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Soda-Ash Wash Food Safe?

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I think it might help to separate the chemistry from the application technique on this one.  Here is my two cents worth anyway.

The chemistry seems pretty straightforward.  Soda ash (sodium carbonate) is about 50% sodium oxide (other 50% burns off as carbon dioxide and water).  Sodium oxide makes a good glass in the right combinations with alumina, silica and other fluxes, as Min has pointed out, and starting at very reasonable temperatures, as LT pointed out.  We know this not least of all because sodium oxide is the primary flux in soda-lime-glass, used all over the world to package food.  However, when the chemistry is wrong, glass may look good and seem hard, but it can be unstable, prone to etching or corrosion (non-hygienic?), soluble, basically not chemically durable.  Glasses too high in sodium can also be brittle, prone to chipping off, etc, ie, not mechanically (as opposed to chemically) durable. 

In principle clay bodies contain the right kinds of materials (alumina, silica, other fluxes) to combine with sodium oxide to make a  good glass on the surface of the pot.  And we know that clay body materials and glaze materials DO combine and intermingle during a stoneware firing to form the glaze/body interlayer, which has its own third chemistry, distinct from either the clay or glaze.   So, it is not inconceivable that a durable glass - not unlike a glaze - could form on the surface of a pot even if we just added a single ingredient (say, sodium oxide) during the firing, under the right conditions, ie  as long as the approximately the right amounts of other necessary materials were present.    In fact, the oxide makeup of soda lime glass itself might not be a bad place to start in thinking how to alter a clay body to move toward a more viable surface glass in soda or salt firing.  Calcium oxide would probably be a good first addition ( I am thinking wood ash helps here), along with upping the silica...

IMO sodium oxide from a "wash" is no less likely than sodium oxide from a "glaze" to flux and combine with clay body materials (with a caveat for thickness, more on this below).   Nor is sodium oxide in vapour form any different from sodium oxide that started out on a pot - they both combine and flux and form eutectics the same way, namely by getting on a pot and becoming involved in the melt. 

Turning to the issue of application, I would argue that this is exactly what is happening in salt and soda firings.  The vapour glazing deposits a little bit of soda or salt at a time with repeated dosing.  Although much of the vapour goes up the chimney, settles on the wall of the kiln, kiln furniture, etc..  the sodium that does make it on to the pot encounters an active melt underway, and readily joins in.  The sodium vapour settling on the pots is so thin that it CAN effectively combine with the body materials to achieve eutectics and form a very thin but relatively robust glass/glaze.  Put another way, all we are seeing on the finished pot is the interlayer - without a glaze above it.  Such a surface may or may not be food-safe in the strictest terms, but it may well be robust enough to last for a long time as, say, a salt-glazed clay water pipe.

If we were willing to throw salt or soda into our electric kilns, and provide a draft to move the resulting vapour around, I would wager that the effect would be similar (anyone try this?)

To increase the chances of a good glass, I would suggest a good long soak at the top of the firing to homogenize the thin melt certainly wouldn't hurt.   And the more fluxes you had in the clay body (particularly, calcium, which is the "lime" in soda lime glass), the more eutectics combinations you would form and the more likely you would be to get a stable, durable glass.

Back to the OP, if your soda "wash" applied directly to the pot is too thick, then it may begin to behave more like a poorly conceived glaze recipe, ie, chemically unbalanced and without the right blend of fluxes to form a stable glass.

Finally, I would just say that during wood firings in recent years we have added small ceramic bowls filled with table salt around the kiln amongst the stack to get a bit of salt vapour effect on the pots.  Works great, particularly combining with ash from the firing (which is mostly calcium!) 

 

 

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Soda ash melts at 1564F, regardless of whether or not it comes into contact with alumina and silica. Salt/soda vapor will only form glaze at high temperature and when in contact with alumina and silica. They are two different reactions. Under a microscope, salt/soda vapor glaze has no glaze-clay boundary- the glaze is growing out the clay. Soda ash probably does take up some alumina and silica, but not nearly to the extent of  the vapor, and as a glaze it is much too high in sodium. If you were to fire some soda ash onto a slab, it would still be somewhat soluble, not at all durable. By contrast, salt/soda vapor glaze is quite durable. To get the same effect as salt vapor, you would have to apply very, very thin layers of soda ash, firing each layer as it is applied.

The type of kiln doesn't matter- electric or gas or wood- in terms of forming the salt glaze. As long as it's hot enough, it'll work. We don't use electric kilns because they can't handle the corrosive nature of the vapor.

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1 hour ago, neilestrick said:

 As long as it's hot enough, it'll work. 

 

I am loving all this discussion! But I still have some questions to fully understand this:

I have seen salt and soda fired work at cone 6, it looked just as good as the soda work I have seen at cone 10+ as well. I handled both the pots in person at a large pottery show. I looked up the artist, although I don't remember their names now. But I still don't understand how dumping soda into a kiln creates a different chemical reaction than soda ash being on a pot creates. I really am not arguing, just trying to understand the chemistry of what is happening that is different.  I get that the sodium is melting the alumina and silica in the body and forming a glaze, but why isn't that happening with soda wash being brushed over it? 

If the clay body is vitrifying at cone 6, then the silica and alumina in the body are melting with fluxes in the clay to form a good pot, right or wrong? Why isn't the soda ash on the pot at cone 6 electric interacting with that? This is the part I don't understand, I really just want to understand the chemistry of why one works and the other is claimed not to, although we obviously all have various opinions about it.

Is it because the soda ash is melting too soon and all the flux power is gone at cone 6 when the clay body starts to fully vitrify? If that is the case then could one make soda ash ware with clay that vitrifies at the same temp as soda ash starts to melt? 

I am going to run my test again soon and I will post the tiles I get from the soda wash. I have seen plenty of sheens that a wash can provide, this wasn't anything like that. I was brushing on multiple layers of soda ash wash and then firing them to a hot cone 6. The soda ash physically changed the surface of the clay to a smoother feeling, more than just a flashing of color.  If I hadn't thrown away the darn tiles I would post them. I had too many tiles on my desk and I had looked through them and noted my results down so I threw them away. I make too many tiles to keep them all, and I, of course, didn't save any of the soda ones... *facepalm*

As far as the surface goes, I don't see how it could be negative in any way, but maybe durability issues? Soda ash is completely safe for a food ware, and clay can be used bare with no glaze. I don't see any downsides besides destroying elements quicker, but as John Baymore had posted before, if you're doing something worth it, pay for the elements with increased prices. 

Edited by Joseph F
further thoughts

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Okay! So being a knucklehead, I went to double check my tiles on my shelf and I found a tile that I did a crackle slip test on, then remembered I had also brushed some soda ash on it. However, I remembered that I only did soda ash on the corner of the tile just to see what it would do with the black slip formulation. This was just a single brush stroke of a highly saturated soda ash wash.

MVIMG_20180130_142813.jpg.d76e8b9a6f61f3f812919cf2588e29ad.jpg

I started in the corner and brushed across. You can see the difference between it going from a plain black clay slip, as it moves from the bottom to the top where it looks almost glassy. Are we saying that is a sheen? I can see the middle part being called a sheen, but that top corner edge part looks pretty darn reflective to me. I am not saying this is a stable good glass by any means, but that it is forming some type of glass. The tiles I had looked like this on the entire tile, with the same flashing on the parts that were less brushed. The corner is smooth to the touch, very different than the sheen part in the middle.

Sorry for the blurry picture. I couldn't get it to focus well on the glossy part.

Edited by Joseph F

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3 hours ago, Min said:

You go first. ;)

OK -I plan on doing just that in about 6 weeks.Sort of.

I'm converting a large oval electric (all elements ripped out and electrics removed)

This kiln is then going to have 4 gas burners entering is two sides.

The plan is to line  3inch brick with a fiber liner and coat that. The kiln will be an updraft -then we will salt it heavily . 

I'm doing this at a workshop on a island mid pacific  in March . I 'm guess in the kiln will get about4-6 fires in before its dead.

I did a similar conversion at this art center 4 years ago April.That kiln is long dead and gone-this will be the new one.

No soda ash here its going to cone 10 and get salted.

I'll post details as it gets closer.

Edited by Mark C.

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26 minutes ago, neilestrick said:

There is no denying that the soda ash will melt and form a glass of some sort, but it won't be a good durable glass. It will be way too high in sodium.

Good stuff! What makes the difference here between one form being good and the other not? I think that is what I am still missing. Why is the wash worse than the spray?  Chemistry wise it seems so curious. Literally soda ash sprayed into a kiln is somehow massively different than the same thing applied with a brush before the firing? I know it isn't temp because I was reading more and there are people soda firing earthenware. I think this is why I am so confused. It has to be something with the exact timing of the top temp fluxing the clay body? Otherwise I don't see the difference.

I plan to soak the next set of soda ash tiles I make in vinegar, lemon juice and dishwasher detergent. Curious how bad they fail. I think this is the only way I'm gonna be satisfied. I will put the above tile in lemon juice tonight.

I have read so many things as facts then later discovered they indeed are not the case through my own trials and experiments.

Edited by Joseph F
further thoughts

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I think the idea is that instead of all the soda ash being on the pot all at once it is slowly added through a few hours or however long of the firing. I'm still not sure this means you couldn't get very close to the same results with the right amount of soda ash and length of firing. 

Does the chlorine in salt make any difference? I would think it might compared to soda.  I only had the chance to salt fire once, never tried soda.

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I think the difference is that the vapor works in such small amount that it forms a glass that is much more in balance as a good glaze, whereas the soda ash provides too much sodium, making a glass that is still somewhat soluble. Like I said before, if you could apply super tiny amounts of soda ash wash, and fire each application, you could theoretically get a similar effect to vapor glazing. But if you apply one thin enough coating of soda ash to be in balance with the alumina and silica as a good glaze, it won't form a sufficiently thick layer of glaze. It's the continuous tiny bit of melting that occurs over an hour or two of salt/soda vapor that builds up the glaze layer, with the soda constantly eating away a little more of the surface of the clay and melting the alumina and silica.

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3 hours ago, Joseph F said:

If I hadn't thrown away the darn tiles I would post them. I had too many tiles on my desk and I had looked through them and noted my results down so I threw them away. I make too many tiles to keep them all, and I, of course, didn't save any of the soda ones... *facepalm*

How do you throw them away? Poor little things :( I have two boxes full of them :lol:

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Thanks Neil. I think that makes it easier for me to grasp what is happening differently between the two situations. I wonder what the effect of the glass deteriorating is in the surface appearance is with the wash. I imagine it is similar to other bad glazes but without coloring or anything negative to reach out what the downsides are.

I also wonder if one could mix in a small percentage of alumina and silica to almost balance out the application differences but not go all the way into a real glaze territory in the wash formulation. A lot to ponder to destroy my elements faster.

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5 hours ago, High Bridge Pottery said:

How do you throw them away? Poor little things :( I have two boxes full of them :lol:

Man. I fire a few hundred tiles a week when I am going full steam. If I kept all those tiles I wouldn't have room in my garage.

Edited by Joseph F

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9 hours ago, Joseph F said:

 I still don't understand how dumping soda into a kiln creates a different chemical reaction than soda ash being on a pot creates.

It doesn't.  It is the same chemical reaction.  The difference in the two processes is about sizes and amounts of materials involved, not their chemistry.

In a salt or soda firing, when salt or soda is introduced (mind you not onto the pots themselves, usually nearby in a open part of the kiln chamber in my experience), it immediately starts to volatilize and disassociate.  Small solid bits of salt or soda quickly melt and break down into much smaller particles which get airborne, and at the same time disassociate at the molecular level into by-products, namely sodium oxide and chlorine gas (salt firing), or carbon dioxide and water (soda firing).  These newly created sub-materials float around inside the kiln chamber like other gases, hitting stuff.  The chlorine gas, carbon dioxide and water never find a resting place and make their way out the chimney.   Much of the sodium oxide in these gases ALSO disappears up the chimney, but some small amount of it eventually land on different parts of pots, and starts interacting and fluxing what it finds there. 

But that is the point - imagine how thin a layer we must be talking about - a layer a few microns thick laid down for each dosing of salt?  That is nothing.  No wonder we have to dose multiple times in the firing.  The actual final shiny surface on the pot ends up being thicker because it is not just sodium oxide, but rather a new composition of sodium oxide and the things it fluxed, aluminium oxide, silica oxide, as well as other materials that were in the clay, and possibly byproducts of the fuel (eg, wood ash).  These materials all contribute to the final surface.  As Neil said, it looks like the glaze is growing out of the clay body, and it kind of is.

Now think about a soda wash applied with a brush.  Think about how different this application is to vapour glazing.   The amount of sodium oxide we are talking about in a soda wash applied with a brush is probably HUNDREDS of times more material.  Yes the water evaporates, and yes only 50% of the dried soda ash ends up staying on the pot as sodium oxide, but we are talking way more sodium oxide than a few microns laid down by vapor. 

And as already pointed out, herein lies the problem, and the difference between a wash and a vapour application of soda.  In all probability there is far more sodium oxide in a wash than can be reached/intermingled/taken up by the alumina and silica.  So all that leftover sodium oxide just sits around on top of the pot,  melted down, maybe attached a little bit to something besides just other molecules of sodium oxide, maybe not.  In either case, what remains it is highly unlikely to be a good glass.  Only the bits of sodium oxide at the bottom of the wash covering, the bits very near to the alumina and silica and other fluxes, were lucky enough to get involved in a "good" melt, achieve a eutectic, and form a good glass. 

Or at least that is my take.

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11 hours ago, Joseph F said:

I have seen salt and soda fired work at cone 6, it looked just as good as the soda work I have seen at cone 10+ as well. ...If the clay body is vitrifying at cone 6, then the silica and alumina in the body are melting with fluxes in the clay to form a good pot, right or wrong? Why isn't the soda ash on the pot at cone 6 electric interacting with that?.....Is it because the soda ash is melting too soon and all the flux power is gone at cone 6 when the clay body starts to fully vitrify? If that is the case then could one make soda ash ware with clay that vitrifies at the same temp as soda ash starts to melt?

Sorry, Joseph chopped out a few quotes from your post and strung them together to get at the cone 6 point, hope you don't mind?  Havent changed what you said, though (I don't think !)

 There are many combinations of sodium oxide, silica dioxide and alumina oxide which start melting below 1000C.   Here is one particular eutectic from Digitalfire, seems like a frit: 

Soda-Alumina-Silica

Oxide Analysis Formula
Na2O 17.80% 1.000
Al2O3 5.10% 0.174
SiO2 77.10% 4.468
Oxide Weight 348.31
Formula Weight 348.31
.
GSPT - Frit Softening Point 800C

But just because something melts down and smooths over does not necessarily make it a "good" glass.  What I have seen from industry suggests that one can indeed make good glasses at low temperatures,  the recipes and process are tightly controlled to produce glasses with certain defined characteristics for use in specific applications.  Boro-silicate glasses (eg, Pyrex) get soft around 800C, and I think we would call them food safe, at least in some uses.   However, this glass has other tradeoffs.   Traditional soda lime glass is manufactured at around 1450C.

As far as when the soda ash delivers its fluxing power: The soda ash decomposes into sodium oxide and by-products early on in the firing, and the fluxing power of the sodium oxide released lasts all the way up to the top temperature of the firing.  However, since sodium oxide starts to volatilize at around 1100C, some of it will turn into gas and be lost up the chimney.  However, whatever is already involved in a melt process will stay put and keep fluxing.  Its power does not dissipate as temperature gets higher. 

And yes, I think there is scope to tweak clay bodies to make a soda ash ware.... !  It seems to me the practical constraint is really our clay bodies and what heatwork they need to mature. 

 

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2 hours ago, Benzine said:

Can it be any soda, or does it have to be name brand?  Do different flavors, lead to different results?  Should we perform the "Pepsi Challenge" with pottery? /end sarcasm

baking soda is soda ash with a sodium ion replaced by hydrogen.  it converts to soda ash  at about 50 C.    There are some laundry powders that are essentially baking soda / soda ash with perfume.    Different flavors might have different amounts of sodium (the element) per unit weight of 'stuff' used.  Convert to grams of Sodium used, not grams of 'stuff' used and the results should be the same. 

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

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.

Tom,

Please post the citation or a link to this report.  

LT

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