Rex Johnson Posted September 30, 2015 Report Share Posted September 30, 2015 O.K., I'm still experimenting with a few glazes ^6-^10. (below). I'm satisfied with the glazes and their outcome for the most part. Using the Olympic 2728 kiln is finicky as some of you know, fires hot on the bottom by about a full cone from the top. That being the case, using mostly ^6 and ^6-10 glazes shouldn't be an issue either. Since Laguna Clay is where I get my clay locally I'm using B-Mix ^5 -^6 WC-401 and ( WC-436 ) (w/grog) from Laguna Clay. The problem is the ^5-^6 clay evidently can't take the heat. At ^6 and above it starts to melt, warp, fracture the pots feet, and on the bottom the clay is blistering (bumps) under the glaze. Maybe I should be using ^10 B-Mix. You tell me. One fellow said if I go to cone 10 clay there may be a problem with the glaze fitting the clay body. But who makes a cone 6-10 clay body (???) The glazes I've been using are those from the CA Tried and True PDF. Also a couple Coyote ^6 glazes Temmoku Glaze- "A Cone 10 recipe that works equally well at Cone 6; yields yellow “tea dust†crystals inreduction." Malcolm Davis Shino - "cone 6 or 10 or anything in between... and quite possibly even lower. " Coyote Shino ^6 Coyote Iron Phoenix ^6 Pete's Black Gloss ^5-^10 I have my schedule down, slow fired (7+ hours) with a 1 hour soak at ^6. These are reduction at 1650 for 30 minutes and a very slow cool down once shut down. Third time hasn't been a charm but the glazes are really nice. Top shelf but fractured foot from the clay actually sticking to the shelf. (B-mix 401 w/o grog) Mid/top Temmoku w/Pete's Black this one survived without incident (B-Mix 436 w/grog) Bottom shelf, Malcolm's Shino w/Temmoku (B-Mix 436 w/grog) but warped foot, and you can see the clay body blisters in this pic... Link to comment Share on other sites More sharing options...
Mark C. Posted September 30, 2015 Report Share Posted September 30, 2015 Do not use cone 10 clay for cone 6 work as the clay will not be vitrified. Your 7 hour fire is pretty fast-depending on how fast you are going thru quartz inversion (1000-1100) this may be why you are getting cracking-also cooling to fast can cause cracking. The last point is that top bowl foot crack can happen as the heat can be to fast on top of your kiln. Usually these cracks are from the above issues.I do not think its the glaze cuasing the cracks Mark Link to comment Share on other sites More sharing options...
Rex Johnson Posted September 30, 2015 Author Report Share Posted September 30, 2015 No, I don't either Mark. But the clay is getting that taffy look, like it's over vitrifying if that's a proper description. The clay isn't actually cracking but actually wants to attach to the shelf and chips, or actually fractures the foot around the rim of the foot , again like breaking a piece of hard taffy. All I can think is the clay isn't fit for this temp. Link to comment Share on other sites More sharing options...
Mark C. Posted September 30, 2015 Report Share Posted September 30, 2015 Since I do not work in the temp range I can only add that a change of clay body seems needed-I would look for another cone 6 body.Hopefully you are not going hotter than 6 with this body? Mark Link to comment Share on other sites More sharing options...
Rex Johnson Posted September 30, 2015 Author Report Share Posted September 30, 2015 Oh it is going past cone 6 on the bottom for sure. That said, even spot on (free standing cones) on top the clay is still sticky. Here's a rough firing schedule form memory: Candle 1-2 hours to 500-600 degrees. 600-1200 degrees about 2 hours 1200-1650 about 1.5 hours hold at 1650-1700 (reduction) for 30-45 minutes 1700 > 2260 2 hours soak for 1 hour at 2260 Turn off and cover burner ports and flue at this point cone 6 is bent on top and completely down on the botttom cooldown is 24 hours Link to comment Share on other sites More sharing options...
Rex Johnson Posted September 30, 2015 Author Report Share Posted September 30, 2015 ...and thus is my dilemma, if Laguna's rated B-Mix is cone 5-6 can't handle cone 6, and they don't make a cone 6+ I'm shtuck. Link to comment Share on other sites More sharing options...
Mark C. Posted September 30, 2015 Report Share Posted September 30, 2015 You live in the land of clay choices-try an ardvark body or a clay palnaet body or switch to pocelains-or lower your firing temp? finding the right body takes work for sure. Mark Link to comment Share on other sites More sharing options...
neilestrick Posted September 30, 2015 Report Share Posted September 30, 2015 If you're holding at cone 6 for an hour, then you're going well past cone 6. Are you using cones? Is there any particular reason you're holding for that long? Link to comment Share on other sites More sharing options...
Rex Johnson Posted September 30, 2015 Author Report Share Posted September 30, 2015 True that. Definetly pushing past ^6 on the bottom. I am using free standing cones. Do you think it's simply over firing? Is soaking for an hour too long then? Link to comment Share on other sites More sharing options...
David Woodin Posted September 30, 2015 Report Share Posted September 30, 2015 If you are going to the top temperature @ 280 deg per hour and soaking for one hour than the top temperature should be about 2223 deg F not 2260 deg F. David Link to comment Share on other sites More sharing options...
Rex Johnson Posted September 30, 2015 Author Report Share Posted September 30, 2015 so I am just firing too hot then. these cone charts are confusing, maybe that's the problem first one is simple... but this one is from Orton says 2269 for large self supporting cones... Link to comment Share on other sites More sharing options...
claygram Posted September 30, 2015 Report Share Posted September 30, 2015 I tried Laguna b-mix cone a few years ago and stopped using it because it tends to stick to the kiln shelf - pulling small chunks out of foot rings during the glaze firing. I just have a regular kiln sitter and do very straight forward firings always the same--no soaking or whatever .. Read more recently that this was a problem with that clay for what reason I do not know. Many more nice clays out there to try--good luck Link to comment Share on other sites More sharing options...
neilestrick Posted September 30, 2015 Report Share Posted September 30, 2015 Ignore the charts, ignore the pyrometer. Put large cones in the kiln and watch them. That's the only way to be sure. Link to comment Share on other sites More sharing options...
Rex Johnson Posted September 30, 2015 Author Report Share Posted September 30, 2015 I tried Laguna b-mix cone a few years ago and stopped using it because it tends to stick to the kiln shelf - pulling small chunks out of foot rings during the glaze firing. I just have a regular kiln sitter and do very straight forward firings always the same--no soaking or whatever .. Read more recently that this was a problem with that clay for what reason I do not know. Many more nice clays out there to try--good luck wow, really?...and I thought it was just me. That is exactly what's happening, though firing a bit too hot won't help there. I'll have to see if anyone else has this problem... Hmmmm.... http://www.potters.org/subject103162.htm Link to comment Share on other sites More sharing options...
Mark C. Posted September 30, 2015 Report Share Posted September 30, 2015 Neil has it right-juist pay attention to large cones they tell the whole story. Try some alumina in your wax to keep feet from sticking-also have some alumina in you kiln wash then feet will not stick. I have a lifetime of sticky feet experience with porcelain at cone 11-B mix is just white stoneware-sounds like you are overfiring it.If you like your glazes at that temp switch clays. Mark Link to comment Share on other sites More sharing options...
David Woodin Posted September 30, 2015 Report Share Posted September 30, 2015 The problem is the one hour soak, you have to substract about 48 deg F from the top temperature if you are going at a 280 deg ramp. If going at 270 deg F ramp per hour than for a 60 minute soak subtract 45 deg F from the 2269 Temperature, 2269-45= 2224 deg F. IF you are afraid of pinholes etc than slow down the ramp rate. David Link to comment Share on other sites More sharing options...
Rex Johnson Posted October 1, 2015 Author Report Share Posted October 1, 2015 Thanks for the input all. I will do another test in a couple weeks. Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.