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I have been having an issue with the bottoms of my pots getting big chips out of them during a glaze firing.

The chipped parts are not really stuck the the kiln shelf, they brush off easily, so I don't think it's a glazing issue. Normally I dip glaze pots with clear and wipe off the bottom with a sponge. I wonder if a little residual glaze is still there and sticking?

 

This is happening with my cone 6 porcelain (standard 365) and a black stoneware clay. Both have a high shrink rate. 

 

I do use kiln wash. Do I need to apply kiln wash for every firing? It seems like that must be the issue but on some shelves the wash is getting so thick that it flakes off. Should I be removing the excess and re applying fresh kiln wash?

 

 

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It is called plucking, often happens with a porcelain or a porcelaineous stoneware. From your description, it sounds like you do not wax bottoms. That could be a first step. Or add some alumina hydroxide hydrate to wax -- the alumina in the wax will help keep the wares from sticking to the kiln wash. Or, use wadding.

 

Edited 5/12

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Looking at your kiln shelf pic it also looks like your glaze is running more than you are allowing for. During cooling, if the glaze is stuck to the shelf you will lose a little of the pot. I don't have much experience with porcelain, but know that many times they will run unless the glaze is properly adjusted for the clay.  Knowing that I am not that experienced, I could be completely off base, but that is my opinion. Bruce is also correct about the term, plucking, and that give you a starting point for an internet search.

 

 

best,

Pres

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What's your kiln wash recipe? I would scrape/grind off the old stuff and re-apply, couple coats, let dry a few minutes between. Kiln wash gets less effective the longer it's been on the shelves when you have glazes that fume a bit. Are those dark circles glaze or clay? Is there glaze right down to the bottom of the walls?

 

BTW, this is the kiln wash recipe I use, works well. 

50 alumina hydrate

25 epk

25 calcined epk

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I just buy pre-mixed powdered kiln wash but I also have alumina hydrate so I'll try adding a bit of that to the mixture. I don't normally wax bottoms, but I'll give that a try too to see if it helps. I usually wipe the bottoms plus at least 1/4 inch up to the bottom. The black signature is underglaze so that isn't the issue. 

 

Do I need to apply fresh kiln wash every glaze firing?

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I just buy pre-mixed powdered kiln wash but I also have alumina hydrate so I'll try adding a bit of that to the mixture. I don't normally wax bottoms, but I'll give that a try too to see if it helps. I usually wipe the bottoms plus at least 1/4 inch up to the bottom. The black signature is underglaze so that isn't the issue. 

 

Do I need to apply fresh kiln wash every glaze firing?

 

Premixed commercial kiln wash is junk. Add about 25% more alumina to it and it'll work.

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Grind off that junk commercial kiln wash and use the recipe Min suggested and this will stop happing f4rom that moment on.

That is the same recipe I use with all my porcelain fire to cone 11 with zero plucking .

The alumina is the key thing you need-commercial wash is not good.

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I make 1 inch by 1/4 inch clay "coins" and put some of  them under all of my  pieces in the kiln. This allows heat circulation under the bottom and keeps it 1/4 inch above the shelf. Never get any sticking.

 

PS... You still need kiln wash on the shelves for the pot that drips.

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you mention that the chips "brush off" easily.  do be very careful with the colored circles.  do not touch with your bare hand because you could slice a deep cut into your hand of fingers.   if glaze sticks it can leave very sharp lines that you cannot see well from above and you may be tempted to try to brush it off with your bare skin.

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