: OffCenter - Viewing Profile

OffCenter, on 22 May 2013 - 07:21 PM, said:

Jim,

You likely are right is the is cone 6 B-mix.

One of the problems with using commercially prepared "stock" clay bodies is that, because they are proprietary formulations, the manufacturers will not share the actual recipes. So we really do not know what is in there and what the proportions are. Makes troubleshooting difficult.

That being said, in order to get a cone 6 clay to really vitrify, the usual high fire (cone 9-10) kind of choice of a potash feldspar to supply the fluxing oxides for the body is not ususally going to work all that well. The proportions of fluxing oxides relative to silica and alumina in feldspars is not ususally providing the "oomph" to get the melt active within the body matrix. So to accomplish this task, the usual "substitution" that happens is to use some or all Nepheline Syenite as a partial or full substitution for a potash feldspar contribution in the body.

Nepheline Syenite is a SODA based material. THIS is where the potential issues begin. The potential issue with this is that Nepheline Syenite is slightly soluble in water (great for carbon trap shinos). And as the Ph of the water supply in the clay body changes.... the level of this solubility changes too.

So....... what I am driving at here is that it is possible that the older clay that has sat around a long time wet before use, and then wet formed and then slowly dried, and then picked up atmosphereic moisture when it was "dry", and then had water from YOUR water source (not the suppliers.....which MIGHT be Ph controlled) added to it.... the water chemistry of the re-mixed body is subtly changing as some sodium ions disolve out of the neph sy content.

This will cause the clay to go "short"...... and eventually when it gets bad enough exhibit some thixotropic qualities. So that might be part or ALL of what you are describing. It fits the situation that the beer and organic stuff growing does not help.

If you are not using a splash pan (like I do not) that likely means that you are throwing pretty dry (like I do). With your skill base, I am guessing that you are a pretty qucik thrower..... not working and working and working and working a piece to death. It seems to me that kind of forming profile would NOT wash out a huge amount of the finer particle clay materials to the extent necessatry to cause the shortness. THAT answer does not make sense to me.

Out of curiosity, do you find that if you let the bags of clay you get from the supplier sit around for a LONG time wet, and that the clay exhibits any similar "short" characteristics? Is it more plastic when you just get it than it is a year later (you might not order that far out)?


best,



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

I finally have a little time to read the posts in the communities, and I love that there are so many talented people in the group. I notice that when I view profiles it helps to visualize and TRY to solve problems if I can get an idea of what the final products you make look like. Please take advantage of the gallery benefits of your Ceramics Arts Daily accounts. This would be of good use for people that don't have web pages, and at least Ceramics Arts Daily participants can view your work. Never know, might work like free advertising!OH! I FORGOT! Please let me know when your update your gallery!

The relief work on good Yixing is not sprigged. Each element of such designs is hand done with remarkable precision (and the ability to replicate items at a very high level).

I was watching an undergrad class on a very similar type of surface technique at Wuxi Institute of Arts and Technology (in Yixing) a couple of weeks ago, and the painstaking time taken with every movement and the intense focus of each student was what struck me the most.

While the cheap stuff might involve sprigging.... the decent pots don't.

WuxiInstitute-SurfaceDecoration1.jpg

WuxiInstitute-SurfaceDecoration2.jpg



best,

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

image.jpg

I'm having some problems with bisque firings and would like to run this by you...

I use stoneware which I underglaze at leather hard, then I sgraffito the piece. I let it get to bone dry and do the bisque in an old kiln - it has 4 on/off switches; the manual said to flip one switch an hour and then wait till the kiln sitter turns it off (^04). then I apply clear glaze to the cooled work and fire to ^5 using a newer kiln with an electronic controller.

Then I started having a problem. After the glaze firing some of my pieces had areas, usually circular, where the it looks like the glaze has pulled away, taking the under glaze with it. So there's a small area, sometimes several all near each other, where the clay body is exposed. In the photo, hopefully you can see the white spots, center right. Because I use their products, I called Amaco for help. I was told my bisque firing was too fast, that out gassing was not complete. And I was told a bisque to ^04 should take 10-12 hours! So now, hearing about these fast bisques, I'm confused.

I was an advocate of bisqueing as fast as possible - my little old kiln could fire off in 5 hours. But lately that doesn't seem to be working for me, so I'm going to to do a really slow bisque next time, as Amaco recommends. But why is it that some of you have no problems with a fast bisque? Wouldn't incomplete out gassing affect your glazes? Any thoughts on this? Thanks!