EmmaC Posted January 31, 2022 Report Share Posted January 31, 2022 Hi there, I've been having issues with hairline cracks on the joins of my handles. I don't really have cracks on the join between the handle and the body, it's the cracks on the handles. I use stoneware fire to cone 9, usually I put a white slip on greenware then paint the pieces with engobe. I put the painted pieces in a plastic box ( in the box I also cover my work with a plastic and mist the box so the box stay somehow moist) for a few days before I take them out and put them on a shelf. I think they are drying pretty slow in this way but i still have hairline cracks here and there... i'm devastated as i spent quit amount of time on each mugs and these imperfections is killings me Could anyone tell me what else I could do to prevent the cracks from happening? Thank you!! Emma Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted January 31, 2022 Report Share Posted January 31, 2022 It probably isn’t how you’re drying it, from the description you’ve given, If it’s happening in the same place every time, it’s more likely a construction issue. You need to just take a tool that will fit into that crease and compress the join a bit. The round end of a paintbrush handle or a small wooden tool, something like that should work. The cracking that’s happening in the second image on the inside of the loop: I don’t think I’d worry about that one too much. It’s evidence of the material, and is not a structural flaw of any kind. If you don’t like them for aesthetic reasons, you can take a damp paintbrush and just wipe them away when the clay is still soft. I like synthetic watercolour brushes for jobs like this. Link to comment Share on other sites More sharing options...
EmmaC Posted January 31, 2022 Author Report Share Posted January 31, 2022 Hi @Callie Beller Diesel I used a wooden tool to compress the joins after i attached the handle, but maybe I did not do it enough... and I am thinking maybe the join part is too wet or is it possible it is the slip that's too thick? When i make this shape of handles I always have the cracks like this. for the second image, I didn't see the crack until the mug is bisqued and I tried to crease the crack but with the transparent glaze the crack still shown up:( Link to comment Share on other sites More sharing options...
Mark C. Posted February 1, 2022 Report Share Posted February 1, 2022 Ok you say its a cone 9 clay body (what color or better yet is it fine or groged clay) then you coat with a white slip (also made from a cone 9 clay body? I need more exact info on this part. Best practice is both bodies be the same firing temps. My guess with some of the info missing is the two -clay and slip may be an issue. . Is this issue getting more cracks after firing or all in the 1st drying period ?Have you tried drying slowly under plastic lifting it slowly also those colors do not look like cone 9 more like low fire cone 06 or midrange cone 6 so something is up there as well . Yellows and reds tend to burn out at cone 9. Link to comment Share on other sites More sharing options...
EmmaC Posted February 1, 2022 Author Report Share Posted February 1, 2022 Hi @Mark C. the clay I used is PRAI with tiny groges (Drying shrinkage: ≈8.5% Firing shrinkage (1300ºC): 7.0%), the slip is from SAFC porcelain (drying shrinkage (110 °C): 2.5 – 4.0 %, firing shrinkage (1280 °C): 11.5 – 13.7 %). They can both be fired to 1280c. I am in France and many studios use SAFC porcelain as a white slip base that's why i'm using it. I am still quit new to pottery, is the shrinkage rate between the clay and the slip i used too different? I usually have no issue with the slip on my pieces, only this specific handle design is having issue. i'm thinking if the shrinkage rate also caused the tiny cracks line on my handle joins ? (see the middle picture on my original post). The colorant are all up to cone 9/10 ( France doesn't really have cone 6 clays unfortunately , I know i could fire my clay to only cone 6 but i want to get low porosity as i mostly make tableware. Link to comment Share on other sites More sharing options...
Hulk Posted February 1, 2022 Report Share Posted February 1, 2022 Cool handles! I'm using slip in joining because it provides filling - particularly where it is difficult to reach with a finger or tool - and hence is easier; the slip squishes out, then I wipe away all but the tiny bit* that's left behind with a wet finger. Where a finger doesn't reach, a damp brush does the trick. I've had much better luck using slip made from the same clay. I scrape off and collect slip from my hands when throwing at the wheel, in a small bowl that sits next to the wheel**. A moat of water surrounding the mound of slip keeps it from drying out. The next day, the mound of slip is available for joining handles, knobs, spouts, etc. Slip that is too thin shrinks more, hence, a crack may form in the fillet! Slip that will squeeze out, just, but no more, works better. Once fully dry, if a bit of a crack has formed, I'll dampen it with a brush, pick up some slip with a wet brush, then fill and smooth the crack, voilà! From there, if a bit of crack should appear after bisque, dampen and fill again will usually suffice, although the trash can may win there, depending how big is the crack. Same clay Wipe back to small (smallest!) fillet Use thicker/dryer slip Fill any small surface cracks that may appear If cracks indicate a bad join, consider trashing the piece; tiny cracks in the slip fillet (only) are easily mended. Hope that helps! *or fillet, "a concave junction formed where two surfaces meet (as at an angle)" It's much more trouble (for me) to smooth the edges of the join together by working the clay; I'm not very skilled there, heh. Another option may be to leave the curled in edge at the join, which doesn't work for me... **I'm using two bowls, set within easy reach, one for bits and blobs of clay that will go to reclaim, the second for collecting a mound of slip, which, of course, continues to dry out once deposited there; I'm looking for stiff slip that will squeeze out of the join, just, no wetter than necessary... Link to comment Share on other sites More sharing options...
Mark C. Posted February 1, 2022 Report Share Posted February 1, 2022 My main point is the slip and the clay should be the same shrinkage for best results-yours is not. Its easy to make a slip from the clay you use but I'm guessing you want a white slip on a not so white clay body?I would find a white slip (or white clay) witha more close shrinkage rate to your clay body Those colors -reds and yellows are fired to cone 9? Link to comment Share on other sites More sharing options...
Min Posted February 1, 2022 Report Share Posted February 1, 2022 @Mark C., I've seen red and yellow underglazes with Cd inclusion stains survive cone 10+ in a soda firing. 5 hours ago, EmmaC said: I am still quit new to pottery, is the shrinkage rate between the clay and the slip i used too different? I usually have no issue with the slip on my pieces, only this specific handle design is having issue. i'm thinking if the shrinkage rate also caused the tiny cracks line on my handle joins ? (see the middle picture on my original post). Try reducing the amount of water in your slip. To have it the same fluidity but with less water add just a drop or two of darvan / dispex to about 200ml of very thick slip. Give it a good stir and see if it's fluid or not, if it isn't then add another drop of darvan / dispex and stir it up again. The darvan / dispex is a deflocculant, it will allow you to have a fluid slip with a lower water content than a similarly fluid slip with water only. It could just be that the slip is going on too thick in the area between the coils in that green handle. Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted February 1, 2022 Report Share Posted February 1, 2022 I agree with Min. The join is probably a bit wet. If you’re going over the spots that the handle joins the mug with a tool and you’re not getting cracking there, that says to me you just need to touch up those other points in a similar fashion. It has the added benefit of removing some of the excess moisture. Link to comment Share on other sites More sharing options...
EmmaC Posted February 1, 2022 Author Report Share Posted February 1, 2022 @Hulk Thank you for the tips!! I actually use magic water for the joins (and fillets haha thanks for the explanation :) I will add your tips into my making process! @Mark C. yes red and yellow to cone 9/10 is not a problem here. i would say violet and green are the hardest to go up to high temperature, they only really work below cone7 and i still haven't found these 2 colorants that I want. @Min & @Callie Beller Diesel this is definitely the step i'm gonna take next! just one question - is it ok to just use Sodium silicate instead of darvan? Thank you! Link to comment Share on other sites More sharing options...
Callie Beller Diesel Posted February 1, 2022 Report Share Posted February 1, 2022 Sodium silicate can be a stronger deflocculant than Darvan. If that’s the thing you have on hand though, just use a VERY light hand. Link to comment Share on other sites More sharing options...
EmmaC Posted February 1, 2022 Author Report Share Posted February 1, 2022 @Callie Beller Diesel got it... I will use it carefully!! Thank you Link to comment Share on other sites More sharing options...
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