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Hairline Cracks During Bisque Firing- Why?


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Well, next dilemma in the tile endeavor. Thanks for your patience and assistance. I have had one completely successful (glazed too) tile out of many tries. I was hoping to have 2 more today. I opened the kiln of my bisque firing and found my 2 tiles with hairline cracks either from the bottom inward or the sides inward. History of the process, they are 7x7 hand pressed molded tiles. They are about 3/4" thick and were given plenty of drying time before the cone 04 bisque firing. I have a small kiln - it is about 9x9 (octagon shape) and I placed one tile on the bottom and the next on a shelf that sits about 2" above that and place the next tile on it. My friend thought maybe there was not enough air circulation. The kiln was completely closed from the very beginning of the firing. Any ideas come to mind as to why this happened and what to do to prevent it? This happened one other time before and I just ignored the crack and glazed it hoping the glaze would cover/fix it but instead the crack made its way almost all the way to the other side. Help - please?

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I agree with Chris that it may be stress. You can try repairing it with paper clay and re-bisque.

I would recommend not firing the tiles flat on the shelf. Either hand roll or extrude some coils to get the tiles off the shelf and let the heat surround it more evenly. Electric kilns don't really have much in the way of air circulation as in a draft. It is more radiating heat..but you want that to be even. That is why there are tile setters. Even heat for flat surfaces is helpful.

Marcia

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I have had one successful tile - glaze and everything!

How do I repair with paper clay? I am sorry - but I am pretty new to all this. My friend told me I choose one of the most difficult things (molded tiles) to do. Hence the reason she does not do them.

Does it have anything to do with how I put the clay into the mold? I started with slabs but got a lot of creases, so someone suggested I just pull off pieces of clay and push into mold and keep doing this until I build it all up. This seems to help a bit with eliminating most creases. After that, I then cover it with canvas and pound on it with the rubber mallet, then put a block of wood down on top of it and canvas and pound down on it more. Could it have anything to do with adding a little water to try to smooth out some of the creases when it came out of the mold. I know I did work on the edges of the tile a bit.

As far as the kiln goes, I am not sure, but I may have put the "successful" tile on my stilts (that I use when I am glazing) so that they were raised off the shelf/bottom a little bit.

How about the firing process? I read somewhere about firing too quick. I just put the cone in, turned it all on and walked away (about 2 hours later - all complete - then let cool completely). Would it help if I left the "peep" hole open and/or vented the top and/or just flipped the bottom switch and later come back and flip the upper switch?

What are tile setters? Do them have them to fit a 7x7 tile?

Thank you so much for your time and thoughts. I really, really appreciate it! :)

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2 hour firing? Sounds very fast. slow down. The coils are rounded and let the tile move as it shrinks. The stilts do not. They are used for pieces that are already bisqued and are going through a glaze firing at the same temperature or lower..so no shrinkage movement. Stilts warp pieces that are shrinking on them.<div>Removing tiles from the mold could create stress. There are some very sophisticated methods of doing this including having the mold plumbed for compressed air to blow air to release the tile. Another method is to rammed the tile using fairly dry stiff clay and 500 pounds pressure from a ram press. OR you could slip cast with a low shrinkage recipe. These are some of the ways to avoid pulling clay from a mold. <br><br>repairing with paper clay:<br>Try to make a a few tablespoons worth at a time. It molds so no need to make more until needed.  Take some of your dried clay and pulverize it with a rolling pin or spoon. Try for a 1/4 cup.  In a container, soak toilet paper...maybe a couple of feet or a yard or so. Tear it up into small pieces and soak over night. Squeeze some of the water out of it. In a blender put the toilet paper mash, add some water so the tp can be fluid, add a little vinegar (1/2-1 tsp or so) , some karo syrup (1/4-1/2 tsp or so) , a dash of sodium silicate (a drop or two) and  then clay. Blend to a paste consistency. You want the clay to be the prime ingredient. The toilet paper can be 15-25% of the volume. By squeezing it, you can visualize this estimate.<br>When filling bisque cracks you may need to widen the crack in order to fill it. You can use a sharp instrument like a dental tool. Before applying the paste, wet the bisque surface so it doesn't suck in the moisture from the paste too quickly. Fill in the cracks. You can even re-attach broken parts using this system. <br>Re-bisque fire the repaired piece.<br><br>Marcia

 

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I looked at your tile photos again. The crack could have happened in the firing but also, it could have happened when drying and didn't show until the firing. Try waxing the edges of your tiles before they dry. This will help with edges drying too fast.

Marcia

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Well, next dilemma in the tile endeavor. Thanks for your patience and assistance. I have had one completely successful (glazed too) tile out of many tries. I was hoping to have 2 more today. I opened the kiln of my bisque firing and found my 2 tiles with hairline cracks either from the bottom inward or the sides inward. History of the process, they are 7x7 hand pressed molded tiles. They are about 3/4" thick and were given plenty of drying time before the cone 04 bisque firing. I have a small kiln - it is about 9x9 (octagon shape) and I placed one tile on the bottom and the next on a shelf that sits about 2" above that and place the next tile on it. My friend thought maybe there was not enough air circulation. The kiln was completely closed from the very beginning of the firing. Any ideas come to mind as to why this happened and what to do to prevent it? This happened one other time before and I just ignored the crack and glazed it hoping the glaze would cover/fix it but instead the crack made its way almost all the way to the other side. Help - please?

 

 

Are you sure that the cracks were not caused by the tile being unable to move in the kiln while firing and cooling? You may try to fire on a shelf with some course grog underneath the pieces to allow them to move. I would also try this trick under them while they are drying, and let them dry for several days slowly.

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I have seen this happen many times on flat pieces what our students have done best thing I know of to stop this from happening is

 

A. Let the tile completely dry out I mean bone dry before you fire it and I would also do this in a slow manner say covered with a trash bag.

 

B. don't fire to fast. On most clays that I have used a good rule of thumb is if your kiln is heating up more that 125 degrees an hour that is to fast

 

C. Place the tile on a flat self with a nice layer of grog under it. The grog will act like ball bearings and allow the tile to move around during the shrinkage that occurs while you are bisque firing

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  • 7 years later...

I work in the studio at my former college and have had 4 large hand built bowls come out of the bisque with large cracks in the bottom side of all 4. Since I've not had that many problems befor in the 5 years of working in this studio I'm thinking the problem could be either this semesters clay or the student not firing the pots correctly. 

My question is why hasn't this new professor done anything about the issue and I'm very conflicted weather to ask her or maybe offer to assist in the firing.

Any advice would be appreciated.

 

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If you're compressing your bottoms well and not lettting water sit in the bottom, i would blame the clay.  I've never heard of a firing process introducing cracks, my bisque casualties have always been massive and violent.  If it were from stacking improperly I'd expect the crack to form on the rim instead of the bottom.  If your facility has been recycling clay it might be losing too many fines and needs them replaced (glazenerd's recipe is I think 60% ball clay, 20% silica, 20% feldspar @ 1 cup per gallon of reclaim)

I'd definitely ask the professor though, it's probably something they've seen before and can remedy.

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11 minutes ago, Oldmuddy said:

Actually i don't because the assigned student does unloading after class.

I think i should mention the problem I'm having with professor. I will ask about the cooling process. Thanks for your help.

 

If you're adding a foot ring to the bottoms of the bowls while the bowl is leather hard, the foot ring will shrink more than the bowl, and can put stress on it. Just one possibility. What direction do the cracks go- up and down or around?

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The book Curt mentioned, Frank Hamer's the Potters  Dictionary of  Materials and Techniques is really good and has a thorough explanation of cracks in greenware, bisque, glaze firing. You can read the entire section on cracks and their causes in a Google book preview here.  If you can't post a picture perhaps this excerpt will help.

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