ABlanc Posted August 22, 2022 Report Share Posted August 22, 2022 Is there a doctor in the house? (Wish my first post was not about cracks!) The details: B Mix with clear glaze, no cracks after bisque on this particular piece, happened after glaze firing. However, other similar pieces in design have come out of bisque with hair line fractures. There was plenty of drying time and no visible cracking while drying with any of the pieces. Any ideas? Quote Link to comment Share on other sites More sharing options...
Pres Posted August 22, 2022 Report Share Posted August 22, 2022 @ABlanc Considering the type of crack, and the crazing of the glaze, IMHO this is a cooling dunt, where the kiln was cooled a little to quickly for large flat pieces of this sort. I used to get some cooling dunts in the early years from overzealous cooling of the kiln, or opening too soon. How are you firing the kiln? Is it on a kiln setter, or a controller with a ramp up and down set up? best, Pres Babs 1 Quote Link to comment Share on other sites More sharing options...
Min Posted August 22, 2022 Report Share Posted August 22, 2022 Hi @ABlanc and welcome to the forum. It's hard to tell for sure on your images but is the glaze edge rounded over along the cracks? Looks like it might be, especially on the last image on the far right side of the crack. If so then those cracks were there before the pots started cooling down, probably in the bisque or greenware. Quote Link to comment Share on other sites More sharing options...
Dick White Posted August 23, 2022 Report Share Posted August 23, 2022 Are you wedging the clay before rolling your slabs? It is a misconception that clay is perfect straight out of the bag. The pugging process during production of the clay body introduces spiral tension/compression rings in the block, which if rolled straight into a slab could result in latent fractures along those rings. Quote Link to comment Share on other sites More sharing options...
ABlanc Posted August 23, 2022 Author Report Share Posted August 23, 2022 We are doing a ramp up and down during firing - we have other work that is for sure cracking due to dunting so we added a ramp down on everything. I will check if the glaze edge is rounded over when back in the studio tomorrow. We certainly have other pieces that are just like this in design - large flat wall hanging pieces - that are coming out of bisque with hair line fractures. This particular piece didn’t appear to have any cracks after bisque though. We are not wedging this kind of flat slab work out of the bag. That is interesting to think about. One thought I had was if it was somehow due to the clay being high fire and the glaze being low fire and the mismatch of that. The clay is thus being under fired. Also B mix might just not be the best choice for large flat surfaces? Okay! Back to tearing my hair out! Quote Link to comment Share on other sites More sharing options...
Min Posted August 23, 2022 Report Share Posted August 23, 2022 1 hour ago, Dick White said: Are you wedging the clay before rolling your slabs? It is a misconception that clay is perfect straight out of the bag. The pugging process during production of the clay body introduces spiral tension/compression rings in the block, which if rolled straight into a slab could result in latent fractures along those rings. It's funny you should mention this. I was using a bag of clay today that had sat around for about a year, still lovely and soft but those rings from the pugger's auger really showed up in it. When this clay is "fresher" they don't show at all (even though they are still there). 6 minutes ago, ABlanc said: One thought I had was if it was somehow due to the clay being high fire and the glaze being low fire and the mismatch of that. The clay is thus being under fired. Don't think low fire glaze firing would crack the pots like that. Does the crazing bother you or are you okay with it given the pieces are decorative? 10 minutes ago, ABlanc said: Also B mix might just not be the best choice for large flat surfaces? With grog or sand might be a better choice. Quote Link to comment Share on other sites More sharing options...
Mark C. Posted August 23, 2022 Report Share Posted August 23, 2022 (edited) looks like a cooling crack to me Also high fire body is not mature and a bit punky also amy be a factor ? The high fire clay, low fire glaze is not an issue with moisture weeping? can you explain the forming technique as this could be the factor ? Looks a bit like toilet lid-what is the size of the piece? Edited August 23, 2022 by Mark C. Quote Link to comment Share on other sites More sharing options...
ABlanc Posted August 23, 2022 Author Report Share Posted August 23, 2022 Those spirals, wow, interesting how you can really see them there. We like the crazing and the reason we are under firing is because we like the color of this clear low fire glaze paired with the creamy color of the B mix. And the B mix is just always available. We did order some with grog to test out, still waiting for it. Definitely not a toilet seat lol. It’s a wall sconce or will be. It’s about about 12 inches in length and only sits off the all about an inch. The back side “walls” are for hiding LEDs/horse hair - see pic. Those walls are slab strips cut and attached to the back. Here is pic of similar sized finished piece. Our success rate (I.e. no cracks) has been dismal with these pieces. They either crack like this or have hairline cracks after bisque. Quote Link to comment Share on other sites More sharing options...
kswan Posted August 23, 2022 Report Share Posted August 23, 2022 I was making large wall hanging pieces with B Mix 5 and it just doesn't want to be that large and flat. I ended up going with paper clay. I first mixed my own batch of it but ended up buying premade. Jerry Bennett of Philadelphia has an online course for making paper clay that I found very helpful. I still use the paper clay for bisqued press molds and things that want to warp. Quote Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted August 23, 2022 Report Share Posted August 23, 2022 2 hours ago, ABlanc said: They either crack like this or have hairline cracks after bisque. This is making me think that it’s more likely something that’s happening in the forming stages, and the ones that work may still have hidden flaws. According to Hamer and Hamer, dunting often starts with how a piece is made, whether it happens in the cooling or on the heat up. One way to confirm this would be to take any bisqued pieces you have and do a ping test. Just flick it and see if it rings or if it clicks. Click=crack you can’t see. Roberta12, Babs and Pres 3 Quote Link to comment Share on other sites More sharing options...
Pres Posted August 23, 2022 Report Share Posted August 23, 2022 15 minutes ago, Callie Beller Diesel said: One way to confirm this would be to take any bisqued pieces you have and do a ping test. Just flick it and see if it rings or if it clicks. Click=crack you can’t see. I have been using this test for years with patens/plates, as there are times a hairline crack cannot be seen, but can be heard! best, Pres Roberta12 and Babs 2 Quote Link to comment Share on other sites More sharing options...
ABlanc Posted August 23, 2022 Author Report Share Posted August 23, 2022 Here are some pics of another similar piece with post bisque firing crack that runs on the front and then continuously up the back (I marked alongside in yellow so you can see better) Just the wrong clay for the job perhaps? Quote Link to comment Share on other sites More sharing options...
Pres Posted August 23, 2022 Report Share Posted August 23, 2022 I am beginning to wonder if the clay is too dense for the object? Maybe it needs some grog to open it up a bit and allow for some change in the expansion/contraction of the body in firing. best, Pres Quote Link to comment Share on other sites More sharing options...
Pres Posted August 23, 2022 Report Share Posted August 23, 2022 Another thought is the construction process: cast?, press mold? best, Pres Quote Link to comment Share on other sites More sharing options...
ABlanc Posted August 23, 2022 Author Report Share Posted August 23, 2022 4 minutes ago, Pres said: Another thought is the construction process: cast?, press mold? best, Pres Meaning is it or meaning it should be? These are hand formed out of slabs. Quote Link to comment Share on other sites More sharing options...
PeterH Posted August 23, 2022 Report Share Posted August 23, 2022 How do you support the pieces when firing; flat on the shelf, on a waster, on sand, ...? Babs 1 Quote Link to comment Share on other sites More sharing options...
ABlanc Posted August 23, 2022 Author Report Share Posted August 23, 2022 1 minute ago, PeterH said: How do you support the pieces when firing; flat on the shelf, on a waster, on sand, ...? Flat with sand and right side up, meaning the back with the "walls" is on the shelf. Quote Link to comment Share on other sites More sharing options...
Mark C. Posted August 23, 2022 Report Share Posted August 23, 2022 (edited) I think B mix is the wrong clay for this. It does not slab well (staying flat) like to cracks as well and drys super uneven. I work in porcelain 95% when I need to do slab work I choose other white bodies I make larger press molded baking dishes -that body is white -its 1/2 porcealin and 1/2 wso called 1/2 and 1/2 the WC number is 382. Its made for slab torture. I suggest a body thats more up to the task any white body with finer white grog will work as well. I also will add as other below me on the waster slabs or sand or grog to allow the slab to move as it shrinks in the fire Edited August 23, 2022 by Mark C. Magnolia Mud Research and Callie Beller Diesel 2 Quote Link to comment Share on other sites More sharing options...
ABlanc Posted August 23, 2022 Author Report Share Posted August 23, 2022 1 hour ago, Mark C. said: I think B mix is the wrong clay for this. It does not slab well (staying flat) like to cracks as well and drys super uneven. I work in porcelain 95% when I need to do slab work I choose other white bodies I make larger press molded baking dishes -that body is white -its 1/2 porcealin and 1/2 wso called 1/2 and 1/2 the WC number is 382. Its made for slab torture. I suggest a body thats more up to the task any white boidy with finer white grog will work as well. This was what I was worried would be the ultimate answer. I say that because we really want to use our low fire glaze and also have the clay fire to a buff color, not white. Quote Link to comment Share on other sites More sharing options...
Jeff Longtin Posted August 23, 2022 Report Share Posted August 23, 2022 To pick up on what the Pres mentioned...my gut tells me the slab is being overworked. (In the making process.) Too many "walls" are being attached and its causing stress in the firing. The walls may be necessary for strength and to hide things, but the attachment process is too stressful. Can I presume the slab is flat on a surface and you're pressing the walls into it? The question would be how can you reduce the stress when going thru this process? My gut tells me you should create a form that is intact on its own, essentially, and then put the shaped slab OVER it. That way taking stress off the slab itself. Pres, Mark C., Kelly in AK and 2 others 5 Quote Link to comment Share on other sites More sharing options...
Pres Posted August 23, 2022 Report Share Posted August 23, 2022 Another thought here would be to salt the kiln shelf with grog both for the bisque and the glaze firing. It may, not guaranteed, ease the stress enough. I used to keep a salt shaker with grog in it to salt the shelves when teaching in HS. It requires vacuuming carefully afterward, but in many case was helpful. best, Pres Quote Link to comment Share on other sites More sharing options...
Bill Kielb Posted August 23, 2022 Report Share Posted August 23, 2022 6 hours ago, ABlanc said: Those spirals, wow, interesting how you can really see them there. We like the crazing and the reason we are under firing is because we like the color of this clear low fire glaze paired with the creamy color of the B mix. And the B mix is just always available. We did order some with grog to test out, still waiting for it. One thing to add a good glaze fit generally strengthens the ware. Crazed glazes can weaken it by providing a nice starting path for any defect to occur. Depending on the joinery, this shape is hard to build and keep the shrinkage rates reasonably the same. I am curious to the order of assembly making these and if any molds were used. Roberta12 1 Quote Link to comment Share on other sites More sharing options...
Babs Posted August 23, 2022 Report Share Posted August 23, 2022 (edited) How are you drying this multi layered slab? What is your firing schedule? Had a teacher who said many times, to non listening teenagers, " It is not the material but the operator", i.e. the process, design etc being applied so before troting down to the store, check your steps, or it could be Murphy.... Happening all over your kiln? Edited August 23, 2022 by Babs Quote Link to comment Share on other sites More sharing options...
Roberta12 Posted August 23, 2022 Report Share Posted August 23, 2022 I agree with @Bill Kielb about the crazing being a weak area. And I agree with everyone that mentioned Bmix 5 not being the best clay choice. If you can find something off white or buff with grog in it, that might reduce some errors. Anytime I have tried to put a "notch" in a flat piece, it seems to give me a weak area for cracking. I am able to get an off white clay (Laguna) via Denver that is groggy and truly bullet proof. Very forgiving. If you can find something of that nature, you might be able to execute that design which I LOVE btw! clever idea! Pres 1 Quote Link to comment Share on other sites More sharing options...
Mark C. Posted August 24, 2022 Report Share Posted August 24, 2022 Buff clays are very abuntant Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.