shiro Posted December 23, 2011 Report Share Posted December 23, 2011 Hello all, I have a question about glazed items cracking in the kiln. I'm a volunteer for a charity with a ceramic workshop but I'm still learning the ropes about ceramics myself. The problem is some wall plaques that we make. They bisque fire just fine but the last few times we've glaze fired them they have all cracked, even if they are of different designs. The crack is very clean with no splinters usually cracking in just one place into two clean peices. I'm pretty sure it's not bubble related or it would have exploded on the bisque firing and there's no sign of any bubbles in the breaks. The glazes are ones we often use and aside from the cracks the fired peices look great and the glaze has fired fine. Other items being fired at the same time have all been fine. In the past we have glaze fired these same wall plaques with no problems. Our electric kiln was inherited from another charity and is very slow and needs the elements replaced (which is happening after the holidays) But even despite the slowness it's been firing things fine until these recent breakages. If anyone can explain why this is happening, it'd be very helpful! Thanks! Link to comment Share on other sites More sharing options...
Pres Posted December 23, 2011 Report Share Posted December 23, 2011 Hello all, I have a question about glazed items cracking in the kiln. I'm a volunteer for a charity with a ceramic workshop but I'm still learning the ropes about ceramics myself. The problem is some wall plaques that we make. They bisque fire just fine but the last few times we've glaze fired them they have all cracked, even if they are of different designs. The crack is very clean with no splinters usually cracking in just one place into two clean peices. I'm pretty sure it's not bubble related or it would have exploded on the bisque firing and there's no sign of any bubbles in the breaks. The glazes are ones we often use and aside from the cracks the fired peices look great and the glaze has fired fine. Other items being fired at the same time have all been fine. In the past we have glaze fired these same wall plaques with no problems. Our electric kiln was inherited from another charity and is very slow and needs the elements replaced (which is happening after the holidays) But even despite the slowness it's been firing things fine until these recent breakages. If anyone can explain why this is happening, it'd be very helpful! Thanks! Sounds like some sort of cooling dunt. How are you placing these on the shelves, what sort of firing range are you using,and what is your firing cycle. Does it include a cool down? Link to comment Share on other sites More sharing options...
shiro Posted December 23, 2011 Author Report Share Posted December 23, 2011 Sounds like some sort of cooling dunt. How are you placing these on the shelves, what sort of firing range are you using,and what is your firing cycle. Does it include a cool down? Thanks for the reply. ^^ We've tried placing them direct on the shelves and tried placing them on stands on the shelves, both times they still cracked. I'm afraid for fireing range, this is where my knowledge ends. My mentor who taught me most of the basics only taught me how to load, unload and turn the kiln on, and since she left neither me or the project manager has any other knowledge on the kiln. " We haven't touched the setting since it was first set up pretty much. We make stuff out of terracotta stoneware mostly with some low temperate glaze stuff so the kiln is set to accomidate that as far as I know. I do know the kiln is taking 3 days to complete it's firing, but not sure how much of that is cooling down. Link to comment Share on other sites More sharing options...
Devany Posted December 23, 2011 Report Share Posted December 23, 2011 Cracking is a common problem for flat pieces, usually because the clay was not compressed well. When rolling slabs be sure to go in both directions. It shows up in the glaze firing because that is where most shrinking occurs (vitrification) at the higher temperature. Cracking can show up before that too, especially when a flat piece is not dried evenly. I do not work with low fire, but three days sounds like a very long time. I fire at ^10 and the actual firing time is usually about 8 hours with another 8-10 for cooling, unless you count the candling which is 6 hours before firing. Sounds like some sort of cooling dunt. How are you placing these on the shelves, what sort of firing range are you using,and what is your firing cycle. Does it include a cool down? Thanks for the reply. ^^ We've tried placing them direct on the shelves and tried placing them on stands on the shelves, both times they still cracked. I'm afraid for fireing range, this is where my knowledge ends. My mentor who taught me most of the basics only taught me how to load, unload and turn the kiln on, and since she left neither me or the project manager has any other knowledge on the kiln. " We haven't touched the setting since it was first set up pretty much. We make stuff out of terracotta stoneware mostly with some low temperate glaze stuff so the kiln is set to accomidate that as far as I know. I do know the kiln is taking 3 days to complete it's firing, but not sure how much of that is cooling down. Link to comment Share on other sites More sharing options...
Diana Ferreira Posted December 23, 2011 Report Share Posted December 23, 2011 do you use alumina on the shelve to aid 'movement' of the tiles? Link to comment Share on other sites More sharing options...
trina Posted December 23, 2011 Report Share Posted December 23, 2011 Hi, I agree that uncompressed clay is a problem, but also it is worthwhile on flat tile like pieces to carve a hatch pattern on the back when the piece is leathery hard. This helps if the piece is not the same thickness overall so it removes some of the clay in the thicker areas, and if you do get a crack it often is unable to totally spread across the whole piece. I know a crack of any size is a drag but I have found with my students that if it stays in one piece and they can take it home and not all the work is lost they consider it a success. Hope that helps a little. Trina Link to comment Share on other sites More sharing options...
shiro Posted December 24, 2011 Author Report Share Posted December 24, 2011 I think it could be a compressing problem. Since I can't always keep an eye on everyone I usually give plaque making to people who I've already shown how to do it, but maybe they have forgotten to roll both ways. We'll have another go after the holidays and see if that's the case. Thanks for the tip on carving hatch pattern on the back Trina, we'll try that too. Diana, we don't use alumina, they are just laid on the shelf. But previously, (for years before I was recruited) they have been fired that way with no trouble. Devany, yes 3 days is ages, before the elements went we were able to put the kiln on in the morning and it was ready to unload the next day. Can't wait till the kiln is serviced so it's working right again! Thanks for the replies, I really apprecaite the help. Link to comment Share on other sites More sharing options...
Guest JBaymore Posted December 24, 2011 Report Share Posted December 24, 2011 With flat items glazed only on one side... I'd also wonder about the possibility of a difference in the Coefficient of Reversible Thermal Expansion (C.O.E.) between the glaze coating and the clay body. If the glaze is putting the body in significant compression on one side, it can cause dunting cracks upon cooling. Please look at the cracked pieces carefully. Is the crack through the glaze sharp and precisely defined at the edge of the glass? Or is the glaze layer slightly rounded or pulled away (really tiny amount) from the edgers of the crack? If it is sharp, it is a cooling crack, happening after the super-cooled liquid glaze has set. The would be a key point in see ing if the cracking is happening on the heating phase up to and including firing end point, or on the cooling phase. best, ....................john Link to comment Share on other sites More sharing options...
shiro Posted December 28, 2011 Author Report Share Posted December 28, 2011 Ok, thanks for the tip, I'll have a close look at the cracked pieces when we get back after the holidays. Good to know what to look out for to help diagnose the problem! Link to comment Share on other sites More sharing options...
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