Jump to content
Sirdripsalot

The Great Dunting Mystery. Please help!!!

Recommended Posts

Hello all! I am a potter of several years but know that I still have a lifetime of learning to do. I have a very particular aesthetic I try to accomplish with my mugs-- essentially they are liner glazed interior with a thick coating of multiple glazes on the exterior. Many potters do this, amiright? I have been creating these intricate glazes myself for 3 years now, without issue. It seems like all of a sudden (last 4 months) I have had customers come to me saying that their piece has been fine during multiple uses, when all of a sudden they pour their hot liquid in, get a ping, and a horizontal crack appears on the vessel moving along in a ring. To me, this sounds like thermal shock dunting.  Right now, I am feeling lost among the vast amount of variables and don't know where to begin trying to solve the problem.  I will list all of the variables/info I have collected, and hope someone out there can tell me where to focus my attention. I appreciate this community, immensely-- Thank you in advance!!

 

-I fire with Laguna Bmix 5 and Standard 225. Both mature at cone 6. I fire to cone 6. Majority of issues have been with the 225, with one or two bmix pieces. I worry it is a clay body issue.

-My firing cycle is quite standard and the same each time. I fire at a medium rate with no slow cool (I can provide the schedule if needed).

-I throw my mugs thin.

-My liner glaze is always the same. Black Licorice, Cone 6 (Mastering Cone 6 glazes). Recipe below. I do add Epsom Salt to this to prevent settling. 

-The exterior glaze always has a bit of the glossy liner glaze above and a matte as the undercoat. Then-- there are SO MANY different glazes I use from there in my laying techniques-- application and amounts differ from piece to piece. 

-Each time the issue has happened with a mug, the glaze has been thick and the issue arose when the customer added freshly boiled water. My glaze is alwayyyys thick on the exterior though...

 

What does everyone recommend? If I had my biggest wish come true, there would be an adjustment to the Liner Glaze (better COE)  that would allow a more balanced thickness of glaze overall in the piece. Can you see an adjustment I can make in the recipe below? Is there a better black recipe for this? 

Or, Should I throw thicker to ease the tension of exterior vs interior glazing? 

Orrrr... Am I missing something entirely, hah!

 

Black Licorice

Material Amount
 
26
 
26
 
22
 
17
 
5
 
4
Total base recipe 100
 
9
 
 Cobalt Oxide  2

Share this post


Link to post
Share on other sites

Thick multiple liner glazes may be stressing out your clay body.

you have two choices the way I see it -0usually dunting is a body fault and you need to change bodies

but maybe in your case your to thick and to many interior glazes may be the issue

You need to test a thinner single applacation liner abnd see if this dunts if it does you need to change clay bodies

Share this post


Link to post
Share on other sites
42 minutes ago, Mark C. said:

Thick multiple liner glazes may be stressing out your clay body.

you have two choices the way I see it -0usually dunting is a body fault and you need to change bodies

but maybe in your case your to thick and to many interior glazes may be the issue

You need to test a thinner single applacation liner abnd see if this dunts if it does you need to change clay bodies

I think you have it mixed up-- my liner glaze is just one coat of black, the exterior has multiple layers. Does this make a difference in your recommendations?

 

Share this post


Link to post
Share on other sites

Uneven stress between inner and outer glazes will cause this. It doesn't matter if the uneven stress is inside to outside, or outside to inside. Keep your single inner layer of Licorice but go easy on the multiple thick layers on the outside. If you must layer the glazes on the outside of your mugs, do it with thinner coats so that the total glaze thickness approximates the single layer inside.

Share this post


Link to post
Share on other sites

Couple thoughts, first off, I've never seen the Ron Roy Licorice glaze using 2 cobalt oxide. I believe it started out using 1 cobalt carbonate then a version of it using 1.5 cobalt carbonate but never 2 cobalt oxide. To switch from carbonate to oxide multiply by 63 and divide by 93 so if you take the higher amount of 1.5 cobalt carb and want to use cobalt oxide instead you would need 1.02 grams so basically 1/2 what you are using now. You don't want to be using heavy metals in higher amounts than necessary.

@Hulk, it wouldn't be accurate to compare the coe values for a few reasons. Outside glazes include a matte, coe figures don't work for mattes as some of the materials precipitate out of the glaze matrix to form the matte surface or they are matte because they are actually underfired gloss glazes which again means the coe figure isn't accurate as it isn't a fully melted gloss glaze.

I agree with what Mark and Dick said, decrease the total thickness of the areas using multiple glazes. To check your mugs before selling or using them take a few of them,  the ones with the thinnest walls, and freeze them overnight then put them in the sink and pour boiling water into them. 

Share this post


Link to post
Share on other sites
15 hours ago, Sirdripsalot said:

Thank you all for your insight, it really means a lot. I plan to do a series of test tiles and experiments with glaze thickness varying on both the inside and outside of thin ware. I'll write a note here with my findings when done! ☺️

I think you'll need to do tests on your mug shapes, rather than tiles, because your cracks run horizontally on those thrown shapes. Flat tiles won't behave the same way under stress.

Guessing you don't want to modify your exterior glaze technique, so I'd recommend that you throw the forms a little thicker, to stand up to the stress put on them from unequal glaze thickness between inside and out.

Share this post


Link to post
Share on other sites

It needs to be mugs not tiles for sure. Tiles will not dunt the same as your mugs. The form is the part of the issue in dunting.

you should not sell any pots until you fix this issue

selling defective pottery will kill you in the long run

Edited by Mark C.

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.


×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.