Suggestions for an ultra-fine temper.

[EarthToMatthew]

Hey everybody, I've recently ventured into processing my own clay. I've been digging it up from a water bank, running it through a sieve and letting it drip dry from a pillowcase. Afterwards I add some bentonite to increase plasticity. My aim is a really smooth clay. I've experimented using diatomaceous earth as a temper but it ruins the texture for me.  I know that some people use wood ash/volcanic ash as a temper- I figure this would be the best bet for my desired texture. Does anyone here have any input on the subject? Do you use ash as a temper? Do you have a better suggestion? Thanks! 

[Babs]

8 hours ago, EarthToMatthew said:

Hey everybody, I've recently ventured into processing my own clay. I've been digging it up from a water bank, running it through a sieve and letting it drip dry from a pillowcase. Afterwards I add some bentonite to increase plasticity. My aim is a really smooth clay. I've experimented using diatomaceous earth as a temper but it ruins the texture for me.  I know that some people use wood ash/volcanic ash as a temper- I figure this would be the best bet for my desired texture. Does anyone here have any input on the subject? Do you use ash as a temper? Do you have a better suggestion? Thanks! 

I think there was a post on that relatively recently.

Try the wild clay posts at beginning of this forum.

Maybe search " temper" in this forum too.

[PeterH]

3 hours ago, Babs said:

I think there was a post on that relatively recently.

Might have been this one, where the choice of terminology seemed to be a hindrance rather than a help.

I'll freely admit that I'm not sure what temper means, and I'm pretty sure that it means different things to different people.

One meaning is:
Temper is non-plastic material that is added to clay to keep it from cracking when it dries. It is most often sand, ground stone or ground fired ceramics but historically a wide range of materials have been used for temper. 
e.g. see
Tempering Pottery Clay https://ancientpottery.how/tempering-pottery-clay/
Temper (pottery) https://en.wikipedia.org/wiki/Temper_(pottery)

Another meaning  relates to the non-pottery specific definition of tempered as
limited or controlled, or made less extreme https://dictionary.cambridge.org/dictionary/english/tempered

... and I expect that there are others.

OTOH you seem to want an additive to achieve a really smooth clay. So I don't think its currently  cracking, and you are trying to make its smoothness more extreme.  So I suppose I'm asking if something like "less gritty" or "more plastic" would capture your intentions better (and leave less room for misunderstandings).

PS What mesh-size is your final sieve?
Particle Size in How to Find and Test Your Own Native Clays
https://digitalfire.com/article/how+to+find+and+test+your+own+native+clays
But for fine functional ware, you will need a way to screen out +100 mesh sizes (see links below).

[EarthToMatthew]

3 hours ago, PeterH said:

OTOH you seem to want an additive to achieve a really smooth clay. So I don't think its currently  cracking, and you are trying to make its smoothness more extreme.  So I suppose I'm asking if something like "less gritty" or "more plastic" would capture your intentions better (and leave less room for misunderstandings).

Yes that's correct, I'm making sculptures so I need a really smooth clay that can facilitate fine details. The clay body I've been buying from a supplier has no tooth or grit to it at all, and I'm hoping to replicate it. You're also correct about my usage of the term temper. I need a non plastic material that will prevent cracking and shrinkage but doesn't ruin the smooth consistency. My final sieve is done with an 80 mesh. 

Thanks for sharing that article, I'll read through it right now!

[Piedmont Pottery]

Kyanite is available in 200 mesh and would not detract from the smoothness of your clay.

[glazenerd]

Matthew:

where you collected gives indication of mesh size. Most clay collected along water banks are obviously sedimentary, but pending how fast the water flows; usually find larger particle size. If you find a location along the creek/river where the current is slow; then you will most likely find smaller particle size clay. Did you test the clay without any additions before making the decision to add bentonite? Bentonite can be used, but it is not the best solution to create plasticity for many reasons. If you are after smooth; then theoretically you must increase small particle percentages (under 2 microns). Temper by wild clay definition is any large particle (20-40 mesh) added to create malleability, in lieu of plasticizers. 

You are actually talking about particle size distribution (PSD); a clay theorem that formulates a body based on %’s of large, medium, and small particles sizes. You are starting at 80 mesh from the description you posted. At this point, adding high percentages of fine mesh mullite, molochite, or kyanite would inhibit plasticity. All there of these are used to increase cone value, increase strength via mullite %, and or reduce warping when drying or firing. I do not know your firing method, or your peak cone value; so adding silica or feldspar would be a wild guess at this point. You can start with 60% wild clay, 20% Imco400 or Kentucky glaze#1, and 20% OM4 ball clay. In addition to smoothing out your clay; both additives will increase your dry time if you are doing large format pieces.

Tom

Edited February 1, 2023 by glazenerd
Typo- sure auto correct works. Yeah right.

[Min]

+1 for a fine mesh kyanite. @EarthToMatthew, good article on kyanite additions to clay below, worth a read to see if it sounds like what you are looking for.

https://digitalfire.com/material/kyanite

[EarthToMatthew]

20 hours ago, glazenerd said:

Matthew:

where you collected gives indication of mesh size. Most clay collected along water banks are obviously sedimentary, but pending how fast the water flows; usually find larger particle size. If you find a location along the creek/river where the current is slow; then you will most likely find smaller particle size clay. Did you test the clay without any additions before making the decision to add bentonite? Bentonite can be used, but it is not the best solution to create plasticity for many reasons. If you are after smooth; then theoretically you must increase small particle percentages (under 2 microns). Temper by wild clay definition is any large particle (20-40 mesh) added to create malleability, in lieu of plasticizers. 

You are actually talking about particle size distribution (PSD); a clay theorem that formulates a body based on %’s of large, medium, and small particles sizes. You are starting at 80 mesh from the description you posted. At this point, adding high percentages of fine mesh mullite, molochite, or kyanite would inhibit plasticity. All there of these are used to increase cone value, increase strength via mullite %, and or reduce warping when drying or firing. I do not know your firing method, or your peak cone value; so adding silica or feldspar would be a wild guess at this point. You can start with 60% wild clay, 20% Imco400 or Kentucky glaze#1, and 20% OM4 ball clay. In addition to smoothing out your clay; both additives will increase your dry time if you are doing large format pieces.

Tom

Hey Tom, thanks for your insight, I definitely have a lot of research to do on this topic. I fire in an electric kiln to cone 6.  I'll start looking into your suggestions I appreciate you giving me a good place to start.

Thanks to the other folks who suggested 200 mesh Kyanite, it's definitely much cheaper than the wood ash I've been looking at. Time to start experimenting!

Edited February 2, 2023 by EarthToMatthew

[glazenerd]

Wood ash is not a good clay addition; caustic levels of fluxes.

[Kelly in AK]

Agree with @glazenerd, wood ash is caustic. Also agree with above comments about kyanite. You’re after a fine mesh non plastic to inhibit shrinkage cracks, but testing the clay without additions is a good place to start. Too much non plastic material can lead to “splitting” cracks, which are a different beast that happen while your making your work.

https://digitalfire.com/glossary/splitting