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Newbie And Having Trouble :(


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Hello all, I'm new with ceramics and I enrolled a few months ago to a class in a clay studio where I have been learning one little step at the time and I love it so far :)

In the other hand, I am doing a legacy project at my son's elementary school as a volunteer. It's a tile mural. 120 tiles that make a big design: 83 where made by the children and I made the rest to complete the 120.

I did the tiles one by one out of clay 04-6. I followed carefully the rule of keep an eye at air bubbles ;)
The kids designed their own tile and then I bisque fired them and they turned out nice! Then we glazed them and they turned out not so nice :( Some of them look matte and feel rough to the touch. A very few of them look nice, soft and shiny.

I am using a new electric kiln at school. I bisque fired the tiles in setting "cone 05 low bisque" and then we used low fire glaze lead free, 2 to 3 coats. And fired them "cone 05 low fire glaze" setting.

My question is... Can I reglaze them? Just like that? Glaze again over the rough surface? and in setting cone 05 low fire glaze again?

Thank you very much!! Any information is highly appreciated!

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Sounds like maybe the glaze wasn't thick enough. Most commercial glazes need 3 coats, sometimes 4. You can glaze over them again and re-fire them. Fire slowly, as some clay bodies don't like to be re-fired.

 

I assume your mention of air bubbles has to do with thinking they will cause pieces to blow up? Not true. Think about adding handles or other attachments to pots- we score the clay, which makes tons of little air pockets which don't blow up. Steam is what causes things to blow up in the kiln. Air only expands about 1.5 times when heated up to firing temps. That's not enough to blow apart the clay. However water turning to steam expands 1,700 times. That is enough to blow clay apart. Bone dry pots have some water in them. It's impossible to have 0% moisture in the clay when it's in a room that's 30-80% humidity. So that water must evaporate at the start of the firing before it turns to steam. Pots that are thin have no problem drying completely at the start of a firing. Thick pots take longer to dry, so at typical firing speed a thick piece may not get totally dry before the kiln gets hot enough to turn the remaining water to steam, and BOOM! Kids pots tend to be thick, so they are more likely to blow up if the teacher doesn't slow down the firing or give them a good preheat in the kiln.

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Thank you Neil!

I think you're right. Some kids said they glazed their tiles 2 coats or 3, but probably they just did 2, they seemed to be well covered, though. But that must be the case.
So, I can just add more glaze on top and fire them again :) Should I set the kiln as "low glaze cone 04" this time?

Thank you for the air bubbles and steam information, it's VERY helpful! :)

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if you find it hard to apply more glaze to the fired pieces, heat them first.  a hair dryer works, putting them into an oven works.  it does not require very much heat to get them warm enough to accept more glaze.  once you have a thin layer of new glaze on the piece, you can add more without heating.

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