Soda-Ash Wash Food Safe?

[RogueArtist]

Hello everyone,

I've been reading about using a soda-ash wash fired to cone 5-6 on bare clay to give it a slight shine. Would this be ok to use on dinnerware as long as the clay beneath was vitrified? Would the surface be too soft and scratch/fade/dissolve in the dishwasher? Even if I use a liner on the inside I'm still worried about the outside surface changing over time.

Does anyone here used soda/salt glazes on functional work? How do they behave? Any information/experience would be appreciated!

[Joseph Fireborn]

Edited: decided to let others with more experience answer. 

[Mark C.]

This is not a yes or no question or answer. The thing is the clay has to be completely vitrified as you noted . Soda-ash also needs to be mature  and the surface needs to be one that cleans easy and not change in a dishwasher . If you are making for yourself thats one thing-selling it is another.Testing is the only answer that makes sense .

The words food safe are really very vague and have little meaning. Its more like will it wash and hold up well to use thats more important as well as not leaking or leaching.

I'm not a cone 5-6 guy in any sense of the word-maybe some mid range fire folks can chine in on this.

 

[Min]

Glazes are made durable by having adequate amounts of both silica and alumina. Given that soda ash is 100% flux (sodium) and does not contain either silica nor alumina then you can pretty much determine that a wash is not going to stand up to daily use without suffering a change. There is nothing nasty to leach out of the soda ash wash pot so from purely a leaching point of view that wouldn't be my concern. 

Simple tests to rule out a glaze, take 2 glazed test pots and half submerge one in household vinegar for several days. Take the other test pot and either leave it in the dishwasher for 2 or 3 months of regular use or simulate this by simmering the test pot in a soda ash solution. 50 grams soda ash : 1 litre of water, simmer for 6 hours. Rinse off the test pots, dry them thoroughly and check for loss of gloss / colour between the original glaze and the vinegar and soda ash treated parts of the glaze. Any change is going to indicate the glaze won't stand up to daily use. These tests just rule out what isn't going to be durable, for ruling in a glaze lab testing is the only way to be sure. 

[glazenerd]

Mark:

i will chime in as a cone 3 to 7 guy. At cone 6, a properly formulated glaze will melt completely and be food safe in regards to leaching, grazing,  etc. The problem with cone six is the clay bodies; many of which are not properly formulated. Flux levels are off, particle distribution is off, firing schedules are off; all combining that create problems. What most see as glaze problems, I see as clay problems: but I am admittedly the oddball in that belief. Then again I can make a glaze crackle on one clay, and reformulate the clay to be smooth on another. Zero absorption porcelain or stoneware cone six bodies are not that tough to produce: they are just 10-15 cents a pound more to produce. That is the real issue: buying 40 cent clay and expecting it to perform like 90 cent clay.

okay Joel, as per earlier agreement: controversial post.  

[Sputty]

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[Min]

1 hour ago, Sputty said:

Well, I dunno.

Think about salt glaze: the glaze is made by sodium in the kiln atmosphere reacting with silica and alumina on the surface of the clay body. Doesn't a soda-wash do the same thing, chemistry-wise? Where is the difference?

A salt-glazed item is pretty durable, in my experience.

I agree that salt fired pots are durable with the sodium aluminum silicate glaze formed on them in the firing. I don’t think the alumina and silica from the clay body interact with the sodium to the same degree with a wash in cone 6 electric firings as happens in a salt (or soda) firing. 

[Marcia Selsor]

I agree with Min. The soda used in soda firings fluxes with the alumina and silica in the clay to create a glazed surface. A soda ash wash is not fluxing the clay body. More like a thin coloration on the surface.

Marcia

 

[neilestrick]

Agreed. Soda ash is a surface treatment, salt/soda firing glaze is the soda combining with the alumina and silica of the body. Salt fired glaze is one of the most durable surfaces you can make.

[Sputty]

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[High Bridge Pottery]

13 hours ago, glazenerd said:

Mark:

i will chime in as a cone 3 to 7 guy. At cone 6, a properly formulated glaze will melt completely and be food safe in regards to leaching, grazing,  etc. The problem with cone six is the clay bodies; many of which are not properly formulated. Flux levels are off, particle distribution is off, firing schedules are off; all combining that create problems. What most see as glaze problems, I see as clay problems: but I am admittedly the oddball in that belief. Then again I can make a glaze crackle on one clay, and reformulate the clay to be smooth on another. Zero absorption porcelain or stoneware cone six bodies are not that tough to produce: they are just 10-15 cents a pound more to produce. That is the real issue: buying 40 cent clay and expecting it to perform like 90 cent clay.

okay Joel, as per earlier agreement: controversial post.  

I think you are the oddball by having an interest in the clay body, I blame your crystal addiction. I hope to one day become versed in the art of clay body.

On topic.

It is food safe in real terms of not leaching Lead or Cadmium, if it is hygienic to use that is another question. I don't think I would use it on the inside but I see no issue with the outside even if it does slowly change how it looks.

[RogueArtist]

6 hours ago, Min said:

7 hours ago, Sputty said:

Well, I dunno.

Think about salt glaze: the glaze is made by sodium in the kiln atmosphere reacting with silica and alumina on the surface of the clay body. Doesn't a soda-wash do the same thing, chemistry-wise? Where is the difference?

A salt-glazed item is pretty durable, in my experience.

I agree that salt fired pots are durable with the sodium aluminum silicate glaze formed on them in the firing. I don’t think the alumina and silica from the clay body interact with the sodium to the same degree with a wash in cone 6 electric firings as happens in a salt (or soda) firing. 

What temperature is a traditional salt/soda firing? Does anyone do cone 6 salt/soda firing (the traditional way in a gas kiln)? Or is cone 6 just not hot enough for the chemicals to interact properly? (Sorry if this is a basic question - I looked online but I can't seem to find a cone # for traditional salt firing!)

PS: Thanks for all the replies!

[Magnolia Mud Research]

For perspective only:

salt glazing was done commercially (early 20th century)  for sewer pipe and architectural work using cone 3 as a firing temperature.  These were large combustion kilns using coal or fuel oil.  
Sodium capture from combustion sources onto kaolin particles is permanent at temperatures above ~800 C. Permanent defined as not soluble in hot water after several hours leaching time.

 

I have no personal data other than cone 10+.  I have done some scouting work at approximately cone 3 in a test kiln about 10 years back.  The process worked fine. Good surfaces, all sculptural items.

 

LT

[Sputty]

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[Mark C.]

I fire my salt kiln to cone 10-I have a friend who has done soda at cone 6 a few times -she switched to cone 10-better results for her my guess is?

or maybe since she needed lots of help to  load and fire ,more people around here use cone 10 clays it was easier to fine folks .

I knew a potter who did super low soda bisques -that stuff was purely decorational in nature.

[High Bridge Pottery]

20 minutes ago, Magnolia Mud Research said:

For perspective only:

salt glazing was done commercially (early 20th century)  for sewer pipe and architectural work using cone 3 as a firing temperature.  These were large combustion kilns using coal or fuel oil.  
Sodium capture from combustion sources onto kaolin particles is permanent at temperatures above ~800 C. Permanent defined as not soluble in hot water after several hours leaching time.

 

I have no personal data other than cone 10+.  I have done some scouting work at approximately cone 3 in a test kiln about 10 years back.  The process worked fine. Good surfaces, all sculptural items.

 

LT

There is a place near me that I visited a good few years ago now that used an old pipe making place to make flower pots. They fired with coal but I am sure it was to cone10. Had a beautiful beehive kiln

[Sputty]

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[High Bridge Pottery]

@Sputty Yes  That's exactly who it is. About 30 miles west of Newcastle upon Tyne

[Sputty]

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[Min]

1 hour ago, RogueArtist said:

What temperature is a traditional salt/soda firing? Does anyone do cone 6 salt/soda firing (the traditional way in a gas kiln)? Or is cone 6 just not hot enough for the chemicals to interact properly? (Sorry if this is a basic question - I looked online but I can't seem to find a cone # for traditional salt firing!)

PS: Thanks for all the replies!

Typically in the cone 10 range. Julia Galloway soda fires at cone 6, hers are the only ^6 soda pots I've seen "in person" and that was quite a few years back. John Britt has a little info on it here for cone 6. (btw, I've used the claybody recipe Galloway posts, it's a lovely clay to work with)

[Joseph Fireborn]

I am a bit confused here. If the soda ash isn't fluxing the silica/alumina in the clay surface then what exactly is it doing? 

Edit: From Min's link to the John Britt Book preview: 

Quote

Upon its introduction, the salt or soda immediately vaporizes and flows vigorously throughout the kiln, coating everything with a thin layer of volatilized sodium oxide, which appears as a thin sheen of soda glass on the clay

So if we are applying soda ash wash, which is just a different delivery method to the ware, what is the difference in the final work? 

The reason I ask is that I have been testing a lot of soda ash lately and I am trying to determine what exactly it is doing. It doesn't appear to just be a flashed surface like spodumene does in a glaze, it smoothes the surface of the tile to the touch compared to a tile without soda ash wash.

 

[Callie Beller Diesel]

If it were me, I'd be worried about the thickness of the wash. Wash implies a really thin layer brushed or sprayed on, not the buildup that you'd get from adding sodium to a kiln's atmosphere over time. 

I support the idea of doing your own testing for porosity, and see how bad it would stain if you leave a cup of coffee or tea on the surface. I'll add that you might want to put a piece in the dishwasher and leave it in there for a good 20-30 washes to see how the commercial detergent affects it.

I did try running some experiments with leaving soda ash soloution in some bisqued stoneware jars overnight, and then glaze firing to a functional cone 9. I wanted to see if I could make some pickling crocks, or even just the weights, sort of in the style of the old salt glazed crocks. I only succeeded in making jars with all kinds of little bloat bubbles all over them . 

[glazenerd]

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.

disclaimer: again I do not salt fire so I am not offering advice on that process. However, I do know about the chemistry of fluxes and their reactions to heat: so my opinion was given only from that perspective.

Nerd

[neilestrick]

If you use soda ash on a pot or in a glaze, as the piece dries the sodium will migrate to the surface, leaving a fuzzy white film. This will melt very early, like by cone 08. In a reduction firing, in order to reduce the clay body, you have to go into reduction very early, like cone 012. Otherwise the soda melts and seals it off, preventing the reduction atmosphere from penetrating into the clay body. This is how carbon trap shino glazes work. You reduce early, then create a bunch of carbon that gets trapped into the melted soda.

If you introduce salt or soda into a kiln at low fire temps, you'll get color flashing, but will not build up a glaze layer like at higher temps. Look up Paul Soldner's work to see some great examples of low fire salt.

Cone 10 salt/soda typically has a heavier glaze buildup than I've seen from cone 6, however I don't know if that's because people are using cone 10 clays and the salt is more reactive if the clay body is closer to its maturation point at the time of salting, or because it is simply more reactive at higher temps. I've only done high fire salt, and I never started salting until cone 9, because I'd get more bang for my buck than starting earlier.

The salt layer does not build up quickly. It happens over a period of time, like an hour or more (depending on the kiln). It attacks the clay bit by bit, gradually forming the glaze layer as the soda vapor combines with the alumina and silica in the clay. Silica on its own will not be affected by soda/salt. Alumina on its own, or too much alumina in the clay, will resist the soda/salt as well.

[Marcia Selsor]

7 hours ago, RogueArtist said:

What temperature is a traditional salt/soda firing? Does anyone do cone 6 salt/soda firing (the traditional way in a gas kiln)? Or is cone 6 just not hot enough for the chemicals to interact properly? (Sorry if this is a basic question - I looked online but I can't seem to find a cone # for traditional salt firing!)

PS: Thanks for all the replies!

yes. Many people do. As long as the clay reached maturity the soda will act accordingly. BUT a soda wash is just a wash. In an atmospheric condition like a soda firing in the traditional sense,  the soda volatilizes etching the surface dramatically. Not so in an oxidation atmosphere.

Marcia