irregular cracks on fired cone 10 pots

[nellhazinski]

I have a problem with irregular cracks forming on some of my fired porcelain bowls and plates.  The bowls are fired to cone 10 and the cracks usually appear months after firing.  The bowls, glazed in celadon have a footed flat bottom and are also glazed  on the under surface inside the foot. Does anyone have ideas about the cause of the cracks?  the first image is a drawn example of the crack;  the 2nd is an actual crack on a bowl.

[Kelly in AK]

In Hamer and Hamer’s “Potters Dictionary of Materials and Techniques” there is a diagram of a crack, labeled “crack f” that looks very similar to your picture. 

The causes the list for this crack in glazed ware mostly have to do with a mismatched COE.

One particular note mentions glaze that is thick in relation to the bottom of a piece, the action of the glaze fluxes the clay of the bottom more than elsewhere. I read that as having to do with the thinness of the bottom as much as thickness of the glaze.

 

 

Edited September 1, 2023 by Kelly in AK

[Babs]

Hamer suggests the gaze layer inside isthick, even thicker than base of pot.

Smash the pot to see if the turned base is thinner tgan rest of pot.

Body contracts more than glaze and so cracks.

Maybe wipe pots with indian ink when come out of kiln,may show the crack actually exists then,or it can happen later. 

[Piedmont Pottery]

Delayed cracks can be indicative of a glaze with a thermal COE lower than the clay body.  You might try to increase your glaze COE a bit to see if the cracks still happen.

[nellhazinski]

interested in this, but what is a thermal COE?  I've thought that the glaze thickness might be the issue, but in some cases the glaze does not appear to be too thick.

[Hulk]

Where there is a very slight difference in COE, a thicker glaze might show a crack where a thinner layer wouldn't (depending on some associated factors...).

COE is fundamental to combating crazing!

Materials expand and contract as they heat up and cool down, respectively.
Where clay and glaze expand and contract at rates that are different (enough), problems happen!
At one extreme, bits of glaze pop off the work, shivering, and at the other extreme, the glaze cracks, crazing.

Crazing can be ameliorated by changing the glaze formulation/composition, also by using a different clay.
You'll hear and read the term "fit" and "glaze fit" - where the glaze "fits" the clay, there aren't shivering or crazing problems.

Here's links to some articles:
Co-efficient of Thermal Expansion (digitalfire.com)
Glaze Crazing (digitalfire.com)
...and a few threads from this Forum:
Crazing Clear Glaze - Clay and Glaze Chemistry - Ceramic Arts Daily Community
Crazing question - Clay and Glaze Chemistry - Ceramic Arts Daily Community

The cracking depicted in the OPs image (first post, above) seems a special case to me - typical crazing forms a pattern, where a large pattern indicates a closer match than a fine pattern. I'm curious to know if the clay is cracking, or is it the glaze?

[nellhazinski]

Thanks for the info.    Looks like I have some work to do.

The glaze is cracking, not the clay.  

[Kelly in AK]

If the bottom is particularly thin that may also have some bearing on the problem. Again to reference Hamer and Hamer’s book, the section on crack diagnosis is outstanding. 

[Min]

40 minutes ago, Kelly in AK said:

If the bottom is particularly thin that may also have some bearing on the problem. Again to reference Hamer and Hamer’s book, the section on crack diagnosis is outstanding. 

What Kelly is referencing is available to see in a Google book preview here. Have a look at crack "F". Highly recommend this book!

https://books.google.ca/books?id=TApnGTVLwxAC&pg=PA86&lpg=PA86&dq=hamer+and+hamer+cracks&source=bl&ots=inS6KQPb4N&sig=ACfU3U3-WW9HznLwIEytHMyGanL2bpymZA&hl=en&sa=X&ved=2ahUKEwiG1JCB0KPgAhWyPH0KHX7EBo4Q6AEwB3oECAcQAQ#v=onepage&q=hamer and hamer cracks&f=false

[oldlady]

dumb question.   now that someone is marketing what they call "celadon" glazes and so many people are brushing bottled glazes, is the glaze you use one you make or something supplied as celadon?   since it appears to be clear and without a hint of blue, i do not know where it gets its name.  

[Bill Kielb]

Just asking, are these finished with a swirl in the bottom as appears in the second picture. It’s really hard to see these cracks, maybe you could post some additional pics as examples here.

[PeterH]

Two ideas you may find useful.

1) Stain the cracks to make them more visible (you may have more than you think).

2) Use a thermal stress-test to make "latent" cracks appear sooner.
... one from Digitalfire https://digitalfire.com/test/iwct

 

 

[nellhazinski]

I want to thank everyone for their responses.   Very helpful.   I actually have the Hamer book and the section on cracks is great.  Years ago,I used the formula "Rhodes" celadon for my blue celadon glaze. This glaze has a lot of Cornwall stone, which I thought may be the problem.  It started to craze in the manner I described, so I increased the silica, but the problem persisted.  I was frustrated and chose to  test other celadons, and found another beautiful one, which is the one I am now struggling with.  This may be a silly question, but does the addition of bentonite lower the COE of the glaze?  This glaze sinks to the bottom almost immediately when mixed, so I've been adding perhaps too much bentonite.

I'll be researching on Digitalfire per Hulk's links.   Thanks so much for these.  

[Min]

7 hours ago, nellhazinski said:

does the addition of bentonite lower the COE of the glaze?

Not to any appreciable amount in the typical amounts used. Post your celadon recipe, someone might have thoughts on how to fix the crazing. (also which claybody you are using it on)