docweathers Posted November 26, 2016 Report Share Posted November 26, 2016 I got some serious cracking on my last firing. I could use some advice. LRW 11/11/16: Firing schedule Ramp 1: 100ºF/hour to 190ºF Hold 3 hours Ramp 2: 500ºF/hour to 1968ºF 5 min hold Ramp 3: 150ºF to 2190ºF 5 min hold Ramp 4: natural cooling to 1900ºF No hold Ramp 5: 100ºF/hour to 1400ºF No hold, kiln off. Kiln Skutt km1227 with computer-controller It is located in a large closed building where there are no drafts. Clay G mix 6 with grog from Georgie's Bisque to 04 Glaze fire to 6 Plugs pulled at 1000° Lid cracked 1 inch at 380° ugly pictures attached Link to comment Share on other sites More sharing options...
Mark C. Posted November 26, 2016 Report Share Posted November 26, 2016 Doc I cannot see how long it took to get to cone6? whats your guess?in hours Do not pull your plugs at any temps.Let cool without drafts. Keep kiln closed up until lower than 300 Degrees -keep closed longer with platters Where was this platter in kiln Middle ? top?Covered with a shelve? bottom? One last note but its my bias -I'm not a Georgies clay body fan but as I said its just my experience which their bodes (high fire only) they may make a mean cone 6 body Link to comment Share on other sites More sharing options...
glazenerd Posted November 26, 2016 Report Share Posted November 26, 2016 Cracked before you even glazed fired them. Fissures in the clay, and the mechanical stress of the glaze finished the job. Need to do the ping test: hold the piece with two fingers and gently tap it with something metal (softly!) If it pings it is solid, if it pongs it has a fissure. I use a metal glass cutter, with the metal ball on one end. $3 Nerd. Link to comment Share on other sites More sharing options...
Babs Posted November 26, 2016 Report Share Posted November 26, 2016 Glaze Nerd how do you get to that? Glaze edge looks sharp, cracking on the way down maybe but hard to see Link to comment Share on other sites More sharing options...
Mark C. Posted November 26, 2016 Report Share Posted November 26, 2016 That platter looks like it was tortured a bit to make it that shape to start with. Any chance it is trying to get back to the shape it once was (clay has memory) Link to comment Share on other sites More sharing options...
bciskepottery Posted November 26, 2016 Report Share Posted November 26, 2016 Is the bottom thicker than the walls? If so, you could have uneven heating/cooling. Link to comment Share on other sites More sharing options...
Marcia Selsor Posted November 26, 2016 Report Share Posted November 26, 2016 it also looks like hunting . How fast did it cool or did it get a cold shock.? Marcia Link to comment Share on other sites More sharing options...
Marcia Selsor Posted November 26, 2016 Report Share Posted November 26, 2016 The thickness looks uneven too. As both Mark and Bciske mention. Looks like the crack may have started where you indented the lip but the bottom looks very heavy. I have a chuck for trimming such shapes to avoid stressing lips. Highly recommend using something to support the pot when trimming more delicate shapes. Marcia Link to comment Share on other sites More sharing options...
glazenerd Posted November 26, 2016 Report Share Posted November 26, 2016 Babs: Seen that crack pattern too many times: all I need to see. It is the classic bow/hook pattern: an arc towards one end that straightens out to the other. Where the hook/bow crack is: is also where the piece was making the most contact with the shelf. The body had stress points in it from making (will let others address that cure) and as the glaze began to harden upon cooling: it finally snapped it apart. Notice how open the gap is on the one end? If Doc would have been standing next to the kiln: he would have heard that one go. Nerd by the way Doc: get some silica under those pieces next time: reduce shelf drag. Link to comment Share on other sites More sharing options...
docweathers Posted November 26, 2016 Author Report Share Posted November 26, 2016 wow, thanks for all the good advice.I can see where I made a lot of mistakes. I'm just a raw beginner at this stuff Actually there were a couple of platters that broke in a similar pattern. The bottom was probably a little thicker than the sides on both of them. the whole firing process takes about 16 hours. I would guess it takes about six hours to go from 190 to 2190. The platters were on the top two shelves. the top was not covered with a shelf. it cooled pretty fast from 2190 to 1900 then it was slow after that. I assume the ping test is done on the bisque or do you mean after glaze firing. I do have one of those glass cutters like you mentioned. I can see where I need to change a lot of stuff. My welding is so much easier than glazing. tthanks all Link to comment Share on other sites More sharing options...
neilestrick Posted November 26, 2016 Report Share Posted November 26, 2016 It's too thick. Make it thinner, and make the foot a lot narrower to get ore weight off. Link to comment Share on other sites More sharing options...
glazenerd Posted November 26, 2016 Report Share Posted November 26, 2016 Ping test at bisq/ before glaze. Nerd Link to comment Share on other sites More sharing options...
Babs Posted November 26, 2016 Report Share Posted November 26, 2016 Babs: Seen that crack pattern too many times: all I need to see. It is the classic bow/hook pattern: an arc towards one end that straightens out to the other. Where the hook/bow crack is: is also where the piece was making the most contact with the shelf. The body had stress points in it from making (will let others address that cure) and as the glaze began to harden upon cooling: it finally snapped it apart. Notice how open the gap is on the one end? If Doc would have been standing next to the kiln: he would have heard that one go. Nerd by the way Doc: get some silica under those pieces next time: reduce shelf drag. Yes, dunting, I was referring to your conclusion that it was already cracked prior to glazing..... It does look a very tortured piece as MarkC writes. It's just that I have had pieces do this which have been intact till the glaze firing and the crack occurs when the kiln drops quickly, fibre kiln the culprit. Link to comment Share on other sites More sharing options...
docweathers Posted November 26, 2016 Author Report Share Posted November 26, 2016 I have an extension of my original question This was just a test firing of a bunch of little 12 inch bowls. I have a bunch of similarly shaped 23+ inch .bowls. What additional advice would you folks have for firing these monsters without getting cracks in them, given that I implement all of the great suggestions I've gotten. Link to comment Share on other sites More sharing options...
bciskepottery Posted November 27, 2016 Report Share Posted November 27, 2016 Put either a layer of grog or silica sand beneath the platters or put them on clay coils or slats (bisque fired) so they are sitting above the kiln shelf; that will help allow them to heat and cool more evenly. Sitting on the shelf, they bottoms of the platters stay hotter as they absorb heat from the shelf while cooling. Link to comment Share on other sites More sharing options...
docweathers Posted November 27, 2016 Author Report Share Posted November 27, 2016 that's a good idea. Thanks Link to comment Share on other sites More sharing options...
Babs Posted November 27, 2016 Report Share Posted November 27, 2016 Bisque fire them stacked like Marcia does her slabs. Sale the incurve gently and when the clay is pos. Softer than with these Uniformity of thickness On a chuck to trim And al of the above Link to comment Share on other sites More sharing options...
Mark C. Posted November 27, 2016 Report Share Posted November 27, 2016 Go slower with fire and make sure they are covered by a top shelve.(I never heard how long your fire was?)Make sure they can move as Bruce said in above post. Make sure they are BONE DRY Let them cool SLOWER and do not open anything including peeps until cool. Link to comment Share on other sites More sharing options...
glazenerd Posted November 28, 2016 Report Share Posted November 28, 2016 Doc: Obviously you need to scale this principle according to size, weight, etc..... but you get the idea. QUARTZ INVERSION 573C Program 100F from 1000 to 1100F to avoid cracking a piece during this critical bisq temp. The larger the piece, the more surface/shelf contact it makes: the more critical quartz inversion becomes. Nerd Link to comment Share on other sites More sharing options...
Babs Posted November 29, 2016 Report Share Posted November 29, 2016 Dry them on a similar elevated rack prior to bisquing if rims are fragile, or not.. Couild be aggravated by thinner rim drying quicker that base. Link to comment Share on other sites More sharing options...
Mark C. Posted November 29, 2016 Report Share Posted November 29, 2016 Nerd is that a slip cast or pressed sink? Link to comment Share on other sites More sharing options...
docweathers Posted November 29, 2016 Author Report Share Posted November 29, 2016 I think these suggestions will greatly improve my odds of avoiding cracks. Thanks Link to comment Share on other sites More sharing options...
glazenerd Posted November 29, 2016 Report Share Posted November 29, 2016 Master mold.. body pressed- drain assembly cast- and then I cracked it!!!! Link to comment Share on other sites More sharing options...
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