AsHeE Posted September 11, 2012 Report Share Posted September 11, 2012 I'm seeking some advice regarding the pre-heating of pieces for raku firing. Now that the temperatures are not as warm as they had been, how do folks ensure that when a load is switched, the piece added does not break? My last firing was a bust. I had a pot so large it gave us about 2" on either side of the kiln. I used Continental Clay Raku glaze and some of the Spectrum series (I've had great luck and amazing metallics with the latter). The inside bottom of the pot did not mature and the glaze bubbled. I assumed it was the size of the piece. So, from 1650F, I allowed the kiln to cool to 800F, hoping that this would ensure that the relief I had made would take the shock. Both were made from a raku clay body. As soon as I placed it in the kiln, it partially broke. I removed it and quickly attempted to mend it as a last effort with no success. Then, as the temp increased, it broke in half. I've not revisited my raku kiln since, because of the risk. I have an art show in under two weeks. How can I ensure the ware is warm enough to place inside the kiln? Simply by setting it on top? I think that 800F was far lower than I should have had to reduce the temp to. Can someone help? I cannot attach photos at this time. :-/ Thank you!! Link to comment Share on other sites More sharing options...
minspargal Posted September 11, 2012 Report Share Posted September 11, 2012 I have never preheated ware for the Raku kiln, but then i live in Texas. I've never (up to this point in time) had any thing break in the raku kiln either. Link to comment Share on other sites More sharing options...
Chris Campbell Posted September 11, 2012 Report Share Posted September 11, 2012 I have raku fired in cold weather so I dont think the cold is the issue. I just ran the first firing very slowly and put the pots for the next load on top to pre heat. What was your clay body? What is its cone range and what temp was it bisque fired to? How long before firing did you apply the glaze? With that large pot, I would have applied it the night before to make sure it and the pot were dry. Link to comment Share on other sites More sharing options...
macdoodle Posted September 11, 2012 Report Share Posted September 11, 2012 glaze -at least- the day before and preheat in the kiln for 15-20 minutes before raising the temp is what I was taught . For new pieces in a recent fired kiln we changed the brick/tile/ etc the pieces sat directly on so they were cool as the piece and would heat gradually with the next preheat/fire. Link to comment Share on other sites More sharing options...
Marcia Selsor Posted September 11, 2012 Report Share Posted September 11, 2012 I always glaze a day before as well. I also have fired in the cold...as cold as -20 degrees F in Montana. That is minus 20! I found I got great color in the environment. For your relief, I am not sure how thick and thin the various thicknesses may be. That could be your cracking problem. For something like that, put it in the first batch and fired a little slower. I don't think leaving it in the kiln to 800is a good idea because you miss the good reduction temperature. Are you talking 800 F.? You have also passed through Quartz inversion. MacDoodle mentions a good point. placing pieces on bricks. I use bits of fiber too sometimes or coils just to get the piece up off the shelf and to have the heat circulate. I think you could have used that for the large piece that had a bubbled glaze on the inside. If the bottom was flush with the shelf, it may not have gotten hot enough on the bottom. Marcia From AIR Vallauris, France Link to comment Share on other sites More sharing options...
dave the potter Posted September 12, 2012 Report Share Posted September 12, 2012 The clay that you use is really important. After years of struggling w/cracks both heating and cooling, I started using the Raku clay from Highwater Clay. Problem solved. The biggest problem cured by preheating for me is glaze crawl. Don't remember ever blowing anything up from being too wet. Thinner, consistant thickness also helps w/cracking. Link to comment Share on other sites More sharing options...
AsHeE Posted September 12, 2012 Author Report Share Posted September 12, 2012 Thanks, everyone. This particular piece was a relief made with Raku clay and I did glaze it a day earlier. My first two Raku approaches went seamlessly, so I did not factor in the temperature change for the third. I fired the first piece to 1650F and then let the kiln cool to 800F before putting in the next piece in an attempt to avoid cracking (I think there was some confusion with that). I do pull pieces as soon as they are glowing and glossy, to reduce. I am still working on my experimentation with different reduction materials, so I'm very new into the Raku world, but it's tough to experiment when the pieces break! :-/ I still reduced it...even though it was in 3 pieces, and it's beautiful...I wish my iPad would allow me to attach a picture. The studio I was working out of recently closed, so I have been recently forced to pay-per-pound at a completely different studio until I can close on my house and my kiln arrives. I was a bit rushed to continually produce, as I have a 6-day show in less than two weeks. I don't NEED Raku pieces for it, but I do have some very beautiful metallic pieces and wanted to expand my collection. Since that incident, however, I've not returned for fear of breakage. I am a hand-builder (slab pots and sculptural pieces), so it can take me a while for one piece. Anyway, to answer the question, she bisque-fires at 06. I have had problems, however, with glaze coloring, warping of my clay, and pooling of glaze (none of which I have had before), so it has been a bit frustrating. Can't wait until my kiln arrives!!! I really appreciate all of the help!!! Now that it is cooling even further in Wisconsin, I will certainly be better prepared. Link to comment Share on other sites More sharing options...
Cass Posted September 12, 2012 Report Share Posted September 12, 2012 i always put biggest pieces into a cold kiln, then small work in subsequent loads in a cooler kiln glazing big stuff the day before is also a good call laguna WSO stoneware is a superb, reliable clay for all sizes of raku...dry big stuff slowly, like, weeks...bisque slowly, cool bisque slowly...patience is a big factor in raku even though it happens fast Link to comment Share on other sites More sharing options...
neilestrick Posted September 12, 2012 Report Share Posted September 12, 2012 I would let the kiln cool down a bit lower than 800 degrees. Then when you put pieces in the kiln, make sure you are not setting them on a hot shelf. I put a piece of kiln brick in the kiln for each pot to sit on. When the pots come out, so do the bricks. When new pots go in, so go in cool bricks for them to sit on. Then let the pot sit in the warm kiln for a few minutes before closing the door. Then give it a few more minutes before starting up the burners. The cracking is not at all related to the pot being wet from the glazing. It would blow up if that were the problem, not just cracking in half. Link to comment Share on other sites More sharing options...
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