Porcelain Throwing Method

[Chris Campbell]

I just returned from the North Carolina Potters Conference. Our presenters were three established potters from Japan ... just amazing to watch as they worked.
There were many lessons learned but one excited all the throwers so I thought I would share.
From Fuku Fukumoto ... Google her images and enjoy the Artwork.

She centers and cones her porcelain, then cuts it off the wheel, turns it upside down and centers and cones again.
(many thought this reversed the twist that the first centering and coning process put into the clay) She just said it further compacts the porcelain making it easier to throw. The other presenters agreed that they knew many porcelain throwers who did this but did not do it themselves.

I am not a big time thrower so I do not have an opinion ... also, their porcelain is made from stone and is so grog free it is like butter ... 180 mesh as opposed to our 60 or so.

NOTE : see my later post ... on realizing their clay was stone based, so this could be why it works for them. I have to admit if I was a thrower I would definitely try it just to see what happens, but it also could be a useless extra step for clay.

[Roberta12]

Oh Chris!  Beautiful work!  I use a little bit of midfire porcelain.  I will try that technique.   Thanks for sharing!

 

Roberta

[ChenowethArts]

She centers and cones her porcelain, then cuts it off the wheel, turns it upside down and centers and cones again.

(many thought this reversed the twist that the first centering and coning process put into the clay) She just said it further compacts the porcelain making it easier to throw. The other presenters agreed that they knew many porcelain throwers who did this but did not do it themselves.

 

I am not a big time thrower so I do not have an opinion ... also, their porcelain is made from stone and is so grog free it is like butter ... 180 mesh as opposed to our 60 or so. 

 

@Chris Campbell

 

I have never heard of this technique, but it does warrant experimentation...fascinating concept!

Any idea what the porcelain recipe is? Is it made using a traditional sliphouse method (180-200 mesh)?

 

-Paul

[Evelyne Schoenmann]

Beautiful work. Thanks for sharing Chris!

 

I wonder what Antoinette (Badenhorst) says to that throwing technique. Antoinette, are you around?

[Chris Campbell]

I just realized that there is one crucial thing that might make this a 'no go' ... Their porcelain is made from stone not clay so this could be why it works for them and maybe not for clay based porcelain.

 

Yes Paul, is is around 180 - 200 mesh.

[Guest JBaymore]

There are many techniques that are used in Japan to deal with VERY different clay body types (from typical American) that are done to make it survive or work better.  If I use some of them here with our clays... it often doesn't work.  And if I handle the clay there like I do here..... often it doesn't work.

 

Example, there is one body I use in Japan that if you put it out into the sun to dry... it cracks almost instantly.  Anotehr if you cover it with plastic... it cracks.

 

best,

 

.........................john

[Chris Campbell]

I envy you working over in Japan John and picking up on their aesthetic. The same care and attention was put into their pieces whether small or large. Wonderful to watch a real tea ceremony too ... Well, as real as you can be with 200 potters watching your every move!

[Biglou13]

what do mean by stone based porcelain?

[Chris Campbell]

As I understood it, their porcelain body is not clay but very finely crushed 'porcelain stone' ... 180 - 200 mesh versus our clays which are around 50 - 60 mesh size. It feels like butter with no grog at all.

[neilestrick]

Interesting stuff. Thanks for sharing.

 

I'm not sure what you mean by our clays be in the 60 mesh range. A basic porcelain of grolleg, feldspar and silica would be particles in the 150 mesh and smaller range. White stoneware clays tend to be even finer grained due to the high amount of ball clays used in them. Are you referring to stoneware clays that use fireclay?

[Guest JBaymore]

"True" porcelain is a composition of just a kaolin and a single ground rock  - called p'tunse.  It is a high silica content feldspathic based rock.  Often also called "Porcelain Stone"   ç£å™¨çŸ³ )  Neil's basic "recipe" above would approximate a true porcelain (no ball clay) ....in basic chemical composition. Not particle size or distribution.

 

Might be a "Lost in Translation" moment.   

 

In Japan clay bodies are almost always wet blunged with a great excess of water from far less pure materials, through repeated smaller mesh screenings as it is moved to different blunging batches, and then is filter pressed to remove the last excess water.  This process produces really good quality clay out of materials that we would think of as "inferior" or "primitive" by our industrially refined standards. 

 

In America we tend to mix clay direct to the plastic state from industrially beneficiated (dried and airfloated, etc) clays with just enough water to make it workable.  This is NOT the way to make really good clay.  It is the way to make cheap (production-wise) clay. 

 

If we took the same kaolin and ground rock the Japanese (and Chinese) use for porcelain, and mixed up a body the way US suppliers typically do.... it'd likely be totally un-useable for forming.

 

The reason this labor and machinery intensive process  'works' in Japan is that the valuation for ceramic work is generally higher there.  And they are willing to have material cost a higher percentage of the sale price (indicating respect for good starting materials). Many ceramic centers mix up their own clays from mostly local materials (hence the visual distinction between pottery "villages" work).  Clay prices from suppliers in Japan in many/most places would shock you.  In America... many, many potters will go to another supplier if the price of a pound of clay is even one cent more.  No incentive in most cases for our suppliers to make better clay.

 

If that clay you got to feel was brought from Japan, Chris, it likely was produced by the blunging and the wet filter press method.  That is likely a portion of the buttery quality you mention.  And the repeated screening and settling process will take out the large particles so that is another part.

 

PS:  For porcelain (in Japanese "Jiki"   ç£å™¨  -gee key-), often in the blunging there is a huge magnet suspended in the tank with the mixing slurry... to take out the hematite (iron) nodlues.

 

best,

 

...................john

[Mark C.]

( filter pressed to remove the last excess water.  This process produces really good quality clay)

The clay I used for many years in the 70's and 80's was all filter pressed. The clay is Quyle kilns

Quyle Kilns still to this day make all their clay this way. (its a small clay company in the Ca, Serria mountains).I stopped used it as they did not make a high fire porcelain and thats what I wanted to focus on.

Its more labor to make . I also know a potter who is getting on in years -an Alfred Grad who blunges his own and has a small filter press-(about 12 feet long).

Its lots of work but the results are fantastic.

I would off to buy it from him but my clay making days are behind me.

He is in San Fran ,Cal area

Mark

[Tyler Miller]

Ever wonder why you get S-cracks?  I’ve often wondered too.  Turns out, it’s because the torque of the wheel is applying uneven strain to the clay column and some clay gets pulled forward, some pulled back.  Bottom compression works to fix this by pushing the clay back into itself, negating the strain.

 

Imagine a giant “s.†It’s not quite the right shape for this, but imagine the two curves are more elongated.  More like the shape of the two leading edges of an old fashioned airplane propeller.  That’s line takes the shape of where the forces in your clay column differ.  The clay on the inside of the curve is pulled back, the clay on the outside, pushed forward.  Or pulled forward and pushed back.

 

Now, imagine you took your centred clay column and flipped it top to bottom and threw as you normally would.  It would reverse the forces and push the pre-strained clay back onto itself, eliminating the need for bottom compression.  Ideal for throwing bowls off the hump, or for when you’re throwing shapes that prevent you from proper bottom compression (e.g. a proper Roman storage amphora)

 

 

It is not a material-specific technique, but one usable on any clay body or throwing technique prone to give you grief in the form of s-cracks.

 

Oh, and as far as potters are concerned petuntse = cornwall stone.  They’re technically different minerals from a geological standpoint (but then again, petuntse is a family of minerals of different geological types), but when crushed up and mixed into a body, they behave pretty much the same.  If you’re old enough to have thrown with a cornwall stone-based porcelain body, you know what throwing with petuntse is like.    Petuntse/Cornwall stone, being very high silica content (75%) feldspathic minerals simply negate the need for a silica addition to the body.  It’s true that different porcelains have different plasticities (indeed the same is true for all kinds of clay bodies), but that’s more a function of the clay content, than the fluxing mineral, or the water introduction method.

 

[lemon]

Interesting, thank you for sharing!

 

This sounds similar to the technique John Britt demonstrates in his video on throwing tumblers.

 

 

 

Lisa

[PRankin]

Thanks for posting that Lisa. He explains it very well why it's done like that.

 

Paul

[Benzine]

Tyler, thanks for the explanation.  I was always under the impression, that the s-crack was caused by uneven thickens on the bottom, leading to the thin spots drying faster and pulling away from the thicker spots.  Which is why I thought the compression solved this, as it evened everything out.  I never thought about  the the alignment of the clay particles.  

 

I enjoy watching John Britt's videos.  He always seems so laid back.  He kind of double compressed the bottom in that clip.  He did the compress, then flip, then he did, what I always do, and use the wood rib on the bottom.  I would imagine the percentage of wares with s-cracks, is near zero, with that method.

Honestly, I can't remember the last time, that I have had even a student work have a s-crack, with just the wood rib compression.

[Evelyne Schoenmann]

Thanks for sharing the Britt video, Lisa.

 

He works nice and fast. But he uses a lot of water?! I don't throw on bats but on the wheel head. I wouldn't be able to take the piece off for hours using water like this. I think I have to drill me some holes in the wheelhead for the bat possibility....

 

Evelyne

[Pieter Mostert]

Tyler, I might be misunderstanding your explanation, but if you flip the clay column, you'll still be twisting the clay in the same direction, so I don't see how this would compress pre-stretched clay.

I was under the impression that S-cracks arise from the centre of the bottom shrinking more than the outside, due to it not being compressed as much. Isn't the S shape just a result of how the clay particles are aligned?

[Chris Campbell]

Thanks for sharing the Britt video, Lisa.

 

He works nice and fast. But he uses a lot of water?! I don't throw on bats but on the wheel head. I wouldn't be able to take the piece off for hours using water like this. I think I have to drill me some holes in the wheelhead for the bat possibility....

 

Evelyne

I think he gets away with using so much water because he throws really fast so it does not stay around long. Sitting water is more of an issue especially with new throwers.

 

As to the alignment of the clay, I thought our type of clay aligned like plates ... Horizontally ... So changing direction is not about alignment but further compression. John Britt seemed to be saying to do this at first but later you would not need to. I would imagine this step would be beneficial for those who throw off the hump since the clay would be compressed top to bottom.

 

The Japanese potter we saw at the conference said ... " save five minutes, lose a pot. Not worth it."

[Tyler Miller]

Tyler, I might be misunderstanding your explanation, but if you flip the clay column, you'll still be twisting the clay in the same direction, so I don't see how this would compress pre-stretched clay.

 

I was under the impression that S-cracks arise from the centre of the bottom shrinking more than the outside, due to it not being compressed as much. Isn't the S shape just a result of how the clay particles are aligned?

 

When you flip the clay, you're changing the direction of the forces it experiences.  Proof:  take a clock, stand in front of it.  The hands move in a clockwise direction.  Now, take the clock off the wall and hold it so that the face is away from you.  The clock's clockwise is your counter clockwise.

 

Apply this principle to a column of clay on your wheel.  You throw it in one direction, and then you flip the column top to bottom.  The direction of the wheel stays the same, but the orientation changes, so that you're now throwing in a direction opposite to your initial throwing.  See what I mean?

 

It's true that s-cracks form during drying due to the stresses of contraction (and, indeed, often unevenly), but those stresses manifest themselves as cracks at the weakest points in the clay body.  The s-crack takes its unique shape because of the "s" shaped weak spot created by the torque of throwing.

[Evelyne Schoenmann]

Chris: and.... John Britt is throwing with a sponge on the outside. Sucks up some of the water I guess.

[neilestrick]

I use tons of water. But I work quickly, and once I've got the walls pulled up I scrape them down with a metal rib as I shape the form.

[Pieter Mostert]

When you flip the clay, you're changing the direction of the forces it experiences.  Proof:  take a clock, stand in front of it.  The hands move in a clockwise direction.  Now, take the clock off the wall and hold it so that the face is away from you.  The clock's clockwise is your counter clockwise.

 

Apply this principle to a column of clay on your wheel.  You throw it in one direction, and then you flip the column top to bottom.  The direction of the wheel stays the same, but the orientation changes, so that you're now throwing in a direction opposite to your initial throwing.  See what I mean?

 

 

OK, I see. You're talking about what happens to a fixed horizontal cross-section. I was thinking of the twisting motion that's applied to the clay column.