Mugs cracked

[Gonepotty]

Hello, wondering if anyone can help please. Mug completely cracked down base and up along sides when I added hot water. Clay body is vitrified, clear gloss liner inside, different glaze outside of mug...I'm suspecting this is the cause...If so, is there any way around this (other than don't have 2 different glazes)...

FYI- i have tried the same glaze combination on a different clay body and had no issues...

Thanks so much 

[PeterH]

49 minutes ago, Gonepotty said:

FYI- i have tried the same glaze combination on a different clay body and had no issues...

Might be worth trying a thermal shock test on them, in case you just haven't had any issues ... yet.

thermal shock test on mug... results
http://www.potters.org/subject09036.htm
... especially Ron Roy's posts

[Gonepotty]

2 hours ago, PeterH said:

Might be worth trying a thermal shock test on them, in case you just haven't had any issues ... yet.

thermal shock test on mug... results
http://www.potters.org/subject09036.htm
... especially Ron Roy's posts

Thanks Peter. I have been the boiling water then cold water test but think I'll try the freezer test now. 

Wondering if the cause is the 2 different glazes?

[Min]

If these are glazes you mixed post the recipes. It could be one glaze has a much lower COE than the other.

I would suggest making about a dozen test cylinders, glaze a few of them inside only with your liner glaze plus a few more with your exterior glaze and a few with the liner plus exterior glaze.  Do the freezer test and see which ones crack. Glaze them heavily on the inside.

It's hard to tell from the photos but is the liner a semi matte glaze or underfired?

[Gonepotty]

1 hour ago, Min said:

If these are glazes you mixed post the recipes. It could be one glaze has a much lower COE than the other.

I would suggest making about a dozen test cylinders, glaze a few of them inside only with your liner glaze plus a few more with your exterior glaze and a few with the liner plus exterior glaze.  Do the freezer test and see which ones crack. Glaze them heavily on the inside.

It's hard to tell from the photos but is the liner a semi matte glaze or underfired?

Thanks Min. Unfortunately I don't have recipes - shop bought. 

Liner is a clear gloss. 

I have used each glaze separately on a mug entirely with no issues but just seems the combinations isn't great. Really excellent tip re the cylinders and trialing out to find the "culprit" . 

Would you repeat the freezer test multiple times or should any issues show up within the first round of testing? 

Appreciate your help as always - thank you 😊 

[Min]

2 minutes ago, Gonepotty said:

Would you repeat the freezer test multiple times or should any issues show up within the first round of testing? 

I used to do it a few times but then read an article that made a good point in that the clay has already undergone stress with the first round of testing so further testing on the same pieces isn't valid. I just test once now.

Another thing I would suggest doing is try tapping the edges of pots / samples with these glazes, like rims or mug handle edges, see if any glaze chips off. Use something dense like the handle of a screwdriver, tap fairly firmly.

 

[Gonepotty]

48 minutes ago, Min said:

I used to do it a few times but then read an article that made a good point in that the clay has already undergone stress with the first round of testing so further testing on the same pieces isn't valid. I just test once now.

Another thing I would suggest doing is try tapping the edges of pots / samples with these glazes, like rims or mug handle edges, see if any glaze chips off. Use something dense like the handle of a screwdriver, tap fairly firmly.

 

What would be an appropriate amount of time in the freezer? 

[Min]

1 hour ago, Gonepotty said:

What would be an appropriate amount of time in the freezer? 

I2 - 24 hours should do it for typical mug wall thickness. (put test pieces in the sink when you pour the boiling water it)

[Gonepotty]

1 hour ago, Min said:

I2 - 24 hours should do it for typical mug wall thickness. (put test pieces in the sink when you pour the boiling water it)

Ok great - Thank you

[Kelly in AK]

I would love to know more about the clay. 

[PeterH]

21 hours ago, Gonepotty said:

Thanks Min. Unfortunately I don't have recipes - shop bought. 

I think the experts would be interested in:
- The manufacturer and names of your clay(s)
- The manufacturer and names of your glazes
- The cone/temperature of your bisque firing
- The cone/temperature of your glaze firings

 

[Callie Beller Diesel]

If the interior glaze is one that fits nicely but the exterior glaze is one that crazes on that particular clay, you can definitely get cracks like that. Check the exterior glaze for hidden crazing: rub some ink or use a permanent marker on it, and wipe away. Do this on one of your stress tested items to find any stuff that shows up belatedly. 
 

Another solution to a slight COE difference is to leave your pot walls a little thicker. It’s not a good idea if there’s too much disparity between the glazes, but if the crack pattern is a wide one and the form permits, you could try it. 

[Gonepotty]

On 11/29/2024 at 2:20 PM, PeterH said:

I think the experts would be interested in:
- The manufacturer and names of your clay(s)
- The manufacturer and names of your glazes
- The cone/temperature of your bisque firing
- The cone/temperature of your glaze firings

 

Clay- scarva ES5.

Clear glaze - Scarva GZ1250

Satin glaze is from CTM- staffordshire satin. 

Bisque- 950c

Firing- cone 8

[Gonepotty]

19 hours ago, Callie Beller Diesel said:

If the interior glaze is one that fits nicely but the exterior glaze is one that crazes on that particular clay, you can definitely get cracks like that. Check the exterior glaze for hidden crazing: rub some ink or use a permanent marker on it, and wipe away. Do this on one of your stress tested items to find any stuff that shows up belatedly. 
 

Another solution to a slight COE difference is to leave your pot walls a little thicker. It’s not a good idea if there’s too much disparity between the glazes, but if the crack pattern is a wide one and the form permits, you could try it. 

Thanks Callie:) 

there is no issues with crazing on either glaze but great tip for any future issues. 

[Gonepotty]

On 11/28/2024 at 7:20 PM, Min said:

I2 - 24 hours should do it for typical mug wall thickness. (put test pieces in the sink when you pour the boiling water it)

I have repeated this test on a different clay body using the same glaze combinations- clear liner and satin exterior. So far (I have tested twice using freezer method), no issues. Could it be that the clay body which cracked doesn't like the glaze combinations?

[Min]

4 hours ago, Gonepotty said:

I have repeated this test on a different clay body using the same glaze combinations- clear liner and satin exterior. So far (I have tested twice using freezer method), no issues. Could it be that the clay body which cracked doesn't like the glaze combinations?

When a claybody can't absorb the stress put upon it then it will crack. IMHO your mug cracked because when the boiling water was poured into it, and the interior glaze expanded then a fraction of a moment later the clay expanded followed by the exterior glaze, there was too great a mismatch of expansion (and contraction upon cooling) between the glaze and the claybody. This was great enough to fracture the glaze and claybody. Like Callie said a thicker clay wall might be able to absorb this stress or a different claybody might be able to. (as you have shown)

Wall thickness, type of clay used, pooling glaze, form of the pot, glaze thickness all play a part. Another claybody might be okay with this combination of glazes ( as with the two tests you have run) but we don't know the other variables, wall thickness etc. If it was me and I got cracking using the same claybody for testing I would remove as many variables from the testing as possible but would test. (use a thin walled cylinder with a heavy interior glaze application) I would be leery to use this combination of glazes on any claybody. Verify firing with cones.

If you can get your hands on a copy of Hamer's "The Potter's Dictionary of Materials and Techniques" there are good sections on dunting and thermal shock.

[Gonepotty]

33 minutes ago, Min said:

When a claybody can't absorb the stress put upon it then it will crack. IMHO your mug cracked because when the boiling water was poured into it, and the interior glaze expanded then a fraction of a moment later the clay expanded followed by the exterior glaze, there was too great a mismatch of expansion (and contraction upon cooling) between the glaze and the claybody. This was great enough to fracture the glaze and claybody. Like Callie said a thicker clay wall might be able to absorb this stress or a different claybody might be able to. (as you have shown)

Wall thickness, type of clay used, pooling glaze, form of the pot, glaze thickness all play a part. Another claybody might be okay with this combination of glazes ( as with the two tests you have run) but we don't know the other variables, wall thickness etc. If it was me and I got cracking using the same claybody for testing I would remove as many variables from the testing as possible but would test. (use a thin walled cylinder with a heavy interior glaze application) I would be leery to use this combination of glazes on any claybody. Verify firing with cones.

If you can get your hands on a copy of Hamer's "The Potter's Dictionary of Materials and Techniques" there are good sections on dunting and thermal shock.

Thanks so much Min. Lots of testing to be done.

[PeterH]

1 hour ago, Min said:

If you can get your hands on a copy of Hamer's "The Potter's Dictionary of Materials and Techniques" there are good sections on dunting and thermal shock.

+1 ... but sadly not cheap https://tinyurl.com/5kwaveut

PS An example entry
From Hamer & Hamer, The Potter's Dictionary of Materials and Techniques
http://ceramicsfieldguide.org/pdf/materials-handouts/ClayCracks.pdf

Edited December 1 by PeterH