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qwrkygrrl

Low Fire Shino - Is It Food Safe?

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Hi everyone,

 

I have been trying to get a definitive answer to whether or not my low fire shino is truly food safe. Here is the recipe:

 

Lithium carb 26

Neph sy 64

EPK 10

 

I know that the powdered form of lithium carbonate is considered toxic and precautions should be used in glaze preparation. However once it it fired to maturity (cone 04 for this glaze) it is converted to lithium oxide. I could me misunderstanding, but I have read that in the oxide form, it is safe.

 

Can anyone shed any light on this for me?

 

I love this glaze for its semi-matte texture, beautiful color development, and interesting reactions with other glazes. If it isn't food safe, I would love to know how I could alter it so that it is.

 

Thank you in advance for your help!

Sloane

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I don't think you would even have to go as far as getting it lab tested. I'm guessing a simple home leaching test with vinegar would rule this glaze out as being durable. Plus, with that much lithium I would test for shivering. Totally agree that lab testing is the definitive test to rule in a glaze as being within reasonable limits but I'm fairly confident that this glaze could be ruled out with a home test. 

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How low should the lithium be? Or whatbisnasafe ratio of lithium to silica to make it food safe?

 

I found another glaze that may work, but again, I'm worried about the lithium:

 

Gertsley borate 32

Lithium carb 9

Whiting 17

Neph sy 4

EPK 4

Silica 35

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Don’t know how much glaze chem you know so apologies in advance if any of this sounds condescending.

 

In basic terms, a glaze (for food bearing surfaces) needs to have a certain minimum amount of both silica and alumina in it. Alumina and silica make the glaze more durable and able keep stuff from leaching out of it. The flux(es) also need to be within reasonable amounts.

 

What this means is anything like cobalt, manganese, copper, etc, whether from an oxide, carbonate or stain is not going to stay locked up in the glaze matrix if the silica and alumina levels fall short. The glaze shouldn't loose gloss or show other signs of deterioration over time. The further away from the minimum target level the more unstable it’s going to be.

 

In the image below, your recipe has been converted to a unity formula. Your glaze make up the figures in the purple box. The other box shows “targets†for a ^04 - ^02 traditional lead-free glaze. Have a look at the alumina (Al2O3) and silica (SiO2) row. I think you can answer your own question about whether this glaze going to be suitable for food.

 

As far as lithium levels go you are going to have to do your own testing here. I don't like seeing more than about 5 or 6% lithium carb, but I've seen glazes use more than that. You are getting into an area of thermal expansion here. Glazes that are very high in lithia are by nature going to have a lower coefficient of expansion, have to test on your claybody to see if it fits. Lithia can cause both crazing and shivering on the same pot if the level is too high. Shivering glazes are dangerous, you can have a sliver of glaze (glass) come off the pot (usually on rims and high points) and get into the food/bevies. This is how I test a glaze for shivering; make a thin walled cylinder and glaze the inside only with a thick layer of glaze, include the rim but not the outside of the pot. Fire the cylinder then put it in the coldest part of a deep freezer overnight. Frozen cylinder goes in the sink then gets filled with boiling water then the cycle is repeated a couple times. Hopefully nothing shivers off. (don't know if that's the best way to test for it but it's what I do, same test to check for dunting with low expansion glazes - pot will split in two if really bad fit)

 

 

Hope some of this made sense.

post-747-0-71914300-1473899839_thumb.png

post-747-0-71914300-1473899839_thumb.png

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Thanks Min!

 

As you surmised, I am not very knowledgeable about glaze chemistry. While I've been working in clay for many years, my focus has been on sculpture and decorative items and have been working with the same glaze for quite a while. Now that I find myself venturing into tableware, I am having to address the issue of food safety and durability. My hope was that I could still use the glazes I am comfortable and familiar with, but it seems that will not be the case. Another consideration is my firing temp. I'm now thinking about bumping up to cone 6 for strength/durability. Since I will need to shift to different glazes anyway, this might make sense.

 

Which program or website did you use to calculate the unity formula for my glaze? I think that could be extremely useful to me as I work though this transition and further educate myself.

 

Thank you again for your patience and help!

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