material, the harder it is to break apart in my hands, if I whack it, I look for conchoidal fractures. If I can break it and crumble it to dust, no matter how much it feels like clay when it’s wet, it’s got too much silt. I guess I’m going on at length here because that one little distinction made such a difference for me. More than any additives or adjustments to processing, my best clay starts with careful picking. Always looking for the motherlode. 

Edited May 13, 2023 by Kelly in AK

[glazenerd]

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

T

[Min]

I wonder what would happen if you apply both a physical change (particle size) plus a chemical change to the clay?

[glazenerd]

Min:

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

Tom