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RogueArtist

Soda-Ash Wash Food Safe?

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On 1/29/2018 at 6:23 PM, glazenerd said:

Joseph: perhaps I can clear up the confusion about the soda vapor not acting as a flux. Coming from the clay side: as you heat clay: the sodium changes from solids to a vapor. As the vapor builds up inside the body, it begins to melt the transitional metals ( iron, zinc,etc.), and as they begin to melt: combined they melt the silica. In short, the melt builds as the temperatures climb.

i do not wood fire, so I have nothing to add. So this is from the chemistry angle only. Throwing sodium in a kiln at peak temp, instantly vaporizes it: leaving a coating. This means it,s ability to incorporate other materials into a melt is already spent. To further define: (U of I study) sodium and potassium dissociate (lose their ability to flux or melt) at 2190f. So the vaporized sodium has already lost it's ability to further flux what it lands on.  I would agree with Min, Callie and the others: I would be testing durability and food safe applications.

Nerd

It's definitely not just a coating, and it's definitely incorporating alumina and silica from the clay body into the melt. There's no clay-glaze boundary when examined under a microscope. Salt glaze has been tested, and it contains alumina and silica, which is coming from the clay body.

Salt glaze is considered to be durable, but it doesn't form well on the inside of pieces, so liner glazes are needed anyway.

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Neil:

some clarification is needed perhaps. I was comparing the melt of a standard firing to that of a c10 salt firing. Vaporized sodium  landing on molten clay perhaps does bond with the alumina on the surface. It was the degree of incorporation I was making reference to. There are abstracts out there that have studied the effects of sodium vapor on clay, but they are written for industrial applications. Secondly, the sodium is already vaporized and injected into the chamber under pressure: so comparison cannot be made. 

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We're all clay people and we tend as a group to think with our stomachs, so here's an oversimplified cooking analogy for the (really cool!) explanations from Neil, Curt and LT have provided:

Vapour glazing is a bit like adding liquid to roux to make a gravy or a white sauce: if you add all your liquid (sodium) to your roux (ware inside a hot kiln) at once, the roux can absorb some of it, and given enough time, it'll probably all incorporate to homogeneity ...eventually... but probably not. But, if you add a little bit of liquid (sodium) at a time and whisk it all in (the 15-20 minute soak in between bombs) you get thicker, more desirable and thoroughly incorporated results because the reaction happens more slowly.  If you're just brushing on a wash, it's like you dump a couple of tablespoons of liquid into your thickener. It'll be dry and the end result won't really resemble gravy (soda or salt glaze).

I think people are having a hard time wrapping their heads around this issue because we tend to forget about the physics involved in the chemical reactions, and what it really means when the organics and carbonates burn off and disassociate in the kiln.  We kind of get into the habit of thinking Ingredient A + Ingredient B + Ingredient C + heat--->glaze, and that said glaze is the exact same all the way through, because we mixed it that way in the bucket. We think of chemical reactions as instant, and tend to forget that some of the reacting going on in a kiln is actually happening pretty slowly, in part because many glazes are quite viscous (or they'd run right off the pots). That's why heat work is so important. That's why application thickness is important. In a kiln that involves vapour glazing, soaking and time and atmosphere are important. Ditto for anything that grows crystals.  Method of application matters.

When you're learning glaze chem, it can be complicated enough to think about glazes from the standpoint of "okay, I need x amount of silica, alumina and flux to make a stable glaze that looks pretty and is easy to apply to my pots, now how do I get that mix from the materials I have from my supplier?"  It can be even more daunting to start thinking about how that glaze reacts with the clay underneath it, and about how things like mesh size  and heat work affect the melt, so that line of thinking usually gets left until later.  This conversation is the "later."

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