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Biglou13

Soda Ash Effect?

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I've been doing /using extensively a shino/like glaze Epk, soda ash, spodumene, nephsy. Primarily in cone 6ox.

 

I've notice the edge where. Glazes stop the clay gets this nice toasty red color. Clay is laguna 900.

 

I'm told its the soda ash that causes thus blush?

 

I'm told that "washing" pieces in dilute shino will cause this blush. (Woodfired redux world)

 

I'm testing a few pieces now.

 

I assuming it's something with soda ash and iron, however in single dipped pieces the blush is only at transition.

 

I'd like to get more of that blush throughout the piece.

 

? What's going on here in the chemistry sense?

 

Shino like glaze

Laguna 900 clay (iron bearing)

Cone 6 ox.

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It is the just the soda that produces the red blush. You will see it with any soda containing material, such as Neph Sye (which Shinos frequently contain) or any other soda spar. You can ramp this effect up by making a saturated soda solution. Add soda ash to hot water until no more will dissolve. The hotter the water, the more soda you can add. Then dip your bisque into this solution and let dry. Glaze as usual. You'll have to play around with how saturated you want your solution and how long to leave the piece in the solution. You can do this for both electric ^6 and ^10 gas reduction firings. Look at the work of Laura Ross from a few years ago, such as this one.... before she built her gas kiln. She was dipping her pieces in a soda solution and applying an ash glaze for electric firing. post-13960-0-00208300-1387379181_thumb.jpg

 

Some additional information about shino: Louise Cort published an article in the 90's, in Studio Potter, I believe about

the analysis of historical shinos from Japan in the Freer Gallery. The formula for a basic shino was found to be approx. 80% Neph Sye/ 20% clay. I think this was companion article to Jim Robison's work on approx. 50 different shinos in the same isssue.  I may be mistaken on that. 

 

Ruth Ballou

 

post-13960-0-00208300-1387379181_thumb.jpg

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Biglou,

 

You are getting vapor glazing (mini-micro-soda kiln firing) along the edges of the glaze layer where the soluble soda ash concentration is located.  The sodium compound vaporizes and "fumes" the clay in the process of outgassing into the kiln atmosphere.  This is literally the same aspect as what happens in a soda kiln in a much larger fashion. 

 

Some of ther sodium is coming from the end of the glaze layer itself.  But some is likely also coming from whre soluble soda compounds have migrated through the porous bisque a tiny bit past the end of the wet glaze layer and crystallized on the surface there. 

 

The sodium oxide molecules are powerful fluxes on silica.  They "steal" some silica from the surface of the alumino-SILICATE clay body.  It forms a low melting Na2O-SiO2 based glass.  This glass is coming from the clay body and if the body containe iron, this is picked up in the melt as a contaminant.  Iron in high soda glasses favors the bright red phase of color..... hence the "blush".

 

You can paint soda ash solution onto the surface of raw bisqued clay and get this effect a bit also.  But you have to be WAY careful and "test, test' test" due to the fact that different clays react differently to this.  It is e asy to go from a nice blush to a bubbly melted cratered surface. 

 

You  can also take and make thick, thick THICK small thimble sized clay cups, bisque them, and then place these cups loaded with soda ash next to pieces and put "blushes" in certain down draft stream areas of the wares.  Sorta' like faux wood fire flashing.  But again care is needed.  In this case you are doing a little tiny "soda firing"... and it can deteriorate the bricks.... particularly insulating firebricks..... as well as electric kiln elements if you do more than the occasional piece or use huge numbers of these little cups.

 

Hope that helps.

 

best,

 

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

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John, if I did that in a sagger in my electric kiln, could I get a similar effect and also protect the kiln?

 

A qualified ..... maybe. :rolleyes:  ;)

 

HOPEFULLY all of the vaporizing soda compounds get "captured" by the lining of the saggar andthe piece in it....before escapiong the saggar.  It definately would contain SOME of the atmospheric sodium compound........ but it will not necessarily totally stop it.  Saggars are  never 100% sealed in practical usage.

 

Also... you'll have a "static atmosphere" in the sagger.. so no circulation pattern to make the sodium "follow" the form like it does in a soda kiln or a wood kiln.  More "splottchie" with the highest level of effect closes to the point of the soda cup and somewhat uniformily decreasing away from the point in all directions based on distance to the form.

 

It also depends on HOW MUCH soda ash (or salt) you use.  A teaspoonfull in a bisque cup is one thing.... a pound in the bottom of the saggar is anotehr.

 

"Test, test, test."

 

best,

 

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

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I've done a lot of Currie grids. The grids are fired in stacks to show any outgassing from materials onto the back of an upper grid. Sodium

outgases from Neph Sye and frits 3195 and 3134. Likely there are more examples. This would be easy to test with your own glazes. Just put them on a low walled test tile and cover with an unglazed slab. So, I'm wondering how much a soda ash soaked pot would out gas and how far. In these pieces, the soda is already on the piece, not in a cup next to it. The area may be fairly limited. 

 

The pieces I've fired in an electric kiln have not caused any problems for the kiln, though it is not something I do a lot. I do wonder how much soda ash in a cup would move around in an electric kiln. Typically, soda ash must be put into a solution first for spraying into a kiln. Or there's a mixture of a couple of things to be tossed into the kin, burrito fashion. 

 

Ruth Ballou

Charleston, SC

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I'm just thinking out of the box here for a moment, could you place two holes in the saggar vessel.  The first communicates with the firing chamber and the second is connected to a tube that vents out of the kiln to form a mini chimney.  Air from the chamber would be drown into the saggar as the the saggar outgassed its content up the chimney.  Would this work to create a draft in the saggar chamber thus allowing better soda dispersion in the saggar yet still protect the firing chamber from soda exposure?     I have never tried this....just wondering if it would work.

 

Jed

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To clarify a bit, I was trying to say that there are a number of materials already in an electric kiln that are outgassing. It seems these don't cause a 

problem. So would a soda ash soaked pot be significantly different? Though I'm all for adding to the knowledge base through experimentation!

 

Ruth 

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I'm just thinking out of the box here for a moment, could you place two holes in the saggar vessel.  The first communicates with the firing chamber and the second is connected to a tube that vents out of the kiln to form a mini chimney.  Air from the chamber would be drown into the saggar as the the saggar outgassed its content up the chimney.  Would this work to create a draft in the saggar chamber thus allowing better soda dispersion in the saggar yet still protect the firing chamber from soda exposure?     I have never tried this....just wondering if it would work.

 

Jed

 

Complicated but yes.

 

best,

 

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

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To clarify a bit, I was trying to say that there are a number of materials already in an electric kiln that are outgassing. It seems these don't cause a

problem. So would a soda ash soaked pot be significantly different? Though I'm all for adding to the knowledge base through experimentation!

 

Ruth

 

It is all about the AMOUNT of this you do. And some of the "normal" and slowly inevetiable deterioration of kiln linings (and electric kiln elements) is from the volatiles of all types that go into the kiln atmosphere...... most notable.... potassium and sodium.

 

One soda ash soaked pot once........ not much. One kiln load of soda ash soaked pots...... more so. Lots of kiln loads of soda ash soaked pots...... lots more.

 

I've been teaching high temperature saggar firing courses for years (since the 70's)........ lots you can do with this general concept.

 

best,

 

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

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I think that's a really good question. Salt firing causes problems, but is it the sodium or the chloride?

 

Sodium. And surprisingly, the level of chlorides and hydrorchloric acid vapor mist in the kiln effleunt of salt kilns has been tested to show lower than expected (by conventianl wisdom / theory) of chlorine. This is still being studied.

 

best,

 

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

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One of the questions at the start of this thread was: what's going on here in the chemistry sense?

 

Well I don't know the full answer, but the superficially similar process leading up to the Hidasuki marks on Bizen ware has been studied.

 

By applying alkali metal fluxes to this particular stoneware (via rice straw, sometimes soaked in salt) a dramatic effect is obtained,

 

 

Simply speaking the iron in the clay is dissolved at high temperatures, and precipitates out as as haematite as things cool.

 

The more complex picture is described in papers such as:

http://pubs.acs.org/doi/abs/10.1021/cm0352145

http://pubs.acs.org/doi/abs/10.1021/ar9001872

... sorry but only the abstracts are available free.

This paper is free, but in Japanese (but with the abstract and figure captions in English):

https://www.jstage.jst.go.jp/article/jjspm/54/2/54_2_75/_pdf

 

Slow cooling is recommended -- colour develops at 10C/min but deeper at 1C/min.

 

Regards, Peter

 

Back to the soda ash effect.

I strongly suspect that you can add boron to the flux to dissolve the iron at lower temperatures without a major change to the colour. Borax

(sodium borate) would seem an obvious source, it's solubility is usually a drawback but in this case you would be mixing it with soda ash..

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