Breakage location

[ronfire]

It never fails that the custom order piece is the only one to break in the glaze fire.

Why does the bottom shelf of the kiln has more pieces fail in the glaze fire than higher in the kiln? Might be that I place the larger piece on the whole shelf at the bottom.

Could it be the bottom of the kiln is cooling to fast?  Bottom shelf is 1" off the bottom, shut off the kiln vent about 1 hour after shutdown.

 

[neilestrick]

Define 'fail'.

[ronfire]

The pieces have large cracks in them. Must be on the cool down as the cracks have sharp edges. The last piece as a sink about 12" diameter, also had 1 in the top of the kiln that was fine. Both are the same size with a hole in the center.

[neilestrick]

What size kiln? Small kilns cool fast. The bottom cools more quickly than the middle and top. If you've only got one pot in the bottom, that's going to cool much faster than the rest of the kiln where there's more mass. Try putting your bowl in the middle, where the heat from the top and bottom will help to slow the cooling rate.

Large bowls, in general, are prone to cracking if they're not very even in thickness. Too thick where the body meets the foot, thin rim and thick bottom, etc, and you'll get problems.

[ronfire]

The kiln is a Skutt 1227 with 3" bricks and a kiln sitter. There was a circle of mugs around the sink so there was a lot of thermal mass available.

I usually place a large object on the bottom because it is the only whole shelf. I have some whole shelves from a smaller kiln that I might use on top of a split shelf so the piece sits on a single shelf with no split in the center.

My wife if now under glazing 2 replacement sinks today for a replacement, and will load higher in the kiln. Now of course both sinks will fire fine.

[GEP]

Does the sink that cracked have an area of extra thickness? Either in the middle area, or around the rim? Knock it with a hammer to break it all the way in half, and look at the cross section. 

[Joseph Fireborn]

^^ what Mea said + post a picture.

[ronfire]

Yes the sink is thicker on the Bottom and  gets progressively thinner to the top.  So it make that much difference on the thickness?

 

[GEP]

3 hours ago, ronfire said:

Yes the sink is thicker on the Bottom and  gets progressively thinner to the top.  So it make that much difference on the thickness?

 

It’s possible, can we see how much the thickness varies? 

This plus the possible rapid cooling might be combining to cause the cracks.

This next one is just a hypothesis, but the hole in the bottom might be contributing to the problem too. For a large pot, the bottom holds the walls in place while the whole pot shrinks. In your case, the edges around your hole might be getting caught on the kiln shelf. Therefore warping rather than shrinking comfortably, and causing stress in the walls. You might consider firing future sinks on a waster cookie?

[Mark C.]

I would always place smaller items on bottom shelve no matter what size kiln.Put sinks and other large items in middle area. slower everything. Sinks particular are prone to issues as they can be uneven and the hole etc.

Place the small items less prone to shock in bottom. Your photo shows me enough to say this.

I have done lots of sinks mostly in the 80's and 90's-

as to the special order -yes its the one that will always do this-thats a reason to not do them-or at least for me.

[Mark C.]

The other thing is items like large sinks and uneven heavier forms need more time going up and cooling down-The firing schedule needs to be adjusted for that.

[Pres]

All of this is good advice, I have thrown some pretty large bowls, and some large jars and vases. Whatever I do large requires a lot of extra care. When almost the only pot in the kiln is a tall jar, really problematic. I have come to use two floors a lot of the time as when one wears I put another on top of it. I have also gotten a thicker than normal lid to keep cooling at a minimum. I fire manually, and do fire down when firing a large piece. As to trimming very careful about thickness variations as these can be culprits in cooling dunts. Often the crack starts when the thinner area cools first, and the thicker area that is at the shelf, and cooling much slower because of retained shelf heat does not cool evenly. Tough lessons.

 

best,

Pres

[ronfire]

I will try to change my sinks to be more even.I cant make them flat bottom on the inside so I will start with trimming a foot for the above counter ones  and place them in the center of the kiln.

Would it be good to place them on clay rollers to help with creep as well as cooling the bottom?

 

[Callie Beller Diesel]

Yes. I second the waster cookie idea, or clay rollers. I was having similar issues with some of my largest bowls, and that helped. Sometimes grog or sand isn't enough. 

[Mark C.]

2 hours ago, ronfire said:

I will try to change my sinks to be more even.I cant make them flat bottom on the inside so I will start with trimming a foot for the above counter ones  and place them in the center of the kiln.

Would it be good to place them on clay rollers to help with creep as well as cooling the bottom?

 

Most of my sinks I fired on a separate  foot ring which fits on my bowl foot and allowed the very bottom  hole to be glazed thru and undersides of hole as well.

That ring took the brunt of the shrinkage and drag on kiln shelve. Uneven sink walls are just part of a design but thats not a bad design. I think that less of an issue really. The change in firing program is way more important.

I would not use rollers but support the entire foot with a separate thrown ring of clay.My sinks were all cone 10 stoneware.

[Mark C.]

A photo of the foot on that sink would help me as well.

I still have my foot support as I used it repeatedly.

[ronfire]

Mine did not have a foot, only a thicker layer of clay to get the inside shape. Next one I will trim a foot into the bottom.

[Mark C.]

For me the foot takes the weight just like any bowl form during firing. After firing in a sink the foot is not needed.but also its not an issue below the counter.

The foot also makes for glazing the bottom everywhere (except foot) which makes a stronger form as well as a easier to clean and seal surface.All these points add to better end product.

Now if this sink is for the sit on top counter that in vogue now well thats something that will need rethinking about.