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Blisters In Electric Kiln


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Hello everyone,


I'm looking for advice on glaze blisters. I have been firing a manual 18" electric kiln for around a year with generally good results except for slight 'orange peel' texture and very small pins that I can't seem to get rid of. My problem now however is with larger burst blisters.


The kiln does not have a vent or a pyrometer, I fire the first half of the cycle with both peepholes out, and the last portion with the top peephole out. The glaze/clay combination in question has fired consistently well until now.


I just replaced the kiln elements. I bought the kiln used and have never before replaced the elements (didn't know any better). One day the kiln didn't go up to temperature and I checked the elements. They were in really bad shape, very bunched in places/fused together, and almost strait in others.


So I replaced the elements and fried an empty kiln to cone 04 load to break in the elements, and then a full bisque fire to cone 04. I glazed the pieces from this bisque and fired to cone 5 following my usual glaze-fire procedure (below)


2.5 hour soak on low to burn out excess moisture from glazing

Put the top knob on 5 and the bottom on 6

1.5 hours to glowing elements put in bottom peephole

4.5 hours to bright red, raised bottom knob to 7

1 hour to very bright red raised top knob to 10 and bottom to high

3 hours lowered top slightly because it seemed to be firing fast

5 hours to end


Total firing time 17.5 hours (about the same time as with my old elements)


According to my cones the top and bottom were both fired identically to cone 5 (not over or under fired at all). The middle shelf did not have a cone on it (my bad I forgot) so I don't know the exact temperature. Usually (with my old elements) the middle shelf fires 1/2 a cone higher than the top and bottom, but with my glazes this is not usually a problem.


My problem with blisters was only on the middle shelf. The top and bottom shelves had the normal small pinning and orange-pealing, and one or two random blisters, but all the pieces on the middle shelf were covered in blisters. The same glaze on the top and bottom shelves fired fine. The blisters are open, I can see the clay through them. The blistering is only on the exterior of the pots and is fairly evenly distributed, although there seem to be a greater number of blisters along the 'hip' of the bowl (where it flares out from the base). There also seem to be slightly more blisters on the sides of ware facing the elements. The colors all developed like they were supposed to.


The glaze that blistered was Mayco Stoned Denim applied to 3 coats on interior and 4 coats on exterior of bowls. The glaze claims to fire fine from cones 5-6 up to cone 10, so I don't think over-firing is the problem. The clay is cone 4-6. I have not had a problem before with this glaze or clay so I think it must be something with the firing process or to do with the new elements.


Sorry for the long post, just trying to give as much information as possible since there are so many variables.

I'd really appreciate some advice on how to fix this problem/why it might have happened.


Thanks in advance!

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I gave up using stoned denim. When I first started potting I bought that glaze before I started making my own glazes. I used it several times and it always did what your describing here. I eventually just sat it on the shelf and gave up. I think the glaze needs a slow cool or a hold on the way down to cool and smooth those blisters over. The only successful stoned denim pot I ever saw was a satin surface, which implies they probably slow cooled it.


Maybe someone else who fires it successfully will chime in, but that was my experience with it. 


Edit: I misread your post, you have used it successfully before.  I would get some cones on that middle shelf and see what your middle shelf fired at. 

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Sorry about your problems with Stoned Denium.  It is one of my favorite glazes.  I fire a 1027 to cone 6 using this glaze dipped. Up to cone 6 in 7 1/2 hours with vent then regular cool down.  Nice shiny glaze.    My only worry is that it seems to be really thick out of the dip, but I have never had a problem with it. 

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I love Stoned Denim. I've never had a problem with it. I dont think I've ever even had a single bubble. Electric kiln, fired ^5-6. I can show a dozen things my dad and I gave made with this glaze that turned out beautiful. But. Do you brush or dip? And what clay(s) are you using?


I brush and I've used it mostly with B-Mix and WS-5.



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Thanks for replying everyone!

Bill, it's one of my favorite glazes too (why its so frustrating now!), it usually comes out great. One question, could you define regular cool down, do you just turn the kiln off or do you step it down?

Giselle, I brush the glaze on, 3 coats inside and 4 outside (because it usually comes out thin outside), but I recently bought some dry so I can try dipping. My clay is a mix of Standard s710, s112 and s308. Mixed because I recycle them together. The recycling batch is the same as previous firings that came out good. BTW, beautiful mug! You slip-trialed the texture?

I thought maybe the problem was with the firing? Because I just replaced the elements?


Thanks again!


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Thanks for the replies everyone!


Neilestrick, I've been using a pre-mixed pint glaze, I just bought the dry but haven't mixed it up yet. Sorry if that wasn't clear.


I'm going to fire again in a few days and try a couple of different things (1/2 hour soak, different glazes on middle shelves, and try the stoned denim in different thicknesses) and take some notes and see where I stand after that.

Wish me luck!

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