Mike@riverrun Posted February 1, 2012 Report Share Posted February 1, 2012 Thank you so much, everyone! After some digging, I found that the stoneware is Laguna's 510, and on their site it's labeled as cone 8 - 10... So the fact that the studio fires everything at cone 6 could certainly be a problem! The glaze is mixed in the studio and I don't have access to the recipes, so I have no idea what kind of shrinkage percent it is prone to. I have also found that the pottery world is quite divided on the issue of crazing and whether or not it weakens pots. Does anyone know of any studies done to try to determine a verdict on this? Some detail on my process: As for making the handles - I make them with fresh, soft clay in a wall-mounted extruder in 2-foot-long pieces laid carefully on a board. I cut a piece off using a knife and then bend it slowly while pressing gently (the spot where it is pinched) to keep the clay from getting any of those soft cracks it can get when being bent at such an angle. Then I give that spot another pinch with my fingers lined up so I get a nice even pinch shape there. After that I simply find a good fit on my mug and slice the ends of the handle at an angle to have a larger connection area where it joins to the mug. I score the connection points (both mug and end of handle) and add a touch of water or slip before pressing the handle ends into place. I tend to trim my mugs as soon as I safely can so that they still have plenty of moisture in them when I attach handles ( - I rarely have any issue with handle attachments separating or cracking away from the mug when I use this timing). I try to let my mugs dry as slowly as possible. As I'm attaching the handles on a batch of mugs, I set each finished one on a thick towel placed around my wheel, so the mugs can set on their sides with the handles going straight up to avoid upward or downward stress/sagging on the handles. When I've got them all attached, I set them upside-down on a board and cover it securely with plastic sheeting. They will stay this way (for 7 - 12 days or so) until nearly bone dry and ready for decoration and bisque firing. I know I have a lot to learn and I really appreciate all the information and suggestions! I will definitely try using a thicker hole in the extruder, as well as pulling handles manually, since I can't change what clay the studio purchases for members to use (although I will certainly be bringing up these issues with the studio's managers). Link to comment Share on other sites More sharing options...
Mike@riverrun Posted February 1, 2012 Report Share Posted February 1, 2012 Aha - I was going to suggest you run some test mugs up to cone 8 or 9 and see if you get full vitrification. Had the same problem with a white stoneware that was 6-10 and the handles broke weeks after use, always just short of the attachment to the pot. Link to comment Share on other sites More sharing options...
Guest LibraryKrystal Posted February 1, 2012 Report Share Posted February 1, 2012 Aha - I was going to suggest you run some test mugs up to cone 8 or 9 and see if you get full vitrification. Had the same problem with a white stoneware that was 6-10 and the handles broke weeks after use, always just short of the attachment to the pot. Mike, I am sorry that this has happened to you, but it is also very helpful to know that this problem (specifically the handles breaking, even in the same place that mine are) with underfired clay has happened to others. I have a question (posted above, but I think maybe nobody's seen it)... If my wares are bisque fired at cone 10, can they then be glaze fired at cone 6 with no problem since they would have reached cone 10 previously? Or would the second firing at lower temp undo the vitrification achieved in bisque firing? Thanks! Link to comment Share on other sites More sharing options...
Lucille Oka Posted February 1, 2012 Report Share Posted February 1, 2012 Once ware is vitrified it cannot be undone. Sort-a like untoasting toast it cannot be done. It is best to test if your cone 6 glaze will give you a good fit on your cone 10 clay. There are no absolutes. Make some test tiles of your clay and fire them to cone 10, apply the cone 6 glaze, fire to cone 6, then decide if you like the results. Link to comment Share on other sites More sharing options...
Guest LibraryKrystal Posted February 1, 2012 Report Share Posted February 1, 2012 UPDATE: I tested the water absorption theory today and that's DEFINITELY the problem. I took a mug that's never been used/never been in water and banged other things against the handle - it stood fast with zero breakage. I then sat that mug in a bowl of water for a few hours. Just now, I took it and tapped a spoon against the handle, and it fell right off. This'll be useful information for getting this problem fixed and I want to thank you all again for your sleuthing! You've helped me immensely. Link to comment Share on other sites More sharing options...
Ben Posted February 2, 2012 Report Share Posted February 2, 2012 "If my wares are bisque fired at cone 10, can they then be glaze fired at cone 6 with no problem since they would have reached cone 10 previously?" Good thinking! There are commercial manufacturers that do something similar (and it is common in earthenware to "bisque high/glaze low). In the industry example I have seen they "bisque" somewhere near the maturation temperature of the clay, then they use a vibratory "tumbler" machine to smooth the pots surface, then they heat the pot and spray the glaze on. This allows them a super smooth surface for the glaze to melt onto and give a smoother surface to the fired glaze. So it is possible, BUT!!! they have incredible facilities set up for this system of glaze application that you may not be able to replicate in your studio AND their clay and glaze match in expansion/contraction so they don't have glaze fit problems. Bisqueing your clay to its proper maturing temperature will make it as strong as it can be without glaze. It will certainly be much stronger than it is when it is underfired. BUT that may not solve the glaze fit problem. The clay will have a different coefficient of expansion when fired at different temps. For example, @ cone 6 we know it does not fit your glaze. You will have to test a sample fired to cone 10, then glazed, to see if it fits your glaze (if you can get the glaze to stick to the pot, see below). In general, glazes stick to pots because the pot absorbs water out of the glaze slurry. If you bisque to full maturation of the clay it will have near zero absorption so you will have to find some other way to stick the glaze to the pot. You could try heating the pot and spraying as described above if the studio is set up for that. Beyond that suggestion I have no personal experience. You might be able to create a glaze that is more like a paint in that it has a glue replacing part of the water as part of its recipe but I have never done that. Maybe others have? I still think that the easiest ceramic/technical solution is using the proper clay. That won't resolve the people issues in dealing with your studio operators though so good luck with that. Either way, think about all the great discussion your post has given birth to. That is why I love forums. "We" are always smarter and always know more than "I". And "we" are always more fun to talk to than "I" as well :-) Link to comment Share on other sites More sharing options...
teardrop Posted February 2, 2012 Report Share Posted February 2, 2012 I would certainly be tracking my time and expense on this and would insist that the studio offer me free kiln/studio time to rectify the mistake and replace the work. I hope you find someone reasonable in the studio who will listen to your concerns. best of luck teardrop Link to comment Share on other sites More sharing options...
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