Temmoku glaze shivering and crawling on stoneware

[Linchimb]

Hi, I am testing the 'Hamada Rust' temmoku glaze on a stoneware clay body. Fired at 1250 with 30min soak. Can a glaze both crawl and shiver? On the base it looks like it crawled showing bare clay but on the rim it looks like it shivered with a small chip (although I did drop it on kitchen table, not sure if it chipped from that)....would really appreciate any suggestions for troubleshooting! The glaze pings a bit after firing on my shelf and eventually settles at some point. I haven't noticed any shivering before on slip cast bodies. 

Here's the receipe: https://glazy.org/recipes/2967  I used Potash Feldspar instead of Custer Feldspar and Gillespie Borate instead of Gerstley Borate.

And how do you test for shivering? On digitalfire it says shivering can be delayed, then I am wondering how would you test for delayed shivering if it can happen at any time after firing?

Many thanks!!

 

 

 

[PeterH]

53 minutes ago, Linchimb said:

And how do you test for shivering? On digitalfire it says shivering can be delayed, then I am wondering how would you test for delayed shivering if it can happen at any time after firing?

Presumably a boiling water:ice water test would increase the stresses and reveal latent flaws.
BWIW - Boiling Water:Ice Water Glaze Fit Test
https://digitalfire.com/test/bwiw
... note that for shivering you want to test cold-to-hot.
This test subjects the clay:glaze interface to a differential thermal stress of 180F (112C†) both hold-to-cold and cold-to-hot (the former accelerates crazing and the latter shivering. This test is needed because, although ware may appear OK when first removed from the kiln, over time less-than-ideal fit will reveal itself. Compatibility between the expansions of clay and glaze are critical, not only to the integrity of the glaze layer, but also functional ware strength. Crazed glazes also provide a channel for water absorption by a porous body (creating conditions for the harboring of bacteria). Shivering glazes can drop chips of razor-sharp glaze flakes into food or drink.
† I assume this should be 100C

Some people use a more severe test: freezer-cold to boiling water.
thermal shock test on mug... results
http://www.potters.org/subject09036.htm
... especially Ron Roy's posts

[Bill Kielb]

6 hours ago, Linchimb said:

On digitalfire it says shivering can be delayed, then I am wondering how would you test for delayed shivering if it can happen at any time after firing?

Just to add to the above, this test is designed to try and reveal a glaze fit problem by cycling the pot through a reasonable operating range of temperatures. It’s intent is to reveal now if a pot likely will fail in the future by adding some stress. There are no precise predictive tests that can predict when a pot will likely craze or shiver.

If the difference in fit between glaze and clay is great enough and it exceeds the glaze / clay interface strength or the glaze strength itself or even the clay strength then these defects can occur.

Over time and through regular use the pot wears making the failure more likely. Since wear varies with individual use there can be no likely predictor other than the test of time, so to speak.

Crawling is more a function of a glazes fire viscosity and fired surface tension. Will it heal or pull away from itself? So in essence crawling can be independent of glaze fit.

BTW, this glaze ought to melt at about cone six, is said to be matte, and it’s si:Al indicates it ought to be more matte than gloss. it’s flux ratio also places it in a range that would indicate it may not be the most durable composition. Since your glaze is very glossy, it may well be fired a bit beyond its firing range.

Firing it to a temperature with a hold is also a mystery to me. Were  you trying to add more heatwork or fire to the next cone higher? Say fire to cone 8 but achieve enough heatwork to drop cone 10?

Edited June 20, 2022 by Bill Kielb

[Linchimb]

35 minutes ago, Bill Kielb said:

BTW, this glaze ought to melt at about cone six, is said to be matte, and it’s si:Al indicates it ought to be more matte than gloss. it’s flux ratio also places it in a range that would indicate it may not be the most durable composition. Since your glaze is very glossy, it may well be fired a bit beyond its firing range.

Hi Bill, thanks for your reply. Actually in John Britt’s book this is a cone 10 glaze and I was thinking whether I under fired….it is very glossy. Maybe I can try a different firing schedule and temperature. 
 

I thought crawling is the opposite of shivering and I was puzzled to see both happening at the same time. maybe the crawling was due to glaze application since temmoku type of glaze needs to be applied fairly thickly and in some areas it’s probably too thick. 

[Linchimb]

2 hours ago, PeterH said:

Presumably a boiling water:ice water test would increase the stresses and reveal latent flaws.
BWIW - Boiling Water:Ice Water Glaze Fit Test
https://digitalfire.com/test/bwiw
... note that for shivering you want to test cold-to-hot.
This test subjects the clay:glaze interface to a differential thermal stress of 180F (112C†) both hold-to-cold and cold-to-hot (the former accelerates crazing and the latter shivering. This test is needed because, although ware may appear OK when first removed from the kiln, over time less-than-ideal fit will reveal itself. Compatibility between the expansions of clay and glaze are critical, not only to the integrity of the glaze layer, but also functional ware strength. Crazed glazes also provide a channel for water absorption by a porous body (creating conditions for the harboring of bacteria). Shivering glazes can drop chips of razor-sharp glaze flakes into food or drink.
† I assume this should be 100C

Some people use a more severe test: freezer-cold to boiling water.
thermal shock test on mug... results
http://www.potters.org/subject09036.htm
... especially Ron Roy's posts

Thanks Peter. I just tried this ice-boiling water test and it seems fine. I might put it in the freezer and see what happens. 

[PeterH]

1 hour ago, Linchimb said:

Thanks Peter. I just tried this ice-boiling water test and it seems fine. I might put it in the freezer and see what happens. 

Let's hope it was just chipped when you dropped it. However if it pings it suggests there is a glaze fit issue. Maybe try the hot to cold test as well to see if its incipient crazing.

BTW maybe try an indian-ink test for crazing as well (some crazes can be hard to see).

... oops don't know what you could use on a black glaze. Although I've seen white indian ink for sale.

Interesting read, but don't like the solvent. Maybe try a white marker pen or a "non-cheapo" white watercolour paint (hopefully has a fine grained pigment).
crazing/tenmoku/dmso
http://www.potters.org/subject60993.htm
I've been working on some tenmokus to fit my porcelain body and getting
these glazes into the right expansion range is tough because they are
fluxed with feldspar and whiting which have a fairly hi expansion. Then,
we have to add a fair chunk of Iron which increases the expansion even
more. If you add a low expander such as magnesia, crystals will develop
unless you cool fast. So it goes! The other thing is "revealing" the
crazing when it happens. Real tough to see on a dark, reflective glaze
that is a tenmoku. So, I've tried some ways of bringing out the craze
lines so I could actually get a handle of the "frequency" of lines. The
finer the craze lines, the farther off from fitting. OK, to the
point! I've been using a mixture of DMSO and talc. Brush it on the
finished piece and wait until it dries, wipe off, and check thru a
magnifying lens. Bingo! As many of you know, DMSO is a very powerful
penetrating solvent and it moves the talc right down into the craze lines
and you will then get a perfect visual of what's going on in terms of glaze
fit.

I just inspected some tiles today that have gone thru the freeze/boil test
and the two week soak test and, at last, I think I have a tenmoku that fits

.. I've no idea what the two week soak test is.

A word of caution: DMSO is indeed a tremendous solvent. It is also
capable of penetrating the skin and carrying anything dissolved in it
(that's dissolved, not suspended, like talc) into the body - so be
careful, wear rubber (nytril) gloves when using it.

 

.

Edited June 20, 2022 by PeterH

[Min]

6 hours ago, Linchimb said:

I used Potash Feldspar instead of Custer Feldspar and Gillespie Borate instead of Gerstley Borate.

 

Custer is a potash feldspar, Gillespie Borate has a slightly different chemistry than Gerstley Borate but more than likely not enough to make a difference in this glaze recipe.

6 hours ago, Linchimb said:

Can a glaze both crawl and shiver? On the base it looks like it crawled showing bare clay but on the rim it looks like it shivered with a small chip (although I did drop it on kitchen table, not sure if it chipped from that)

Most common cause of crawling is just having the glaze on too thickly.

6 hours ago, Linchimb said:

The glaze pings a bit after firing on my shelf and eventually settles at some point.

Only time I've seen both shivering and crazing (the pinging you have means it's crazing, even if you can't see it) is with glazes quite high in lithium. I think the glaze that came off the rim is probably from when you dropped it on the table. Considering both the fact that you are getting crazing plus the high amount of sodium and potassium (means the expansion will be high) I really don't think this is shivering.

If you do want to test for shivering cut some slabs of clay into strips and bend them into circles, join the edges and bisque then glaze the rims. Don't smooth the rims out. Shivering most often happens on edges. After glaze firing tap them with something dense like the handle of a screwdriver. If nothing flakes off you can then stress them further by freezing them then putting them into boiling water. If you still don't see shivering try tapping them again with the screwdriver handle. 

Years ago Ron Roy wrote that he discovered his dark tenmoku glaze was crazing when the steam from a bowl of mashed potatoes highlighted the craze lines.

 

[Linchimb]

Thanks! I just did the freezer to boiled water test and it seems fine. Also did the hot to ice test. And knocked the rim with steak mallet. Nothing happened. One thing I forgot to include in my original post is that yesterday it also cracked on the inside of the cup underneath the chip on the rim. 

[Babs]

Just on functionality, any sharp edge rim, foot, more prone to chipping with use. Glaze adherence can be an issue on a sharp rim also, round those edges. More mouth friendly also.

Is that crack sharp edged  or smooth?

What temp. are you cracking your kiln?

Too hot, yes your glazes wil ping on shelf.

 

[Linchimb]

31 minutes ago, Babs said:

 

Is that crack sharp edged  or smooth?

What temp. are you cracking your kiln?

Too hot, yes your glazes wil ping on shelf.

 

Hi, the rim is reasonably round on this one. I have seen glaze running downward and not covering rim well on some pieces with thin sharp rim. Saying that thin rim on porcelain sometimes works quite well with glaze, maybe because it’s more dense, smooth and can be vitrified (?)

the kiln was actually opened at room temperature. I let it cool down naturally after reaching mature temperature and didn’t slow down cooling. 

[Babs]

4 hours ago, Linchimb said:

Hi, the rim is reasonably round on this one. I have seen glaze running downward and not covering rim well on some pieces with thin sharp rim. Saying that thin rim on porcelain sometimes works quite well with glaze, maybe because it’s 

The image with the crack shows a sharp edge on inner side of lip. It may be stronger if lip compressed to a round shape, less likely to chip imo.

Is the crack got a sharp edge or a smoother feel. Would signify a crack happening on way up or down in firing process or glaze fit so bad it is pulling your cup apart....

[Linchimb]

6 hours ago, Babs said:

The image with the crack shows a sharp edge on inner side of lip. It may be stronger if lip compressed to a round shape, less likely to chip imo.

Is the crack got a sharp edge or a smoother feel. Would signify a crack happening on way up or down in firing process or glaze fit so bad it is pulling your cup apart....

I see! Thanks for the advice.

The crack doesn't feel very sharp

[Babs]

So maybe the crack was already on cup prior to glaze fire, certainly before cooling occurred, because of glaze misfit, maybe not.

What is your firing schedule?

No expert here btw