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rcwilson

Firing Supports For Glazed Work

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What's the best way to prop or support a sculpture in a glaze firing to prevent slumping?  I know greenware props made from the same body as the work are commonly used to prevent slumping/warping during a bisque firing.  But what about slumping during the subsequent cone 6 glaze firing?  If I use a bisqued prop under a glazed part of the work in the glaze firing, how can I keep the prop from fusing to the sculpture. Figure a sculpture about 10" - 12", made from a Laguna cone 5/6 porcelain (no grog), bisqued to cone 07, glazed with a commercial cone 5/6 glaze, then to be glaze fired to cone 5/6 in an electric kiln.  Any ideas would be most appreciated.

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I don't know of anything that you can use as a support on a glazed surface even if it doesn't stick it will mar the surface.  The only thing I can think of that you might try in a test  is a thin areas in the glaze and broken segments of coils from the wiring removed from a kiln when it was rewired.  Next sculpture you make try some clay that has more strength so you don't have to worry about slumping or warping.      Denice

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 (Next sculpture you make try some clay that has more strength so you don't have to worry about slumping or warping.)

This is a key point for sculpture.

I fired a bunch of sculptures for someone about 15 years ago and they never got this point-they always slumped and he refuzed to change bodies. I gave up after 1 year of doing this for him.The right body is one that will not move at your end temp. 

Mark

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Mix some alumina in a small container of wax resist and apply it to the prop that touches your unglazed sculpture. The alumina in the wax resist will keep the porcelain from fusing. At cone 6, you will not be able to have a prop touching a glazed surface.

 

When I was teaching hand-building, students would often want to put some strange appendage or whatever on a form or make a huge item that was taller than the kiln; my first question to them was, "How do you intend to fire it?" Virtually every time the reply was, "I hadn't thought of that." How to incorporate firing is an important part of the overall design. It is also something we often fail to teach to new potters.

 

As Mark indicated, some clay bodies work better for sculpture and have more strength . . . usually those with grog. One option would be to make the item with a groggier body, then cover the item with a porcelain slip to give it a smoother surface.

 

Could you add a picture of the item . . . would be helpful in offering more specific advice/suggestions.

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Thanks for the input.  I'm basically trying to understand the process that Boehm Porcelain uses to produce those impossibly thin leaf, flower, and bird-with-outstretched-wings sculptures.  I'm wondering if I misunderstood their process as to glazing.  I'm now thinking that their process is more like this:  the artists first assembled slipcast porcelain parts by slip-n-score to form the final figures.  These were then once-fired to maturity with no glaze, probably to cone 6.  Because they were so thin and wispy, they required props to prevent slumping.  The props were custom-built from the same porcelain body so as to shrink along with the sculpture.  With no glaze, there was no problem of props fusing to sculpture.  After this body firing, colors were added using overglaze enamels (china paints in an oil medium, same technique as bisque-painting of porcelain doll heads).  This required multiple very-low-temp firings to build up the colors, only to cone 020 or 018.  Overglazing directly on unglazed mature porcelain produced a matte finish with a variety of bright colors.  The overglaze firings were so cool there was no danger of slumping, so no need of props.  Does this sound like a reasonable analysis?  If so, I think my original question is moot, as there is no need of a cone 6 glaze firing that could cause slumping, so no props that could stick in glaze.  Does this make sense?

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Here is a porcelain figure that I believe uses the techniques I just described.  All these figures are modest in size and would easily fit in my electric kiln.  Since we can look at this finished product, we know it can be done.  There are many examples of these figures with more extensive coloration on the propped surfaces.  I'm betting this is done at a very low temp.  Does this seem reasonable?

post-64237-0-52160300-1434151813_thumb.jpg

post-64237-0-52160300-1434151813_thumb.jpg

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I believe your process thoughts are very close to correct. Its low fire work and my guess is its not even gone to cone 6 but a lower cone .I am far from an expert on this sort of commercial wares.

 

Mark

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yes, the first firing was to the highest temperature and decoration came later at lower firing temps.  there are some wonderful videos showing work done in england during the heyday of the potteries at stoke on trent.  the ones i remember were for the wedgewood factory.  the kind of work is not the same as the boehm pieces but it is done in the same series of processes.  try finding it and watch what people used to do at work.  every day.  for years.  they were strong.  watch those guys taking saggars filled with dinner plates up ladders to the top of the kiln.

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A couple of possible relevant quotes on hight-bisque low-glost:

 

from: http://lindaarbuckle.com/handouts/bisque-firing.pdf

Although industrial china is often bisqued high (so it can be
supported while being fired to the clay body’s maturity) then
glazed lower (using binders and gums in the glaze to help it
adhere to a body that is no longer porous), studio potters
usually bisque lower so that the work remains absorbent and
easily glazed.

 

 

And a rather ambiguous quote on the process used by Boehm.

from: http://library.uthscsa.edu/2011/09/boehm-birds/

Fine Porcelain Creation

The name “porcelain†was given to translucent vitrified stoneware in China by the explorer Marco Polo in the 13th century. He thought that it resembled a certain seashell named genus porcellana because of its high gloss and translucency. Porcelain is made up of a high temperature (2400° F) fusion of fine white clay and feldspar. To make a sculpture like the Boehm birds, the figure is first modeled in clay or wax. A mold is made from the figure (or many molds in the case of complex figures) and a cast is made by pouring the fine porcelain mixture into it. After the lining of this mold has hardened, the liquid center is poured out, and the mold is removed. At this time if the model was made in sections, the sections are assembled, and fine details are added by hand. The figure is placed in a kiln for twelve to 24 hours, then cooled for three days. At this time it is in its “bisque†state, and may be colored and then glazed, if desired.

 

I think that this is saying that a high bique was used (vitrifying the body), and a later

glost stage was optional. Obviously there would be the usually difficulties in applying

underglazes and/or glazes to a vitrous body.

... opinions to the contrary welcomed.

 

Regards, Peter

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I was able to find the Wedgwood videos on Youtube - very interesting, and the narration was delightfully droll. Thanks for the tip.

 

Also good to note that unglazed porcelain props may stick to a sculpture during a midrange firing.

 

I'm counting on the china painters to have lots of experience getting pigments to stick to a vitrified body during a glost firing, I guess that's their full-time game.

 

I'm going to give this technique a whirl with low expectations- I've read that even the Boehm ceramicists lose nearly 20% of their work. Lots of opportunity for things to go wrong.

 

I've also just read that Boehm is having terrible financial troubles these days. Really a shame - they may close down in the US and go entirely overseas.

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(I've also just read that Boehm is having terrible financial troubles these days. Really a shame - they may close down in the US and go entirely overseas. )

This has happened already in Britain years ago. Also the market for sales I bet is very weak now.

Mark

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i can still remember the day i found a boehm swan inside a glass case in one of the really fancy hotels in washington, d.c.  i was on my way to lunch with a customer and turned a corner in a darkish hallway and there was a life size almost real feathers and all swan at eye level and it was looking at me. i don't remember anything else for that whole day but that swan had eyes that made it look real.  

 

i felt like a kid in a candy shop.   well, at those prices, candy shoppe.  at several thousand dollars, the buying public is rather limited.  and we are a ripped blue jeans kind of society, most people would not even recognize good stuff on sight.

Rae Reich likes this

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Here is a porcelain figure that I believe uses the techniques I just described.  All these figures are modest in size and would easily fit in my electric kiln.  Since we can look at this finished product, we know it can be done.  There are many examples of these figures with more extensive coloration on the propped surfaces.  I'm betting this is done at a very low temp.  Does this seem reasonable?

 

Incredibly beautiful. Thanks for posting. 

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