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g-bus

Please Help Diagnose Glaze Problem

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Hello,

I just did a glaze firing of some test pieces in my little test kiln and one glaze that I used on a mug came out looking awful. I've used this glaze before with success, so wondering what may have caused the problem. I'm relatively new to working with glazes outside of a classroom setting, but haven't seen results quite like this before. I tried looking it up but nothing really fit this particular issue, or maybe I was phrasing my question wrong. Either way, here are the details:

I was using Mottled Turquoise by Blick (one of a bunch of jars of glaze I acquired after buying out someone's small home studio), cone 4-7. The glaze had pretty much dried out so I added some water and mixed it up using an old handheld immersion blender. I then applied with a brush (label recommended 3-4 coats), however it was pretty lump and didn't go on very smoothly or evenly. So after the first coat I used a slightly damp sponge to try to smooth out the glaze. I then diluted the jar a little more in hopes of attaining smoother coverage and added 2 more coats. I poured some into the inside since brushing it was a pain, and that actually came out pretty OK. Everything looked good and even, so I fired up the kiln. Fired to cone 5, but was kind of a quick fire, about 9 hours. The glaze came out really lumpy and not even close to turquoise, was more brown. Also, when I picked up the mug I could hear and feel it sort of crackling under my fingers. This leads me to believe there are some sort of adhesion issues, and possibly the clay body and glaze shrinking at different rates. The clay is sort of a mystery clay that I got for free, but everything else I used it for has seemed to have come out fine. I had another glaze produce similar results on a clay that I was very familiar with and was fired in a different kiln. The handle on the mug in question looks good, but pretty much everything else looks terrible. The other pieces fired with it were just test tiles with clear glaze over underglazes and all looked fine. Any help would be appreciated. Thanks in advance!

mug1.tiff

mug1.tiff

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I do not deal with premix glazes- others can answer in specifics. However, the pin-holing and severe glaze runs on the left side suggest it was over fired. Same clay in the mug and test tiles? That would tell you something as well.

Nerd

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You may have watered the glaze down too much when rehydrating; looks like the application was too thin and uneven. Too thin will also affect color.

 

The bubbles/blisters may be from to fast cooling. Did you kiln hold at top temperature before turning off and cooling? Dropping the temperature too suddenly can result in small blisters/craters as the boiling glaze is frozen as temperature drops.

 

Your results in a test kiln will likely vary from results in a full size kiln.

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Bruce:

You bring up a good point about the variances between test kilns and larger kilns being different. If you program a ramp of -300F an hour from peak: they will be much closer. It will also solve a few other problems as well.

Nerd

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I likely did all of the things mentioned: applied too thin/diluted too much, cooled too quickly, and possibly overfired because it was closer to the elements than the cones (manual temp control). I just turned it off when it hit temp for a bit, since it was about 4:15 am and had to work in a few hours. And it was a different clay than the test tiles, but I had another piece in there that was made from the same clay that came out fine. But it only had some clear glaze over underglaze and was sitting in the middle of the kiln. Well thanks for the feedback. I'll just chalk that one off to another learning experience.

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looks overfired...maybe it stood too far an the heating. I know this too. I take a soft brush after glazing. Whe the glöaze is dry enough then I clean the glaze with an brush. Then it gets more thin. Maybe you´ll better stand the Mug in the middle of your kiln. O you try to reduce the temparature at holding time for maybe 5-10 min. It works.

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Actually I'm pretty sure I ended up using a different clay for the handle. I was practicing pulling handles and I think I ended up going with one that was from a different bucket of clay (I've got a bunch of free clay from different sources that I've been trying to keep organized, but that's not exactly my strong suit). The handle is actually what the glaze is supposed to look like, so it's possible there were some clay/glaze compatibility issues. However most of the inside of the mug came out OK, so more likely it was my application.

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