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I want to glaze the bottom (and inside) of a bowl and leave the rim unglazed so that it can be placed in the kiln upside down, with the rim resting on the kiln shelf.  The bowl will be fired to cone 10 and I will use a stable glaze.  I am wondering if there is any technical reason that this should not be done.  Maybe this like firing a glazed lid, which I have never done.

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Make a wafer thin circle of clay a titch bigger than your bowl, dry it between batts or whatever so it stays flat then bisque it to same temp as bowl is bisqued to. Glaze fire the 2 pieces together. If your bowl is porcelain it might want to stick a bit to the waste piece so brush it with a wax / alumina hydrate mix before putting the bowl on it.

 

I fire all my covered butter dishes this like, lid separate on a waste piece. They are porcelain and stay true.

(from the kiln this morning)

 

post-747-0-41168800-1463513114_thumb.jpg

post-747-0-41168800-1463513114_thumb.jpg

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Bad idea all around but you will learn why by doing it.I would try it as it will be pushing the envelope and you will learn from it.

My 2 cents-The bowl will distort and my guess the inverted cone shape will also favor a glaze run.

Mins waster ring above will solve the distortion factor.

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I routinely fire glazed cups, bowls, and whatever on seashells filled with wadding.  The shells leaves a mark where it touches the glaze. Soak the marked area in water to crumble the white stuff (aka calcium oxide) that remains after the shells are calcined by the firing.  The mark can be polished with a whetstone.  

 

the rims and walls of bowls and cups will distort if the weight of the object is greater than the strength of the material but that is a pot design problem, not a firing problem. 

 

The technique works fine in a combustion kiln, but there is no obvious reason not to use the technique in an electric kiln. 

 

Testing is advised, and YMMV.

 

LT

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the rims and walls of bowls and cups will distort if the weight of the object is greater than the strength of the material but that is a pot design problem, not a firing problem. 

 

The problem is pyroplasticity. The clay softens as it reaches its maturation point, especially with porcelain. No matter how thick or thin it is, it's likely to warp when there's no bottom.

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Make a wafer thin circle of clay a titch bigger than your bowl, dry it between batts or whatever so it stays flat then bisque it to same temp as bowl is bisqued to. Glaze fire the 2 pieces together. If your bowl is porcelain it might want to stick a bit to the waste piece so brush it with a wax / alumina hydrate mix before putting the bowl on it.

 

I fire all my covered butter dishes this like, lid separate on a waste piece. They are porcelain and stay true.

(from the kiln this morning)

I did not know this. Great tip Min. Thanks.

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I have fired about a dozen bowls in cone ten reduction in a similar fashion and have not noticed any warping. I was using Coleman's porcelain for some and B-mix for others. These were small ( 4"  - 6" diameter) and relatively thick walled, so you might get different results with your own work, but if you think it's going to suit your piece better to glaze the foot it's worth trying.

 

I also have had success using shells with wadding as LT suggested. It will leave some marks, but so long as you plan for that it can become part of the design instead of a flaw.

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Very interesting responses, thank you all so much.  As long as I don't have to worry about explosions, I will try the wafer and be on the alert for warping.

The reason for wanting to glaze the bottom is that I often eat out of bowl while cupping the bottom in my hand.  I thought that the feeling of a continuous curve, uninterrupted by a foot ring, would be nice.  Of course, I could just leave the underside unglazed. But thinking about this I wondered why I never saw a handmade ceramic bowl with a fully glazed bottom and unglazed rim.

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Min-say a little more about the "...wax / alumina hydrate mix ..."  What is a good ratio of wax to alumina and--ignore my ignorance--do you mix wet wax and dry alumina???? I am beginning to work with porcelain (^6/electric) and this sounds useful to me! 

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Min-say a little more about the "...wax / alumina hydrate mix ..."  What is a good ratio of wax to alumina and--ignore my ignorance--do you mix wet wax and dry alumina???? I am beginning to work with porcelain (^6/electric) and this sounds useful to me! 

 

Hi Lee,

 

Just liquid (cold) wax resist and alumina hydrate. I don’t measure but mix up approx 1/4 to 1/2 cup wax to a heaping teaspoon of alumina hydrate, I add some food colouring to it as well so it shows up better on the white clay. Same stuff for brushing inside galleries, or anywhere 2 pieces of clay might stick. Don’t get it on any of the glaze or it will wreck the pot; it will leave a rough patch since the alumina hydrate won’t melt into the glaze. 

 

Porcelain can “pluck†since it has a higher percentage of flux in it than stoneware or earthenware, more flux equals more soft almost melty the clay gets. Keep your kiln wash in good shape too since porcelain pots can pluck on the foot if the wash gets ineffective.

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lbbloom, holding a bowl without a foot might be comfortable in your hand but how did you fill it?  if it holds liquid, where do you put it down?  are you making a donut to hold it when you do put it down?  i just cannot see how it is a practical pot. :unsure:

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An addendum to Min's comments about alumina hydrate in wax. It is a great tool for the times you need it. In addition to the mentioned porcelain usage, bottoms of large flat platters should be waxed with this so the whole platter can shrink without restriction from the friction of the platter bottom on the shelf. You can also use it to wax the gallery of a lidded pot to keep it from sticking.

 

Finally, a note about mixing and application. I like to put some food coloring in the wax so I know where the wax has been applied to the pot. I use green for regular wax and blue for the alumina wax. (Don't use red unless you work exclusively with white clay - the red wax does not show up against the pink color of bisqued brown clay.) And when using the alumina wax, you must constantly stir it to keep the powder alumina hydrate from settling to the bottom of the wax jar. There is no flocculating the wax as with glaze... ;-)

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 But thinking about this I wondered why I never saw a handmade ceramic bowl with a fully glazed bottom and unglazed rim.

I have only seen unglazed rims on bowls in museums i think either from africa or papua new guinea. their bottoms usually have pointy ends. i have not figured out what the purpose of those bowls were and why they were pointy ended.

 

the unfinished rims i would imagine would not look good. would look incomplete.  if you are going to slurp off the bowl you don't want rough rim on your lips. i wonder if cutlery will chip an unglazed rim over time. 

 

what i really do enjoy myself are round bottom bowls without glaze on the outside and glaze right up the rim. when i use nice clay or oxides i really like leaving the outside unglazed because i like the snug feeling as well as a good comfortable grip. i make the clay a little thicker and make cereal bowls to have hot soup in winter. warm bisque bowl on a cold winter morning is wonderful. fits the palm really well. 

 

i just did make a cup with cassius basaltic clay  ^5 glaze and i wiped too much off the rim so the rim is really bare. looks fantastic. but unfortunately because of the exposed rim it has now become a pencil holder rather than what it was originally meant for. 

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The reason for wanting to glaze the bottom is that I often eat out of bowl while cupping the bottom in my hand.  I thought that the feeling of a continuous curve, uninterrupted by a foot ring, would be nice.

 

One way you can approximate the sensation of a curved bottom is to trim it round, and then elevate it with three tiny legs, in the form of little balls or pinched and pulled legs.  All but the very bottom of the feet can be glazed, so that the pot rests in the hand like a glazed ball, with the minor distraction of the feet.  I have frequently used this approach with bowls, and with hand-held pipes.  One little advantage of the three-foot solution is that such bowls don't trap dishwasher water in their foot rims.  Another is that fettling the glazed pots is very easy.  You just wipe a sponge across the bottoms of the feet and you're done.  And to oldlady's point, a three-legged bowl is very stable sitting on the table.

 

One thing you might consider is that if you don't glaze the rim, silverware may make an unpleasant noise as it scrapes across the rim.  On the other hand, some of the greatest bowls ever made were fired rim to rim in stacks.  If you're only using chopsticks, it would be less of an issue, but also remember that you will have your lips against the unglazed rim if you are drinking broth from the bowl.  From an esthetic point of view, I would miss having a glazed rim, because it is such an expressive element in a pot, and especially in a bowl.  Glaze can used to accentuate or define the rim in a great many ways.  And that's probably true of an unglazed rim, too.  Were I trying to design such a bowl, I think I would try to make the naked clay a forthright element of the overall effect.

 

I'm putting feet on a pipe in this snapshot.  I've also recently done a bunch of tiny amuse bouche platelets for my son the chef-- shallow bowls elevated on little feet.

 

gallery_65900_888_66051.jpg

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preeta, the assumption by the "experts" about the pointy ended bowls is that they were formed that way to stand upright in the ashes or stones of an outdoor cooking fire.  it is also possible that they are made to fit inside three legged stands.  

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I think I remember hearing an archaeologist once explain that the pointed bottoms of some amphorae and jars were made so they could be easily buried in sand to help keep the contents cooler in desert climates.

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