Wild Clay Cracks When Bisqued

[alord]

Howdy,

I recently processed some wild clay and have been having issues with it. Originally, I bisque fired it to cone 02 and it melted into the blobs shown in the first image. I then tested a cone 05 firing and the test tile survived, but had a bunch of small cracks. I tried another bisque firing with more test tiles and a thrown piece and they suffered the same fate. Does anyone have any idea what about the clay body could cause this? Is there anything I could add or test for that would help? The clay threw well enough, though I did notice that the greenware dried very slowly and when bone dry was remarkably strong (I literally threw one of the test tiles at the pavement and it didn't break). The clay came from a rocky region of central/western Texas. Any input would be greatly appreciated. 

God bless.

[Callie Beller Diesel]

Hi and welcome to the forum!

We do have a small pinned thread started in the glaze and chemistry section with a few links on working with wild/found/native clays, and you can start there, but I know not everything we have on the forum is in that thread, so I encourage you to do a search from the main page with some of those terms. Others have shared information before.

Definitely go through the testing mentioned on Digitalfire listed here, including checking out the article links at the bottom of the page. Characterizing your clay can help a lot with figuring out why it’s doing what it’s doing and will give you hints about its composition. Checking with your local geological survey can also give you an idea of what the clay’s composition might be, which will give you more info on working properties.

According to the Potter’s Dictionary of Clay and Glazes by Hamer and Hamer, the cracking pattern of your jug is consistent with bisque dunting. They state that bisque dunting either happens when the firing cools too quickly through quartz inversion, or possibly from being not fired enough to achieve enough strength. Are your jug shards really soft and crumbly?

If the speed of the firing is a problem, that could indicate the clay is very high in free silica. If the dunting is an indication of the bisque not being hot enough for strength but you get some pretty extreme bloating only 3 cones hotter, that’s a very narrow firing range. If you want to use this clay, it’s definitely going to have to be amended.

Hamer and Hamer suggest fixing dunting due to to free silica by binding the silica up by adding a flux like feldspar. Given that feldspar can also contain a lot of alumina, that might be a place to start. It could also be you need to use this clay as an ingredient in a more balanced clay recipe, or turn it into a more vitreous decorating slip with a bit of flux.

[alord]

2 hours ago, Callie Beller Diesel said:

Hi and welcome to the forum!

We do have a small pinned thread started in the glaze and chemistry section with a few links on working with wild/found/native clays, and you can start there, but I know not everything we have on the forum is in that thread, so I encourage you to do a search from the main page with some of those terms. Others have shared information before.

Definitely go through the testing mentioned on Digitalfire listed here, including checking out the article links at the bottom of the page. Characterizing your clay can help a lot with figuring out why it’s doing what it’s doing and will give you hints about its composition. Checking with your local geological survey can also give you an idea of what the clay’s composition might be, which will give you more info on working properties.

According to the Potter’s Dictionary of Clay and Glazes by Hamer and Hamer, the cracking pattern of your jug is consistent with bisque dunting. They state that bisque dunting either happens when the firing cools too quickly through quartz inversion, or possibly from being not fired enough to achieve enough strength. Are your jug shards really soft and crumbly?

If the speed of the firing is a problem, that could indicate the clay is very high in free silica. If the dunting is an indication of the bisque not being hot enough for strength but you get some pretty extreme bloating only 3 cones hotter, that’s a very narrow firing range. If you want to use this clay, it’s definitely going to have to be amended.

Hamer and Hamer suggest fixing dunting due to to free silica by binding the silica up by adding a flux like feldspar. Given that feldspar can also contain a lot of alumina, that might be a place to start. It could also be you need to use this clay as an ingredient in a more balanced clay recipe, or turn it into a more vitreous decorating slip with a bit of flux.

Callie,

 

Thank you for your response. I'll read through the Digitalfire article and see what I can glean from it. I'm basically out of this batch of clay, so I'm not really that inclined to do much to it. These results were just a bit unusual for me and I'd like to get an idea of what could cause this. I have thought of incorporating the clay into a glaze or slip and I'm thinking for trouble's sake I may move in that direction.

I was also testing a glaze when I fired these pieces, so I used the "Fast Glaze" setting on the kiln controller; I presume the "Slow Bisque" firing setting may have yielded better results. The jug is not crumbly or soft in any way. It feels like normal bisqueware. Once it's been bisqued, would a glaze firing also need to be slow?

As far as adding a feldspar, the local ceramics shop has a couple (Custer, Minspar, Nepheline Syenite), would it matter much which was added? 

[alord]

Well, to make things more interesting, I found this when I removed a large chunk from the broken creamer. This is looking at the cross-section of the wall of the piece. I'm not sure what would cause this, maybe leftover organics? I couldn't find more examples of this in the other broken pieces of the creamer.

[Hulk]

Interesting, thanks for sharing!

Maybe Black Coring (digitalfire.com)

[Callie Beller Diesel]

Can confirm that is black coring.

A fast glaze cycle isn’t great for bisquing most clay bodies, red ones in particular. I think that this is pretty strong evidence for going too fast through quartz inversion and excess free silica being your cracking source.

If you look through the digitalfire articles, especially the ones on black coring, bloating and clay testing, you’ll find a lot of really helpful info on how to proceed. There’s a really interesting note in the black coring article with some observations about red clays and feldspars making strong high fire stonewares. This could be a really nice clay to add to a clay body to give it some character. It seems to need some feldspar.

[glazenerd]

You are dealing with several issues:

1. Pyroplasticity (melted blob) is caused by low alumina levels in your wild clay; in this case under 15% total alumina. 2. Your clay is iron disulfide: other iron bearing clays do not produce black coring unless sulfide contamination occurs (very rare) Iron disulfide (iron pyrite) will produce sulfur monoxide when fired too quickly; this gas is a powerful flux which produces black glass- commonly called black coring in the pottery biz. Slow bisq speed is 108F per hour; which is the correct speed in this case. 3. The cracking pattern in part is caused by the black coring; which is subject to severe COE expansion/contraction issues. The cracking pattern is also caused in part by the lack of sub-micron (small) particle distribution. Iron bearing clays typically tend to be larger in particle size. Small particle (sub micron) clays add plasticity, but also create adhesion in this case. There are fixes, but it sounds like you are moving on anyway. 

[oldlady]

that photo should be preserved as the best example of black coring photos.   any way to insert it into one of the resources mentioned for future searchers?

Edited February 11, 2023 by oldlady

[Callie Beller Diesel]

@oldlady, I will add a few tags to this post so it’ll show up in search.