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High Bridge Pottery

How Much Do You Stay Within Glaze Limits?

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I have been doing a few melt tests and one came out that I liked the look of (see picture) It was a test of 1g of Cornish Stone and 1g of Whiting. 

 

I put that into a piece of glaze software then added extra feldspar/china clay/quartz to bring the glaze into the limit formulas. After that I ended up with three transparent glazes which lost all that I was looking for. I guess the limits are good when you want a nice transparent glaze.

 

How much do you stay within glaze limit formulas?

 

post-23281-0-46763400-1416573339_thumb.jpg

post-23281-0-46763400-1416573339_thumb.jpg

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Guest JBaymore

It all depends on that for which you are looking.

 

The most "interesting" glaze surfaces typically are those that come from some so-called 'imbalance' in the oxide distribution in the melt, most often causing some chemical unevenness in that melt, the lack of full melting of some raw material component, or the precipitation of some silicate type materials onto the surface in the cooling phase.  Or all of the above.  Or takes advantage of a raw material source for oxides that casues a 'defect' as the glaze is melting ... that we look at as "nice" (ie. American Shino crawling). 

 

Keep in mind the "limits" that everyone talks about are for what might be defined as "good glass".  And that "good glass" is defined by relatively modern industrial standards. The criteria has as much to do with stability, REALLY long term durability, and consistency as it does with any aesthetic qualities.  THOSE are criteria for mode rn industry.  (There is a reason that bathtub and toilet and sink glazes look like they do.)

 

If you are not concerned about the same things...... then the "limits" can apply less and less to what you are doing.

 

If you are making food service wares....... then concepts like the leaching of potentially toxic materials likely should be in your list of desired criteria. 

 

If you are making sculptures for outdoor installations, then stuff like durability in acid rain and pigeon poop likely should be in your list of desired criteria. 

 

If you are making floor tile, then hardness and resistacne to abrasion likely should be in your list of desired criteria.

 

Understanding how the various limit formulas might help you evaluate your list of personal criteria is where the art of USING glaze chermistry software comes in. 

 

Only you can decide what those criteria are. 

 

The only formal "laws" relative to the production of ceramics in the USA at the moment are from the FDA and the State of California... and they pertain to any wares that contain lead or cadmium compounds.  Not hing else is formually regulated.  You also DO have what are known as standards from organizations like ASTM for the labels of things like "microwave safe" and "dishwasher safe".  Of course general liability law says if something you make harms someone... you can be held liable in either civil or even potentially criminal (unlikely) situations.

 

Then there is a piece that is the "moral" dilema.  If you make wares that are somehow "sub-standard" in some way....... and you know that they are....... what do you do with them?  For example, if you have a dinnerware glaze that is drop dead gorgeous........ has NO toxic components...... but it is outside limits.... and the way it is outsisde those limits tells you that compared to a piece of commercial Noritake dinnerware....... the surface will not stand up to repeated washings as well......... what do you do?

 

NO easy answers.

 

Anyone who uses American Shino and sells it is "outside limits".  (Guilty!)  Anyone who woodfires and sells work with natural fly ash deposits is "outside limits".  (Guilty!)

 

best,

 

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

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I am stuck with commercial glazes, but I over-fire them to make them look a bit different. :) They are rated for ^05, but I fire to ^03 to make my terracotta/whiteware more vitreous. Gives one of my glazes a real crazy randomness--it ranges from a tenmoku-ey teadust to a pretty kinda shino look, all depending on kiln location and if it's inside a vessel or on the outside. That one is my favorite!

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Why are you stuck? There is always time to buy some rocks and melt them.

Somebody I know bought 1kg of glaze in dry format and it was £20 or something insane. I had to laugh when a 25kg was over £200 and they were calling it a bargain.

I did try commercial glaze once but never really liked them. They didn't work that well and I couldn't look at one part and think 'I can work with that' and drop other parts I did not like.

 

 

I will start looking at the limit formulas as more of industrial guidelines. I can see this cornish stone/whiting working well on the outside of some of my pieces with a lot more work.

 

It was funny how I went about trying to get inside the limits in 3 different ways by having 3 different recipes and they all came out a transparent, must be an ok glass though.

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1. I do not have the money to buy a bunch of raw ingredients.

 

2. I have no facility to mix them. My living room in my 540 square foot house is my studio.

 

3. I do not have the luxury of wasting clay on really bad clay tests.

 

4. I work in lowfire and don't have the same beautiful range as stoneware.

 

5. I like Amaco's glazes.

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I have pretty much given up on trying to use limits. For one thing, they are all over the map, depending on whose limit formula you use. In the second place, my experience has been that the real ingredients you use to conform to a limit, cause wide variations in glaze fit and texture.. even though they are almost identical as far as oxide composition is concerned. as John says, The only real way to get what you are looking for is to know a little glaze chemistry and  keep experimenting till you zero in on these characteristics.

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I hadn't even thought about trying to get the same values but with different raw materials to see what happens. Good idea.

 

I am still waiting for my beautiful stoneware range. Right now it is blue, blue and probably blue. Haven't found anything with copper carbonate that I like, I have an OK iron rich glaze and then my colourants run out... Started playing with rutile which has brought me a lovely new blue :D

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Guest JBaymore

Been doing a lot of reading today and found this. Fits in well with the topic of limit formula and where they arrived from. Interesting thoughts about B2O3 and if it's flux, stabiliser or glass former.

 

http://www.frogpondpottery.com/articles/glaze-stability-literature.html

 

High Bridge Pottery,

 

Ahhh.... you now have officially headed "down the rabbit hole".  ;)

 

There is no end in sight.  "Resistance is futile".

 

best,

 

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

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HighBridge: If you want a nice copper ^10 redn glaze this one is pretty nice (to my taste anyway). Mottled green breaking clear or strawberry depending on your reduction level.

 

https://www.flickr.com/photos/expatat/18864799983/in/album-72157653239150013/

Just kind of wanted to test whether this link would work too. I just finished photographing/uploading all my glaze tiles. The recipe is there too. Amusingly, this is a reboot of an old cornish stone based recipe I used to use. Let me know if you like it, I'll see if I can find it.

 

As to limit formulas etc. I avoid them like the plague.

Don't misunderstand, they have their place. For me though, my main criteria is the beauty/expression of a glaze. Sadly, there is no formula for that.

It also worries me that the terms "limit formulas" "glaze chemistry" etc. intimidate and limit people from exploring a side of ceramics that is easily as expressive and important as form. Really glaze testing is no more difficult than baking (with added PPE). 

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Sorry, also forgot to mention;

A good short cut around worries about leaching/glaze fit/durability etc. is to use liner glazes on food surfaces and decorative glazes elsewhere, Ie nice solid temoku inside the mug/ gnarly metallic mess on the outside. It's a little harder to do as dipping glaze is off the menu (mostly), but cuts your stress level while still using cool glazes.

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Guest JBaymore

 Really glaze testing is no more difficult than baking (with added PPE). 

 

As I've said many times here before, the best visual glazes are that way because they are NOT typically "within limits" and would not be considered "good glass".  (Look at bathtub, sink, and toilet glazes for examples of industrially accepted "good glass". ;) )  So you are totally dead on with the "use a liner glaze" idea.  Stable liners with nothing toxic in them to start with is a good place to completely eliminate such "risks to the consumer" issues. 

 

(Doesn't deal with what we might call "archival stability".... but that is a separate subject.)

 

A lot depends on how you define the term "glaze testing".  If the "Mark 1 Eyeball Test" is what we are talking about, you are also 100% correct.  And in my 45+ years experience, I'd say that is the main (if not only) test that the vast majority of potters use.

 

(Mark 1 Eyeball Test instructions:  Look at glaze with your eyes...... if it walks like a glaze, talks like a glaze, and quacks like a glaze...... it's a glaze.  Put it on pots.  Alternate Mark 1 Eyeball Test description:  If it looks good, use it.)

 

But a LOT of people start doing this stuff with a serious deficiency in their understanding of the technical side of what they are doing.  And go right to selling their work to the public before they really know much about the work they are selling other than visual characteristics (one important characteristic).  Because it is "art", it is somehow considered to be "outside" what someone would invest into learning about manufacturing some other kind of product before putting it on the market.

 

Not as much of a potential issue when people were using more "folk pottery" kinds of raw materials.  But at least in the West here, the "supermarket society" puts a vast array of industrially refined and beneficiated raw materials and supplies into the hands of most anyone that has $ to spend. So the potential to produce stuff that is either not so good for the maker to handle, or for the consumer to use, is increased.

 

best,

 

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

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And go right to selling their work to the public before they really know much about the work they are selling other than visual characteristics 

I hear you, and wouldn't limit that to beginners either. When I was quite young and not a potter, I bought a bowl in Greece that was two amazing shades of blue. I now know it was high barium matte copper and cobalt. They weren't beginners by any means, but us tourists love those very Greek seaside blue pots. I never did use it for food though. Electric blue just didn't look food safe. I suppose in their defense, maybe high Ba wasn't a no-no in the 80's.

I don't disagree, but feel many get scared off before they even start. As soon as you say chemistry peoples eyes glaze over and that the last glaze they make ;)

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Guest JBaymore

 As soon as you say chemistry peoples eyes glaze over and that the last glaze they make ;)

 

Too true.  I have to deal with that issue all the time with my Ceramic Materials classes at the college.  I try to get my students engaged in the "magic" of the "kitchen" before we get too heavily into the science behind it all.  Sneak it in on them  ;) .

 

Yeah, the barium blue matts.... LOVE those glazes visually.   Used to use them back in college days.  Left that palate behind now though for my own work.  Still like the visual.

 

Interestingly most of the concern about the fired glazes comes from the pottery community, not people like the US FDA, and really was an offshoot (rightly or wrongly) of the concerns about the toxicity of handling the raw material barium carbonate for the potter in the studio. Barium carbonate is a bit of a hazardous material to handle in the raw form.  But barium sulphate, for an example, is ingested regularly by people for medical testing (radio-opaque to x-rays)  (   https://www.cmxmedicalimaging.com/MSDS%20Sheets/BRACCO%20Barium/Barium%20Sulfate%20Suspension%20Products.pdf  ).  Note that the barium sulphate content of the contrast medium is quite low.  So the form and the amount of the barium compound we talk about is important to the accuracy of the discussion.  (Ditto to other ceramic toxicological discussions also.)

 

Once contained in the glassy matrix of a glaze, what then might leach out into food acids/bases is the real question.  And the toxicity of this variation in the potential chemistry has, to my knowledge, not been formally studied as to health impacts.  So it appears that in the studio pottery community the fired glazes have been "classed"  as toxic right along with the single main studio pottery raw material source of barium oxide for glazes.   Without real evidence to back it up well.

 

One other issue here is that barium carbonate does not "like" to dissociate into barium oxide readily (lose the CO2).  Barium oxide is a flux on silica, but barium carbonate is .... well.... barium carbonate.  And it does not really become "active" until higher temperatures, so its inclusion in lower firing glazes is "questionable" as  to fluxing effectiveness.  So in THAT case, the glaze has little minute specs of barium carbonate "floating" in the glaze....... possibly opacifying it slightly.. but not really truly involved in the melt.  THAT situation could have some impacts because it is barium carbonate that is still in there not well bound into the glaze.  Prime leaching candidate.

 

In the case of barium matts....... it is certainly possible that some of the high levels of barium carbonate that are included might not be fully dissociating.  So that makes the glazes "suspect" for sure.  Any surface excess barium compounds that the glaze cannot hold in solution upon cooling SHOULD be impacted by the oxygen in the cooling kiln to make them barium oxide, not barium carbonate.  But again... I've seen no real studies on this subject.

 

Then there is the "shift" of studio potters to strontium carbonate, thinking it is the "safe" alternative.  Well.... no... the raw material is not barium carbonate.  But once again..... to my knowledge there are no studies that shows that is any better (or worse) than barium carbonate in glazes as to leaching issues. 

 

Compare the Barium Carbonate MSDS and the Strontium Carbonate MSDS below.  Note all the "not available"-s on the strontium.  It is not used all that much.... except by us potters.  And there is no money for toxicological studies for potters :rolleyes: .   Note the lack of even an LD/50 on the strontium!!!!!   We know that barium carbonate is somewhat hazardous... we do not really know all that much about strontium carbonate.  But because we don't know.... many assume it is "safe". (Back to that "is the gun loaded" question.)

 

http://www.sciencelab.com/msds.php?msdsId=9927090     Barium carbonate

 

http://www.sciencelab.com/msds.php?msdsId=9925106     Strontium carbonate

 

Now... check out the MDSD for barium oxide below.

 

http://www.sciencelab.com/msds.php?msdsId=9923002

 

If there is uncombined (in the glass matrix) barium compounds on the surface of the glaze, and they do convert to the oxide form upon cooling...... hummmmmmmmmm....raises a LOT of questions about the fired glaze safety.  But again, nothing has been formally studied, to my knowledge. 

 

So.... the safe thing to do with the glazes....... assume the gun is loaded... until someone checks in that chamber and sees that there either is or is not a bullet in there.

 

best,

 

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

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No, I don't use the Ba mattes much either (I tend to fire redn where they aren't quite as nice.). They are a horrible texture for food surfaces anyway. I wouldn't slide a fork across it twice  :blink:

I agree about the form. But that really complicates the question. As an example, even if Ba leaches into soln., sulphate is a very common salt and present in most food.In the presence of sulphate, Ba will precipitate out as BaSO4 immediately, and as established is not problematic...So is it safe?

The difficulty now is the "negative proof". Positive proof is easy, if you shoot an arrow at a person and they die, it's dangerous. If you shot an arrow at a person and they aren't hurt, does that mean shooting arrows at people is safe? How many arrows would you need to fire without hitting before you decide it's safe.

Combine that with the fact that we can now measure chemicals down to amazingly small amounts and it's almost impossible to declare anything absolutely "safe". 

How much arsenic is it safe to have in drinking water? we would all answer none. 30 years ago the least we could measure was 1 ppm (part per million) and now we can easily go to 1 ppt (part per trillion) or less. Water that we would have said had no (measurable) As. Now that same water could have as much as a million ppt As...  Sorry, just realised I was heading off on a rant from my previous life, I'm a retired analytical chemist..

It's a debate we as a society need to work through, hopefully we use data to do it. But I have a feeling I'm preaching to the choir.

So would I drink coffee from a Ba (or lead for that matter) glazed mug myself? (assuming I knew the recipe/firing conditions etc). Yes. Would I ever state that it was safe to do? No. Do I want to be the one in court trying to prove the safety of leachable Ba in glazes? No.

So much easier to stick with liner glazes.

 

As a side note, strontium is used as a toothpaste additive for sensitive teeth.  No, there isn't much low level toxicity data on strontium.?!?

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Guest JBaymore

Well said there, patat.  Of course, I'd expect that from someone with the chem background like yours. 

 

The negative proof is the key, isn't it.  No simple answers.  The deeper you dig, the more nooks and crannies in the cave you find to delve into.

 

I use some lead glazed pieces for food also........ Japanese Aka Raku Chawan for tea ceremony.  I have inspected the gun ;)  and know it well. 

 

For many, many years I have been trying to get accurate information on the toxicological issues in ceramics out to people.  In the MIDDLE GROUND...... not the typical "we're poisoning everyone" hysteria....... and not the "I smoked 100 packs a day and drank 20 gallons of whiskey a day and I'm 83 years old" denial.

 

It's that old 100% correct ceramic answer adage....... "It depends.  It depends."

 

best,

 

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

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Great discussions here. I appreciate all the long post with critical data and good reading info links. 

 

I have been pretty apt to stay within the glaze limits. I have ordered some barium carbonate that I keep on the top shelf of my glaze mixing stuff, but I haven't used it yet, and I am not sure I will until I understand everything a lot better. Every time I read an article or follow a link "down the rabbit hole", I spend a lot of time thinking about what I would want if I was buying a pot, and in truth its safety before aesthetic. 

 

This has put me on a path to some pretty neutral colors, but in combination they can be pretty beautiful. The real trick seems firing the colors the right way, using slow cools and other methods to bring out differences in looks. My next big step is building a spray booth for my shed and spraying glazes as the application is so much more controlled than what I am doing now. 

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there are several designs for a water screen glaze booth.  essentially, the overspray is guided down to a drain by a continuous sheet of water from tiny holes in pipes that circle the back walls of the booth.  a small pump returns the water repeatedly with the heavier materials falling into a catch basin.  Bill Campbell in pennsylvania has one and it's construction was part of an article in ceramics monthly years ago.  maybe someone can find it.

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