Problems With Majolica Glaze

[lazy hen]

I used Linda Arbuckle's majolica glaze recipe on Red Art terra cotta low fire clay.  I fired to cone 04 and the bottoms broke out of all the mugs, with cracks around the cups.  This happened before when I tested the glaze on a high fire porcelain cup, and I thought it was just unsuited to high fire clay, but this clay should match....  What is wrong?

[Babs]

Other glazez working ok on this clay and firing schedule?

[Min]

Could you post a picture of the cracks? Also, any more info on the clay, like which supplier it's from?

[alabama]

Amaco clay?

[Marcia Selsor]

Or the cup forms?

[Babs]

Or a v thin ring just above the foot...trimming..

[lazy hen]

Clay is Red Art from Seattle Pottery Supply cone 06 - 1.  Other glazes in this firing came out fine, fired for about 8 hours, cooled for 20.  The mugs were glazed inside, over the lip and about a half inch down the outside. 

The pictures show the problem very well, but I don't know what is meant by URL and it won't add them without it.    Help doesn't tell me anything.  Anyone know?

thanks

[Sputty]

.

[High Bridge Pottery]

To upload pictures without an URL you need to click the box "More Reply Options" at the bottom of where you write will be another box "Choose Files" where you can navigate to the picture on your computer. Pictures need to be under 1 megabyte in size.

[Marcia Selsor]

Or a v thin ring just above the foot...trimming..

. That is what I meant

[lazy hen]

I will try to add some pictures of the spectacular majolica failure.........  No luck.   Thank you for the directions, but we apparently don't have the program necessary to add pictures here.

 

Also thank you for the link to the article on Shivering; it sounds like it might be the problem, although it hasn't happened with the majolica glaze on flat bowls, glazed on the inside only.

The mugs were not smooth enough on the outside to use anyway.  The effect I'm trying to get is of bare terracotta outside and a smooth, opaque white or pale color inside cups and bowls.   Terra sigillata is probably the answer outside; any ideas for a glaze similar to majolica but not so hard to use?

[Sputty]

.

[High Bridge Pottery]

I will try to add some pictures of the spectacular majolica failure.........  No luck.   Thank you for the directions, but we apparently don't have the program necessary to add pictures here.

 

Also thank you for the link to the article on Shivering; it sounds like it might be the problem, although it hasn't happened with the majolica glaze on flat bowls, glazed on the inside only.

The mugs were not smooth enough on the outside to use anyway.  The effect I'm trying to get is of bare terracotta outside and a smooth, opaque white or pale color inside cups and bowls.   Terra sigillata is probably the answer outside; any ideas for a glaze similar to majolica but not so hard to use?

 

Under the choose file button should be something saying "use basic uploader" or similar. Try clicking that and clicking the choose file button again, browse to the picture and then click the Attach This File button. If that doesn't work I could try and explain uploading pictures elsewhere and getting the URL (web address) but that is slightly more complicated and 100 different ways to upload pictures.

[neilestrick]

90% Ferro Frit 3124 + 10% EPK is a basic low fire clear glaze. Add 12% Zircopax/Superpax to make it white. 

[lazy hen]

Another try at adding pictures:  Maybe this will work.

 

Looking over stuff for a craft fair I came upon some cups glazed last year - same body, same glaze, applied inside and over the lip only, same firing sched and temperature, and no sign of cracks.  I'm still puzzled.

[Sputty]

.

[alabama]

Change clays....make identical cups, same glaze technique.

Make sure clay isn't too stiff/dry and wedge it.

 

Alabama

[Babs]

So no cracking evident after bisque?

[neilestrick]

Looks like glaze stress cracks to me.

[Min]

I agree with the others about the glaze not fitting the clay. I would take one of your mugs from last year that didn’t break and put it in the coldest part of your freezer overnight. Then take it out of the freezer, put it in the sink and immediately pour rapidly boiling water into it. Hopefully it doesn't crack but I think there is a strong possibility it will, unless the clay formula has changed with the new batch. Also, is the glaze thickness about the same on the current cracked mugs as the ones that didn’t? Might be a combo of a couple things going on.

 

Also, does the unglazed terra cotta stain on the outside of the mugs? (Tacoma Clay Art makes some nice terra cotta bodies if you are looking for a new one)

[Marcia Selsor]

Looks like glaze stressed to me too. I agree that terra sig would help keeping the terra cotta from staining. Also, maybe your sharp angle at the bottom may be another problem. We bisqued to ^02-^1 and then glazed around ^04. Do some trial tests and see what happens.

Linda's glaze recipe is a good one.

 

Marcia