Why do my pieces break at decal firing?

[Gazal]

Hi! I desperately need some help please.

I have been making some platters (mainly oval shapes) and plates with mid-fire clay. they all looked good after glaze firing. however after I applied decals and fired them (to 735 Celsius) all of the flat pieces ie platters and plates broke in the kiln!! all the bowls and small pieces survived though.

the pattern for the breakages is kind of similar-I'm attaching some photos that's why I am thinking it would be fixable hopefully in the future?

Does anybody have any ideas why it's happening? it's very annoying that they survive glaze firing (1220 Celsius) and not decal firing after that.

 

Your help would be much appreciated.

 

Thank you 

Ghazal

 

[Rae Reich]

I think you would do better to decal fire the large flat pieces on edge, propped on bricks or other kiln furniture. Try one that way and see. I think that might relieve the cooling stresses that that shape experiences when horizontal. Cool as slowly as possible, don’t let in any drafts. 

[neilestrick]

What firing schedule are you using? Flat pieces do not heat or cool evenly- the top heats up and cools down faster than the bottom- so you have to go slowly. Propping them up on stilts or clay wads or broken pieces of kiln shelves will definitely help, but also don't go too fast for the first couple hundred degrees, and go slowly through quartz inversion (the 525-575C range) during both heating and cooling.

[Min]

Just to add another thing, I wouldn't put the platters/plates on the very top or bottom shelves, temps are more even in the middle section of kilns.

[Pres]

When firing my thrown plates for Communion sets, I have the best luck if the pieces are packed tight in the middle, next shelf just above the top of the plates below. I also have a tendency to leave them until completely cool. Not good to get in a rush with plates or platters.

 

best,

Pres

[oldlady]

it appears that the plates are completely flat, no foot ring.   i fire that kind of work on sand or small grog.  i make a small hill across the shelf and wiggle the plate down on it to level it off.

[Callie Beller Diesel]

Some clay bodies tend to not like being refined multiple times like that, especially if they’ve got a bunch of free silica that hasn’t been taken up by other oxides in the melt. You have to baby these clay bodies through quartz inversion accordingly. So +1 for everything already said about foot rings, kiln packs and firing speeds to get pieces through that zone more evenly. 

[Kelly in AK]

I’m sorry to see what you’re going through. It’s one of the worst things, after getting everything right every step of the way, the last move blows the lot.

Other than seconding advice already given and offering encouragement, I’ll say what I would be looking at if I were there (note: been at it a while, still learning)  

 The decal firing is just above quartz inversion, a nice clear clue. The clay is not liking it. Perhaps it’s a rushed firing cycle, but there’s clearly some tension in that clay/glaze going on. I wouldn’t hesitate to do a test on a piece that survived: put in ice water, take it out and pour boiling water in. Half full, you want to stress it. It just takes a few minutes and one way or another, this test brings great peace. 

In addition to the suggestions already mentioned, consider using a different clay for the large platters or maybe glaze firing them slightly lower.

I’ve had a few bad dunting days myself. Note in the attached photo most shattered pieces are one glaze (the red one) and also flat, plates and trays. 

Edited September 1, 2023 by Kelly in AK