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Glaze Problem on slipcast items


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#21 etched

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Posted 16 March 2013 - 06:28 AM

I was having a similar problem a few months back. I found out that the glaze that I was using a store bought one had one of the components changed. It was no longer available here in Australia and was now from an overseas supplier and it was slightly different. This glaze was now crawling if you used the recommended 2 coats. When you only used one coat it was fine.

could this be similar to what is happening to you?

#22 AtomicAxe

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Posted 17 March 2013 - 10:54 AM

Epson salts in the glaze won't affect this. The amount needed to cause changes is about 200% more than the glaze itself since most will burn off before it even decides to turn to a flux.

To be honest, I highly doubt even soaking it will affect the glaze, since i just spent about 15 minutes staring at the photo ... do you have glaze splatters on the kiln shelf? since the sections are missing, the surface of the clay is glazed thinly and there are no hard ridges, (i.e. if it chipped off in cooling it would be hard edged and would indicate shivering) ... I can almost assure you that your glaze is not firing slowly enough to let the glaze properly deal with gasses so it just pops out as it melts. Inside wouldn't be affected since it's a little more protected from external heat from your elements. Try to fire slower. as this happens when the glaze melts and doesn't have a chance to slowly outgas. Which is the moste likely problem since it looks like it has extremely thick edges around the missing glaze spots. or it happens beforehand you're glaze even gets to properly stick ... at which point ... i would suggest a different glaze ... you will have to experiment with ramping speeds at how slow and when you need to ramp. Sorry man, without seeing you fire, or knowing every inch of your process after you put that glaze on there ... it's scientific method time, chart what you do, and when you do it, then try to change variables. also try to use the slip to make some non-poured items and glaze those to see if it does the same thing as well as thicker pieces.

Personally though, I would probably change both the clay and the glaze to eliminate both variables since something has changed and not for the good. The cost associated with testing will probably not be worth trying to fix the problem unless you want to do it on the side so eventually you can either go back to it or have options for merch. since the problem is probably not the clay, I would use that for items not cast if you do change your slip cast ware process.

#23 Thrown_In_Stone

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Posted 18 March 2013 - 05:32 AM

Epson salts in the glaze won't affect this. The amount needed to cause changes is about 200% more than the glaze itself since most will burn off before it even decides to turn to a flux.

To be honest, I highly doubt even soaking it will affect the glaze, since i just spent about 15 minutes staring at the photo ... do you have glaze splatters on the kiln shelf? since the sections are missing, the surface of the clay is glazed thinly and there are no hard ridges, (i.e. if it chipped off in cooling it would be hard edged and would indicate shivering) ... I can almost assure you that your glaze is not firing slowly enough to let the glaze properly deal with gasses so it just pops out as it melts. Inside wouldn't be affected since it's a little more protected from external heat from your elements. Try to fire slower. as this happens when the glaze melts and doesn't have a chance to slowly outgas. Which is the moste likely problem since it looks like it has extremely thick edges around the missing glaze spots. or it happens beforehand you're glaze even gets to properly stick ... at which point ... i would suggest a different glaze ... you will have to experiment with ramping speeds at how slow and when you need to ramp. Sorry man, without seeing you fire, or knowing every inch of your process after you put that glaze on there ... it's scientific method time, chart what you do, and when you do it, then try to change variables. also try to use the slip to make some non-poured items and glaze those to see if it does the same thing as well as thicker pieces.

Personally though, I would probably change both the clay and the glaze to eliminate both variables since something has changed and not for the good. The cost associated with testing will probably not be worth trying to fix the problem unless you want to do it on the side so eventually you can either go back to it or have options for merch. since the problem is probably not the clay, I would use that for items not cast if you do change your slip cast ware process.





Thanks for your input. No, there's no glaze chips on the shelf at the end of the firing. I put another kiln load on yesterday, I'll see how this one goes.

My normal firing schedule is 3:30 to 500ºc then full power to 1060ºc. The firing normally takes around 9 hours.

The firing from last night I had it on a ramp of 4:45 to 500ºc then full power to 1060ºc with a hold of 2min. I increased the first ramp to ensure it was slower whilst any remaining water was pushed out from the glaze incase this was dislodging it. I guess for what you describe you mean slow the top end.

On my next firing should I slow it down by an extra hour?, ie something like this: 5:00 to 500ºc then 5:00 1060ºc?

I've never used a ramp at the top end, I hope the kiln can cope with it. It seems to go slowly when it gets above about 900. I don't want it to suddenly switch off with an error that it can't heat fast enough. I guess I'll not know until I try.




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