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Seedy Potter

Large Flat Pieces Cracking During Glaze Firing

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I would likely go with adhering post firing; that way, if one layer does crack, you can choose to either use it or discard it.  And, until you get the cracking figured out, I'd play it safe . . . especially since the layers are all flat.  Stacking in the glaze firing will add  weight and bulk to the items and that can affect cooling. 

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A word about putting grog (or sand) on your kiln shelves. I found that it doesn't need to be more than a thin coating. 

The easy way to get an even thin coating is to hold your hand about a foot or more over the shelf and sprinkle as you move

over the entire surface. Of course you are far away from anything that doesn't need grog on it!

Since putting grog on my kiln shelves, I have had no cracking. 

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Here are the two I made with severe cracking on bottom layer. They are about 8" across.

 

Also, getting this figured out is crucial because I had planned to do the same project with the public at the library where I work. Thanks!

 

 

 

 

 

 

 

Beautiful project!

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I have just unloaded a bisque firing with only 2 large hearts and 1 flat tile.  One of the hearts has a fine crack in the same place as my other heart cracks, but with these two i was super careful of handling them, used a round cutter to make the V of the heart and compressed very gently  They had 3 weeks drying as i went away so were untouched while drying.  Why would one crack and not the other???  I had them flat on the kiln shelf with nothing under them.  Should i use sand in a bisque firing as well.  I am not going to glaze and fire this one again now as i have learnt that the crack only gets larger.   feeling frustrated !!!

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Hi Again

I have unloaded the glaze firing and i put sand on my shelf... thankfully no cracks this time, but the sand has stuck to the back of the 3 pieces  and also to some of the shelf,  is this normal & should I try to clean it off the shelf or just put more sand on it for the next firing???  How thick should the layer of sand be? Should you be able to see some the shelf through the sand or should it be totally covered.  What kind of sand should i have used?

I have made some small coil rods to try next time, so will bisque these next time.  Once these are bisqued, Should these be placed under the hearts for the bisque firing as well as the glaze firing?

also you talk about a cookies... are these the same size as the piece to be fired or do you place a few smaller cookies under each item.  Sorry for all the questions but am pretty new and flying solo here with my new kiln.  I have lost many pieces to this cracking business and am very keen to rectify it  :rolleyes:  Can anyone post a picture of the cookies they use?

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Hi Again

I have unloaded the glaze firing and i put sand on my shelf... thankfully no cracks this time, but the sand has stuck to the back of the 3 pieces  and also to some of the shelf,  is this normal & should I try to clean it off the shelf or just put more sand on it for the next firing???  How thick should the layer of sand be? Should you be able to see some the shelf through the sand or should it be totally covered.  What kind of sand should i have used?

I have made some small coil rods to try next time, so will bisque these next time.  Once these are bisqued, Should these be placed under the hearts for the bisque firing as well as the glaze firing?

also you talk about a cookies... are these the same size as the piece to be fired or do you place a few smaller cookies under each item.  Sorry for all the questions but am pretty new and flying solo here with my new kiln.  I have lost many pieces to this cracking business and am very keen to rectify it  :rolleyes:  Can anyone post a picture of the cookies they use?

 

 

Hi Jojess,

I am going to say grog, (because that is what I use), in place of sand, (what you use). When I have grog stuck to the back of my plates, I rub the backs together and it comes off. Or I can use any fired flat bottom to rub off anything that sticks, like grog or kiln wash. The grog should be thin to avoid unevenness. I do leave the grog on my kiln shelves, however, I do rub the sides and bottom with a clean dry green scrubby before placing in the kiln to avoid any stray grains. 

I don't know if you saw it, but I described how to apply the grog to the shelf above. 

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Update on my problems: I tried a small load (two flat items) using various items to lift the pieces off the shelves. One cracked badly and the other came out beautifully. I have noticed that the top shelf pieces have fared the worst so will put a shelf over the top next time. 

I made a bunch of triangular shaped pieces to use for lifting the work off the shelf next time. Fired them as greenware to cone 6 and it worked fine. We'll see what happens when I use those.

 

Also, I called the store where I get all my supplies (Seattle Pottery Supply). The person I talked to did not believe the problem was the contact with the shelf but that they had dried too fast in this dry hot weather we have been having. She said they probably had small cracks after the bisque firing. I do think there may be some truth to this idea. In the past I have had more problems with cracking in the summer months, now that I think about it. Could this be a possibility with the hearts, Jojess?

 

By the way, I used Oatey brand "epoxy putty" to fill the cracks on one of the most rustic textured pieces and it came out great! Not that that's a real solution, but since I am keeping it it works in this case.

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I think the problem may be in too fast or uneven cooling, judging by the top shelf having the most damage. When examining results while still in the kiln, observe where cracks are in relation to open peep holes, too.

 

People who fire regularly with electric can tell you more about when the most crucial part of the cooling cycle is and how to get past it safely.

 

The idea of firing the plates separately, rather than stacked, is something of a work-around, but if firing them together is important to you, I would devise a way to cut down on the combined thicknesses of the center, where cooling is going to be most uneven and stressful.

 

Also, how are you making your slabs? Straight from the bag to the slab roller? Thrown down and stretched? Rolled with a rolling pin? Leaving them a little thicker and with extra compression of the clay when forming might give you some advantage.

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How do you dry your ware? It may also need to be  covered,  lifted of the drying boards art a certain stage of drying, flipped from time to time......  When drying flat or tiles, or in your case, ware that has different thicknesses of clay, needs to be treated carefully.

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Hi Again

I have unloaded the glaze firing and i put sand on my shelf... thankfully no cracks this time, but the sand has stuck to the back of the 3 pieces  and also to some of the shelf,  is this normal & should I try to clean it off the shelf or just put more sand on it for the next firing???  How thick should the layer of sand be? Should you be able to see some the shelf through the sand or should it be totally covered.  What kind of sand should i have used?

I have made some small coil rods to try next time, so will bisque these next time.  Once these are bisqued, Should these be placed under the hearts for the bisque firing as well as the glaze firing?

also you talk about a cookies... are these the same size as the piece to be fired or do you place a few smaller cookies under each item.  Sorry for all the questions but am pretty new and flying solo here with my new kiln.  I have lost many pieces to this cracking business and am very keen to rectify it  :rolleyes:  Can anyone post a picture of the cookies they use?

 

 

Hi Jojess,

I am going to say grog, (because that is what I use), in place of sand, (what you use). When I have grog stuck to the back of my plates, I rub the backs together and it comes off. Or I can use any fired flat bottom to rub off anything that sticks, like grog or kiln wash. The grog should be thin to avoid unevenness. I do leave the grog on my kiln shelves, however, I do rub the sides and bottom with a clean dry green scrubby before placing in the kiln to avoid any stray grains. 

I don't know if you saw it, but I described how to apply the grog to the shelf above. 

 

Hi Karen,  thank you for your hints and tips..i used grog on my shelves for the first time and no cracks in my hearts  yay!!  I am now trying paperclay, rather than the porcelain that i have been using as someone suggested that it maybe more suited to my flat pieces.  I have some more hearts drying so yet to see what they will do in the first firing.

Rae Reich likes this

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we learn with every step. Clay responds to all these little changes. The grog and the paperclay are good improvements. I still like coils because they let the heat circulate and the clay shrink without sticking. I don't like grog because it can get inside the ware below.

Marcia

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Hi Again

I have unloaded the glaze firing and i put sand on my shelf... thankfully no cracks this time, but the sand has stuck to the back of the 3 pieces  and also to some of the shelf,  is this normal & should I try to clean it off the shelf or just put more sand on it for the next firing???  How thick should the layer of sand be? Should you be able to see some the shelf through the sand or should it be totally covered.  What kind of sand should i have used?

I have made some small coil rods to try next time, so will bisque these next time.  Once these are bisqued, Should these be placed under the hearts for the bisque firing as well as the glaze firing?

also you talk about a cookies... are these the same size as the piece to be fired or do you place a few smaller cookies under each item.  Sorry for all the questions but am pretty new and flying solo here with my new kiln.  I have lost many pieces to this cracking business and am very keen to rectify it  :rolleyes:  Can anyone post a picture of the cookies they use?

 

 

Hi Jojess,

I am going to say grog, (because that is what I use), in place of sand, (what you use). When I have grog stuck to the back of my plates, I rub the backs together and it comes off. Or I can use any fired flat bottom to rub off anything that sticks, like grog or kiln wash. The grog should be thin to avoid unevenness. I do leave the grog on my kiln shelves, however, I do rub the sides and bottom with a clean dry green scrubby before placing in the kiln to avoid any stray grains. 

I don't know if you saw it, but I described how to apply the grog to the shelf above. 

 

Hi Karen,  thank you for your hints and tips..i used grog on my shelves for the first time and no cracks in my hearts  yay!!  I am now trying paperclay, rather than the porcelain that i have been using as someone suggested that it maybe more suited to my flat pieces.  I have some more hearts drying so yet to see what they will do in the first firing.

 

 That sounds like a good idea Jo. Let us know how it works. 

jojess likes this

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we learn with every step. Clay responds to all these little changes. The grog and the paperclip are good improvements. I still like coils because they let the heat circulate and the clay shrink without sticking. I don't like grog because it can get inside the ware below.

Marcia

Marcia, when you have time, could you post a picture of your coils? I would greatly appreciate it.

Karen

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I have completed the firing of all of the pieces and have had no improvement on the cracking even though none were flat on the kiln shelves. In fact, I think more pieces cracked this last time than in the two prior loads. I can only surmise that they were dried too fast, had stress from cut points, and possibly had cooling problems. I also wonder if the clay itself was not a good choice. Anyway, I don't think I will attempt this project again. Too many issues, and too sad to hand people pieces with huge cracks in them. I have been firing things for 8 years and have never had SO MANY problems!

 

Anyway - thank you for trying to help me. Your ideas were good and I will remember them in the future.

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Karen,

 

here are the coils under the tiles. They are also under the pot. I use them over repeatedly.

The larger slab is on at least 2 coils.

post-1954-0-39528100-1439343120_thumb.jpgI mentioned earlier of firing slabs on 1/4" coils. Here is a photo of a bisque in my medium size kiln. Just firing some pieces to a higher bisque than my terra sig pieces. There are 15 tiles and 2 levels in this small kiln. The tiles and larger slab are fired upright leaning against a pot and two more tiles are against a post. The photo is after the kiln cooled.

Slabs are nice and flat. no cracks.I fire my larger slabs upright but usually against a column of shelves or bricks in my larger oval kiln.

 

Marcia

Rae Reich and Min like this

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Large Flat Pieces Cracking During Glaze Firing

The title of this thread brings up an issue in another thread about "large platters breaking." In that thread the discussion was about quartz inversion during bisque firing. I bought up the issues raised in the study done by the Quartz  Group that suggested clay/silica still moved after it had gone through the initial inversion at bisque. I fire a large format tile 14 x 16 that I had problems with it splitting during the glaze firing. it is partly clay, partly glaze, partly surface drag, but also movement. It is interesting to see someone else dealing with the same issue. It confirms my long held belief that molecular movement still occurs which chemistry asserts as true, but not a commonly held belief in the clay arts world.

Nerd

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Marcia:

I would agree with that opinion, although I view it as COE #2. COE #1 is clay vs glaze, and COE #2 is air temp vs. shelf temp. The second COE would increase exponentially with the amount of surface making contact with the shelf. I have always thought of the 1400F to 1000F as the break it ramp: because if the air temp is 1200F, under a large piece could easily be 1400F towards the center. I do know at 150F, the shelf can be up to 80F hotter because that I have tested. Perhaps add a semester of "firing flat" to the agenda because the parameters verses firing ovoids is vastly different.

Nerd

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I have often wondered about the effects of shelf heat as compared to heat in the atmosphere. Firing the slabs on the coils makes a lot of sense as far as the Thermal Expansion and contraction goes. The coils and firing on edge will also go much further when considering quartz inversion. Way I look at it, any little bit will help when talking about the survival of a piece. I will have to try this technique if I decide to do any large flat tiles.

 

I may even try the coils under large sculpted pieces that have wide flat bottoms.

 

best,

Pres

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Shelf thermal mass is certainly a factor in slumping glass of a certain size. I worked on a project at my last paid job that was slumping a 24" wide sheet of glass into an 18" deep mold. The top end temperature was only 800, but to anneal evenly, the process took 12 hours.

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One thing I like about Raku, which is very evident in your video Marcia, is as the glazes cool, once the kiln is open, it's like watching a photo develop.  You start to see where different glazes were applied, as they begin to develop their color.

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