Bill Kielb Posted December 24, 2019 Report Share Posted December 24, 2019 On 12/24/2019 at 12:19 PM, neilestrick said: Depends on the size of the kiln and how tight it's packed. My baby kiln can cool from cone 6 to unload in 5-6 hours. My 4 cubic foot can do it in 8 hours if it's packed light. Both of those situations can be too fast for a wide flat piece. I have not experienced it, but maybe. My comment was more on gradient though, which whatever rate the kiln is cooling most of it is cooling at that rate. To be truthful I have really never experienced cristobalite issues as well at cone six another cause cited frequently. I find cristobalite issues to be pretty rare actually. Now as to opening the top of the kiln at some temperature is definitely a killer. Even cooling (uniform) - is definitely a plus for all wares for sure. I have watched folks unload at 250 degrees with no damage, but when they unloaded that hot and put the wares on a metal cart. Crackalama! So evenness is surely important but making sure the ware can grow and shrink freely is a huge one during firing. Interesting to graph a kiln Cooling rate. Right at the end of fire it may cool down at 800 degrees per hour or more initially, and slowly decrease until the very end to maybe 25 degrees per hour or less. What is too fast? I don’t really know actually. Cooling evenly seems important though. Link to comment Share on other sites More sharing options...
Babs Posted December 26, 2019 Report Share Posted December 26, 2019 The bottom of this dish seems much thicker than the walls. How is this form made? Does it ping after the bisque fire? Qhite a lot of manipulation after throwing. I have always found forms which rise abruptly from the flat base more troublesome. I would try to trim bottom leaving a central nub to the same thickness as wall. Link to comment Share on other sites More sharing options...
Sopita on the Rocks! Posted December 26, 2019 Report Share Posted December 26, 2019 6 hours ago, Babs said: The bottom of this dish seems much thicker than the walls. How is this form made? Does it ping after the bisque fire? Qhite a lot of manipulation after throwing. I have always found forms which rise abruptly from the flat base more troublesome. I would try to trim bottom leaving a central nub to the same thickness as wall. Thanks for the input! I think the walls and bottom were pretty consistent in thickness (it was slab built), so don't think the issue lied there, but the change in directions of the form -almost 90degree angle from base to walls makes sense to me as a possible cause of trouble. Will try "spreading" the sides next time to create a slant angle. Also put something beneath it for the firing seems to be a predominant idea. Only have to figure out the Spanish for alumina (I know grogg is "chamote", but not sure about alumina). ☺️ Also might try to throw this shape, rather than slab buid, maybe that'll help as well! Link to comment Share on other sites More sharing options...
Sopita on the Rocks! Posted December 26, 2019 Report Share Posted December 26, 2019 On 12/23/2019 at 1:25 PM, liambesaw said: Can we get a close up picture of the crack, where the glaze and clay interface? And was it split like that when you found it, or have you separated the halves for clarity Here's the closeup! From all the comments I'm getting, seems to me it has to do with separating a bit from the bottom next time (either alumina, grog or some wadding). Also the tension as the form moves while cooling, so will check the times for that as well... And placing it on the topmost shelves rather than bottom... Thanks a lot! Link to comment Share on other sites More sharing options...
Sopita on the Rocks! Posted December 26, 2019 Report Share Posted December 26, 2019 On 12/23/2019 at 3:00 PM, Min said: @Sopita on the Rocks!, if you run your fingertip across the crack is the glaze sharp on the edges of the crack with no smoothing over of the glaze? Also, how fast do you cool the kiln and how hot was it when you removed the peep hole plugs and cracked open the lid? Not sure about the question of the crack/glaze, but attaching a photograph that maybe shows it better. As for the cooling, once it's reached 1240˚C I let it at that temperature for 15', and after that let it cool all night, opening once it's reached 100˚C or less (it was 90 something when I opened it, if I recall correctly). Link to comment Share on other sites More sharing options...
Hulk Posted December 26, 2019 Report Share Posted December 26, 2019 Nice work! ...hope you get the cracking solved. Link to comment Share on other sites More sharing options...
Sopita on the Rocks! Posted December 26, 2019 Report Share Posted December 26, 2019 1 hour ago, Hulk said: Nice work! ...hope you get the cracking solved. thank you very much! Hope so too!!!! Link to comment Share on other sites More sharing options...
Min Posted December 26, 2019 Report Share Posted December 26, 2019 @Sopita on the Rocks!, looks like the glaze is softened / slightly rounded over along the crack on the base. Rounded / softened glaze edge indicates the crack was there before the glaze melted, as the kiln was heating up and before the kiln reached top temperature. Sharp edge indicates the crack occurred on the cool down. It does look like the glaze on the wall has a sharp edge and that there is a glaze thickness difference between the base and the wall, the base being a fair bit thicker. Link to comment Share on other sites More sharing options...
Sopita on the Rocks! Posted December 26, 2019 Report Share Posted December 26, 2019 10 minutes ago, Min said: @Sopita on the Rocks!, looks like the glaze is softened / slightly rounded over along the crack on the base. Rounded / softened glaze edge indicates the crack was there before the glaze melted, as the kiln was heating up and before the kiln reached top temperature. Sharp edge indicates the crack occurred on the cool down. It does look like the glaze on the wall has a sharp edge and that there is a glaze thickness difference between the base and the wall, the base being a fair bit thicker. Thank you so much for these insights! Will try looking at it and paying close attention to this when I get home! I think I understand what you mean. Link to comment Share on other sites More sharing options...
Babs Posted December 26, 2019 Report Share Posted December 26, 2019 When making hanging planters with flat to the wall backs I had a bit of similar cracking happening. Throwing the flat back and attaching front instead of slab work really helped with that problem. Check the bisque is uncracked. Flick the pot and listen for a pinging sound. Your clay slab has been under pressure of manipulation to get from flat to that shape. Fire on bisqued thin sausage coils of clay placed like an asterisk. Link to comment Share on other sites More sharing options...
Chilly Posted December 28, 2019 Report Share Posted December 28, 2019 On 12/26/2019 at 11:57 AM, Sopita on the Rocks! said: Only have to figure out the Spanish for alumina (I know grogg is "chamote", but not sure about alumina). ☺️ Google translate says alúmina Link to comment Share on other sites More sharing options...
Sopita on the Rocks! Posted January 2, 2020 Report Share Posted January 2, 2020 On 12/28/2019 at 8:58 AM, Chilly said: Google translate says alúmina Thanks! YES it's sold as "óxido de aluminio" here! Link to comment Share on other sites More sharing options...
Cline Campbell Pottery Posted January 3, 2020 Report Share Posted January 3, 2020 Does anyone suggest using cookies or coasters under pots? I've been working on some footed pieces and firing them in a shared kiln where sand or grog could cause a problem. I don't know if this would work for something flat. A cookie is a flat piece of the same kind of clay that will shrink at the same rate as the pot and prevent drag. It is usually a waste piece, but could be a plate or trivet. Cynthia Link to comment Share on other sites More sharing options...
oldlady Posted January 3, 2020 Report Share Posted January 3, 2020 yes, though i have not done that, many others here suggest it. Link to comment Share on other sites More sharing options...
Bill Kielb Posted January 3, 2020 Report Share Posted January 3, 2020 47 minutes ago, Cline Campbell Pottery said: Does anyone suggest using cookies or coasters under pots? I've been working on some footed pieces and firing them in a shared kiln where sand or grog could cause a problem. I don't know if this would work for something flat. A cookie is a flat piece of the same kind of clay that will shrink at the same rate as the pot and prevent drag. It is usually a waste piece, but could be a plate or trivet. Cynthia Yes, Works, but sometimes the waster piece drags on the shelf as well so something like alumina wax can still be a good practice on the foot of the ware. Link to comment Share on other sites More sharing options...
neilestrick Posted January 3, 2020 Report Share Posted January 3, 2020 If you use a waster, you'll need alumina on it so it doesn't stick to the pot. A little alumina suspended in wax resist works good. If the waster catches on the shelf it will crack, but the pot should not. Link to comment Share on other sites More sharing options...
Sopita on the Rocks! Posted January 14, 2020 Report Share Posted January 14, 2020 Thanks for all the comments and help! I tried firing again this weekend and used alumina beneath a new dish I made, to great success!!! Link to comment Share on other sites More sharing options...
Bill Kielb Posted January 14, 2020 Report Share Posted January 14, 2020 6 minutes ago, Sopita on the Rocks! said: Thanks for all the comments and help! I tried firing again this weekend and used alumina beneath a new dish I made, to great success!!! Nice work and nice solution! Easy peesy lemon squeezey ! Glad it worked, go make more cool stuff! Link to comment Share on other sites More sharing options...
Shelly1221 Posted September 7, 2020 Report Share Posted September 7, 2020 Help I’ve been working on 3 nativity scenes for Christmas, and the big barn center piece for the manger keeps cracking! I don’t fire them myself but have been going to a ceramic shop and she’s been firing them for me. The first barn had a deep crack( not all the way through) and then the next two had a fine line crack.. I don’t know what’s being done wrong but I’ve put in so much time on each piece ,the last two were already fired in the glaze and I had put the mother of pearl on them and the6 cracked in that firing.. any idea what’s going on? Link to comment Share on other sites More sharing options...
Bill Kielb Posted September 7, 2020 Report Share Posted September 7, 2020 14 minutes ago, Shelly1221 said: any idea what’s going on? Some pictures of the cracks for sure. Hard to say without them Link to comment Share on other sites More sharing options...
Shelly1221 Posted September 7, 2020 Report Share Posted September 7, 2020 5 minutes ago, Bill Kielb said: Some pictures of the cracks for sure. Hard to say without them Link to comment Share on other sites More sharing options...
Shelly1221 Posted September 7, 2020 Report Share Posted September 7, 2020 8 minutes ago, Bill Kielb said: Some pictures of the cracks for sure. Hard to say without them im trying to figure out how to post them here Link to comment Share on other sites More sharing options...
Bill Kielb Posted September 7, 2020 Report Share Posted September 7, 2020 1 minute ago, Shelly1221 said: m trying to figure out how to post them here When you open a new comment at the bottom it says drag files here and / or choose files. Link to comment Share on other sites More sharing options...
Shelly1221 Posted September 7, 2020 Report Share Posted September 7, 2020 It keeps saying the pic file is too large Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted September 7, 2020 Report Share Posted September 7, 2020 It is usually easiest to upload images straight from your phone. Most new models will automatically ask which size of file you’d like to use. If you’ve got an older model, you can email the picture to yourself, and when you hit send the email program will ask you which size you’d like to send. Link to comment Share on other sites More sharing options...
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