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Biglou13

Food Safe Cone 6 (5) Ox Glazes Please

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It seems to be a no fuss, no muss, sort of glaze.  That's all I have used with the fcs, is a standard firing schedule.   I forgot to include just the base recipe in the pictures, but it's a nice brownish.   It doesn't run or move, lighter application gives more of the brown tones in any of the colors.   I had one weird thing happen when I first started using it.   With the copper carb fcs.   The outer layer sort of rubbed off when the customer put cranberries in it.   After having run a lot of tests here at my home, I came to the conclusion it was a anomaly.  I have not had a problem with it since.   I have even layered up the colors.   blue over green is nice.  

 

Roberta

 

I ran the glaze through Insight, the silica level is below limits by a little, plus the alumina is within limits but on the low end. Lithium levels is over when using Leadless Traditional Limits,  Doesn't look like a  durable glaze when you look at the numbers. 

 

 

CaO 0.36*

Li2O 0.25*

MgO 0.20*

K2O 0.07*

Na2O 0.12*

P2O5 0.00*

TiO2 0.01

Al2O3 0.29

B2O3 0.20

SiO2 2.29

Fe2O3 0.05

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Note that that above analysis from John's site is the original with albany slip.

 

One mol of Albany has almost one more mol of silica compared to Alberta.  It is also well higher in alumina and iron.

 

With Albany instead of Alberta... it is a very different glass...... with alumina just over top limit, silica in the mid range of limit.  Those two changes alone affect the stability greatly in a positive direction.  (There is still that ioversupply of lithium and the solubility issue.)

 

The changing of the original Falls Creek recipe using Albany Slip to one using Alberta Slip seems to have been done by someone with what is known as a "materials based" approach to glazes........ which ignores molecular realities a lot.  Alberta Slip is a slip clay sort of like Albany... so just sub them.  It does not exactly work that way.

 

And also note that Ron is the more serious "tech weenie" of the duo. ;)

 

best,

 

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

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I went back and reread a thread about lithium carb.   It was from Nov 2012.   I had asked about that bowl that the cranberries attacked.   The same info from John came up then as now.   I called the author of the article after that happened and she assured me it was something she had never seen.  She urged me to try the glaze again and to run the cranberry test.  Since that keeps coming up negative, I went my merry way and have been using that recipe since then.  I will say that I mostly use it on the outside of things.   But....after reading all of this, I will just use it on non functional pieces and find a new favorite glaze.  Thanks so much to everyone on this forum for keeping me on the straight and narrow. 

 

Big Lou! Disregard what I said about FCS!!!  

 

Moral of the story....really dig into the chemistry of a glaze....even if it is published in a respected magazine!!  :)

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attachicon.gifAlbany.png

 

Here's a comparison of Albany Slip, Alberta Slip and two Albany Slip substitutes.

 

You know if you keep posting interesting things like this I'm never going to get any work done! 

This is what I came up with to more closely match the original albany slip recipe. (I still wouldn't use it though because of the lithium levels)

 

falls creek shino with red art etc

 

Gerstley Borate 18.00

Lithium Carbonate 6.50

Minspar 200 10.80

Silica 8.50

Redart 45.70

EP Kaolin 2.30

Talc 2.30

Calcium Carbonate 5.80

99.90

 

CaO 0.39*

Li2O 0.28*

MgO 0.16*

K2O 0.08*

Na2O 0.08*

P2O5 0.00*

TiO2 0.02

Al2O3 0.31

B2O3 0.22

SiO2 2.65

Fe2O3 0.07

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attachicon.gifAlbany.png

 

Here's a comparison of Albany Slip, Alberta Slip and two Albany Slip substitutes.

 

You know if you keep posting interesting things like this I'm never going to get any work done! 

 

Yes, I will acknowledge here that this site is addictive. I thought it was only me as I have been potting in the wilderness for decades!

I now say, only 1 session and two articles, not strong enough!

EDIT

What, if any, are acceptable levels of Lithium for foodsafety, given all the variancies of firing et al

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Can you just up the silica, rio, alumina... To bring the chemistry closer, and more stable (ish)

If so how much?

I'm sure I'm over simplifying.....

That is easy enough to do but that's not the main issue with this glaze. The lithium is way to high and can cause both crazing and shivering on the same piece. Slivers of glaze can pop off the pot. Bowl of salad with glass on the side type scenario. Crystals growing in raw lithium glaze slurry can be a problem too.

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I haven't had one instance of shivering or crazing.  Not one.   I am not sure why, but I haven't.  I have used these glazes a lot on knitting bowls.  But, I have had the crystals growing in the glaze buckets.....I sieve them out.   Should I not be doing this??  

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By seiving the crystals out you are removing some of the soluble materials from the glaze... and alterring its chemistry in the fired form.  THAT may be why you are not having issues ;) .... the main solubles in that glaze are lithium and sodium compounds.

 

So the calc that shows is not the stuff on the pots.  Which in this case may be better if the look is still good.

 

You can do tests steadily dropping the lithium carb and see what the glaze looks like as that is disappearing.  MAYBE it will be ok without it.  (Haven;t calculated the glaze without the Li2O.)

 

best,

 

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

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can some give me their opinion and or facts on  glazes with lithium.

 

if the chemicals are "glassed"  and aren't leaching  then shouldn't this be safe?

 

It is pretty simple in that black and white case you gave there, Lou.  If the glaze doesn't leach lithium (or anything else), then there is no problem at all.  The "grey area" part of the equation is answering that question........"Is the glaze leaching anything toxic into foodstuff?"

 

The absolute ONLY way to know that for sure is to have random (and regular) samples ascetic acid lab tested to see what is "coming out" of the glaze.  In the USA, other than for lead and cadmium, there are no other "pottery laws" that apply to the leaching of anything out of glazes.  So for everything else, the best potential "guidelines" we have are the US FDA drinking water standards for various compounds.  SInce thiose guidelines are meant for drinking water conmsumption, they are based on large quantites of daily consumption.  So they would be VERY conservative standards to use.  But they can help point you in the right direction.

 

Then you come to the issue of manufacturing controls.  How accurate are your processes (glaze raw materials pre-use testing, glaze mixing, glaze application, kiln firings as to firing cycles, even-ness of chamber, and so on)?  The more accurate these manufacturing controlls are (like in industry), the less often you will have to do lab testing to be sure.  This is because with good controls, that helps assure that the samples tested actually DO apply to the general production runs.

 

And of course you should be having documentation of all of this stuff you are doing on file "just in case".

 

That above being said, there are product liability laws that still do apply to potters.......... which would be tested in an individual court case if a person feels that your product has caused them harm.  If you manufacture a product, and that product injures someone in some way, and they can prove that fact in a legal setting, you are legally liable for damages. (Generally a civil situation.)  If there are things in your professional practice that relate to the safety of your product which you should have been aware about (and it can be proven that you should have been), and you can be proven to have deliberately choosen to ignore that information, then you can also be held responsible for willful negligence above the normal liability issues....... which opens up some other nasty legal headaches and potentiall punitive damage claims (usually really BIG $ above the other damages).

 

Now....... how often this kind of thing actually happens is pretty much reflected in the product liability insurance available for potters.  You can pick up about $2 million in coverage for such issues (and also show and premises liability) for less than $1000 a year.  In the insurance world, cheap.  THAT says that this is not a huge legal issue for potters.

 

We also then come down to the "morality" portion of the question.  A potter has to ask themselves if they are willing to "gamble" with the health and well being of the people who have purchased (or been given) their products.  And how much they are willing to gamble.  There are no "morality police" (at least in this country).....only one's own conscience. 

 

That's the best I can give you.

 

best,

 

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

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