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Why did this happen?


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#1 Pugaboo

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Posted 27 April 2013 - 04:43 PM

I did up 2 8x10 1/2 thick test tiles to test Amaco underglazes. They were both made, painted, bisqued and glaze fired together. Clay is Little Loafers, underglaze is Amaco, clear glaze is Coyote. One of the tiles came through perfectly but the other one cracked, the colors burned out, some seemed to bleed a little and the clay even has a grayish green cast to it rather than the creamy white loafers usually has. The tiles were made by rolling out by hand, then cut at leather hard, the graph lines were inscribed then the underglaze painted on during greenware stage. Both were bisque fired to cone 04 and came through that step looking just fine. Coyote clear glaze was applied to the labeled columns and both tile were glaze fired together to cone 6.

I have attached several files for you to see.

What caused the one tile to fail so badly?

Terry

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#2 Biglou13

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Posted 27 April 2013 - 05:49 PM

Did you make a kiln god or,goddess?

As for crack. It looks like it followed line cut in clay for,part of way. So stress relieved along line.....

Color ?

We're the fired on same shelf next to each other? Contaminants when rolling tiles? 1/2 inch sounds thick maybe drying issue and micro cracks were present after bisque.
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#3 TJR

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Posted 27 April 2013 - 05:58 PM

Pugaboo;
I think Bijou is correct in that the fault line is along your incised line on the second tile. Were the shelves clean? Was there something that caught the second tile on the kiln shelf. That is a lot of work you have done there. There are lots of things that could go wrong. Clay fired too fast? But you said they made it through the bisque O.K.?
It's a mystery. Ain't ceramics grand?
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#4 weeble

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Posted 28 April 2013 - 03:34 AM

The burn out, shrinkage, and clay color difference really look like one was fired hotter than the other. It LOOKS like the one that cracked was fired hotter than the other, was one on a middle/top shelf and the other on the bottom?
Maryjane Carlson

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#5 Pugaboo

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Posted 28 April 2013 - 08:43 AM

Weeble - with my minimal knowledge it also looks to me like one fired hotter than the other. I am mostly going by how much the underglazes burned out since I have used them to cone 6 before and had lovely colors, there's a picture of a tray on the forum here that I used these same colors and brand with. But as I said I am very new and struggling with the final glaze process. Since I am new I don't have my own kiln yet and have joined a studio group and use their kiln there. This unfortunately means I was not present when the kiln was loaded or unloaded so don't know where or how the tiles were placed. There's not much else I can add unfortunately but will continue to test and try to get the results I want.

Not exactly sure what to try next except maybe smaller tiles with just one color applied to them in the 1,2,3 coats and glazed and unglazed areas to see if it happens again that way I might be able to get some of the shades to fire true rather than having the whole thing fail.

It's all trial and error for me at this point!
The world is but a canvas to the imagination - Henry David Thoreau

#6 weeble

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Posted 29 April 2013 - 05:08 AM

Well, its still good information to have! If you are relying on someone else to load the kiln, you run the risk of having your finished pieces fired in the same way as the problem tile, so you may very well see the same results later. As the poor SOB who ends up loading and unloading for our group, I can tell you sometimes help or company is appreciated, so you might ask if you can help load/unload, or just watch so you can learn more about the process. You might ask whoever DID load where the tiles ended up, they might remember.

Another thing that strikes me, you used blues and greens on one tile and reds and browns on another. It could be that the reds and browns are just going to be problematic in that line of underglazes. Or you could have gotten a touch heavier with the clear on one. As has been said, there are a lot of variables, all you can do is track down what variables you can, and try again where you can't pin it down.
Maryjane Carlson

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#7 Pugaboo

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Posted 29 April 2013 - 08:49 AM

Weeble -I agree it is a good idea to know where they are placed. I have volunteered to help load the kiln and have watched several times but as I am the newest member kind of taking things slowly so as not to step on anyone's toes. I was there when the bisque load was loaded and watched and asked questions but the kiln is almost always in use and being there for every loading and unloading has been impossible. I will make a concentrated effort to be there to learn more since I did not know that the position in the kiln was so important. As I said I am VERY new and want to learn as much as I can and I know now to ask questions about where and how things are placed in the kiln and why. So far I've been asking things like how the shelves are placed, what a pyro metric cone is, what a stilt is, the difference between bisque and glaze firing etc. I have watched the director have to chip glaze off shelves when things dripped or melded themselves to a shelf and felt bad for him that's a lot of work! Watching that and knowing I don't quite know what I'm doing I made up little disks of clay and stamped SHELF GUARD on them to place under some of my items just in case they drip to hopefully keep it from the shelf. I am soooo worried I am going to do something wrong and not know it and be responsible for damaging someone else's work or the kiln itself.

On a good note the studio is expanding and adding a second kiln so they will have a seperate bisque and glaze kiln as well as a larger separate glazing room. It's very exciting and everybody is looking forward to it be completed. They also asked for volunteers to make test tiles of all their glazes since they don't really have a system just shelf of little pieces that didn't work and then usually in one clay and not the other. I have volunteered to do this or to help someone that knows more than I and could use an extra pair of hands. Glazing kind of scares me and that is one of the reasons I volunteered; to face my fear head on and also learn as much as I can. I am also one of the few that use the Little Loafers so most of the current samples are not in the clay I am using but the red one instead. Whenever I ask what color is this bucket I get showed a piece but then they have to go but it won't look like that on the Little Loafers. So far I'm doing everything blind which is why I did up the 2 tiles of underglazes so I would know what the colors would look like on the actual clay I am using. It's all test and trial at this point.

I am going to make smaller tiles for each of the colors on the broken tile and run them through that way to see if the colors burn out again. I will also be a little pushier and ask to be there for each loading bisque and glaze that way I can watch where they are placed and learn from that as well. So much to learn!
The world is but a canvas to the imagination - Henry David Thoreau

#8 neilestrick

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Posted 29 April 2013 - 10:15 AM

Any time you put deep lines into a slab like that, you run the risk of it cracking along those lines. As for the discoloration, it's possible that the black underglazes fumed out and discolored the areas around them. Cobalt and copper will do that, and the black probably has one or both. I'm not convinced that's the case, but it's possible. More likely it was over fired like others suggested. I'm not surprised the reds burned out. Amaco underglazes are not necessarily all that stable at cone 6. The clear glaze can also affect color stability, so you may want to try different one. I use Speedball underglazes, which hold color perfectly at cone 6, and are cheaper than Amaco.

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