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SammyJ

Mugs sticking during final firing

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I have researched your topics and did not find anything related to this. Of late in the final firing the edge of some of the mugs stick to the shelves. I have put extra kiln wash on the shelves, I wax the bottoms of the mugs, and I also wipe the glaze off on the edge of the bottom rim. I am so confused. could it be the clay even though it says cone 5-6 maybe is not? Thanks for any advice you can share.

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I have researched your topics and did not find anything related to this. Of late in the final firing the edge of some of the mugs stick to the shelves. I have put extra kiln wash on the shelves, I wax the bottoms of the mugs, and I also wipe the glaze off on the edge of the bottom rim. I am so confused. could it be the clay even though it says cone 5-6 maybe is not? Thanks for any advice you can share.

 

 

I always use stilts when glaze firing. I don't allow glaze on the underside and wipe off about an eigth of an inch all round the bottom. I tend to choose stilts slightly smaller than the diameter of the pot so that the unglazed bottom sits on the stilt points. It's odd that it's suddenly started happening that your mugs are sticking. Have you altered your glaze recipe at all? Are you perhaps using a particularly fluid glaze? At any rate, I feel stilts are the way forward!

 

Christine

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A little more info would be helpful. What type of clay body -- stoneware or porcelain? Is the mug sticking to the shelf and just picking up kiln wash? Is the mug cracking and leaving part of the foot on the shelf? Is the glaze overrunning the foot and adhering to the shelf?

 

Every potter would be well served to have one or both of two books that help you troubleshoot problems: Frank and Janet Hamer's Potter's Dictionary of Materials and Techniques and Harry Fraser's Ceramic Faults and their Remedies.

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I noticed a couple of my thin porcelain pieces stuck during a bisque firing when fired to ^6, so I just readjusted to a ^4 and haven't had any trouble since.

 

Perhaps, your kiln is firing too high (?), or maybe you need to adjust your cone number? It was such a pain, because it looked like I started randomly chiseling sections of the bottom rims on my pieces. Not to mention, I had to take my own chisel to the shelf to get the excess porcelain bits stuck on there.

 

Just a beginner's thought, and hypothesis....

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Sounds like you have experienced 'Plucking' which is the tendency of very vitrified clay to stick to shelves and leave bits behind. Plucking can be a problem with porcelain due to its high level of vitrification and high levels of fluxes in the clay body. Possible remedies include lightly applying alumina on the shelf, using a high-alumina kiln wash, or adding some alumina to wax that is applied to the foot of the piece.

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Sounds like you have experienced 'Plucking' which is the tendency of very vitrified clay to stick to shelves and leave bits behind. Plucking can be a problem with porcelain due to its high level of vitrification and high levels of fluxes in the clay body. Possible remedies include lightly applying alumina on the shelf, using a high-alumina kiln wash, or adding some alumina to wax that is applied to the foot of the piece.

 

 

Good to know! Thanks for sharing!

 

I had never had an issue with my stoneware, so it must just be a porcelain thing.

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Sounds like you have experienced 'Plucking' which is the tendency of very vitrified clay to stick to shelves and leave bits behind. Plucking can be a problem with porcelain due to its high level of vitrification and high levels of fluxes in the clay body. Possible remedies include lightly applying alumina on the shelf, using a high-alumina kiln wash, or adding some alumina to wax that is applied to the foot of the piece.

 

 

Good to know! Thanks for sharing!

 

I had never had an issue with my stoneware, so it must just be a porcelain thing.

 

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Thank you so very much for all of the information. I had for years purchased from one supplier of clay, this last clay was purchased from someone else. I recently found out that all clay for the same temp is really not all the same. The recipe can and usually are different. The end result, I think, is lowering the temp of the kiln somewhat. Again thank each of you for your thoughts and advice.

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... in the final firing the edge of some of the mugs stick to the shelves. ... could it be the clay even though it says cone 5-6 maybe is not?

 

Sammy, are your mugs sticking because of glaze running/sagging onto the shelf, or are they victims of the "plucking" situation someone else mentioned they were having? When I read your original post I assumed it was glaze running and the following comments pertain to that situation...

 

First, I hope you're using witness cones (preferably on every shelf) when you fire so you know whether your kiln is actually firing to the temperature you think it is. Far too often people think they have a glaze problem, only to discover they have a kiln/firing problem... ie, a malfunctioning kiln sitter or thermocouple, a burned out or degraded element, hotter/cooler spots in the kiln, etc. Commercial glazes are usually tolerant of a one-cone variation, but that's not a universal truth for all situations.

 

Second, different clays contain varying amounts of substances which can affect glazes dramatically (eg, color, running, etc.). For example, I recently fired a (new to me) commercial glaze on some b-mix and speckled buff test tiles... the result was a beautiful translucent blue on b-mix and a mediocre green on the speckle buff. It sagged on the b-mix and ran on the speckled buff. While overlaps smoothed out nicely, the color differences where overlapped was VERY noticeable on both tiles... hence, pouring/brushing the glaze could be problematic if one didn't want a somewhat "mottled" color effect. All in all, I concluded this glaze was problematic for my intended purposes but would make a very nice glaze for white clay functional ware that I dip.

 

When you glaze fire there are many actions involved... the heat, the chemical reactions of the glaze, and the chemical reactions of the clay, etc. To some degree, they are interrelated, but not totally. If you are electric firing using commercial glazes, for a given clay you can only control the heat and the thickness of the glaze(s). Layering one glaze on another can make a stable glaze run due to the chemical reactions between glazes and/or the added glaze thickness... we often use this to our advantage to produce attractive visual effects in our glaze work.

 

Lastly, I assume you're clear on the use of wax, but if not... wax is used to mask the foot of the piece when you glaze so that there is no glaze in that area when you fire. Any specks of glaze which "stick" to the wax must be wiped away before firing. Wax does not prevent glaze from "running" during the firing process... wax vaporizes and is long gone at a much lower temperature than the temperature at which the glaze melts and the clay vitrifies. Depending on how much your glaze "moves" (ie, sags, runs) at the temperature you fire will determine how much of the foot of your piece you need to leave unglazed... some glazes need only an eighth-inch, others a quarter-inch, and some more... sometimes several inches.

 

I wouldn't fiddle too much with the firing temperature unless you've confirmed via witness cones you're firing too hot. Since you are making mugs (functional ware) it is best if you fire your clay to it's vitreous state... ie, to the cone rating the clay manufacturer specified (you said 5-6, so 6 is where it probably reaches it most vitreous state). This is important because any glaze defect (an imperceptible pinhole or crack, for example) on the inside of the mug will let liquids come in contact with the bare clay and be absorbed. For example, suppose your customer uses the mug for soup... meat juices might be absorbed and become a potential spot for bacterial growth. The more vitreous the fired clay, the less liquid is absorbed... for functional ware it's best to use clays with absorption specs of 1.5% or less, the lower the better. Don't go overboard worrying about this... just know the risks so you can choose your clays, glazes, and techniques to be as compatible as the situation allows.

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I agree with those who suggested using alumina hydrate. You can just mix up a little with water and brush it on to keep porcelain lids from sticking or brush it on the bottom of the pot to keep the pot from sticking to the shelf or picking up kiln wash. Sometimes I brush on a thin layer of alumina hydrate over the kiln wash on shelves. And, as suggested above, mixing alumina hydrate into your wax resist works well, too.

 

Jim

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