Leaching & Unsafe Glaze Surface

[Linnet]

I was once told that if a glaze that should fire to a gloss finish has any amount of matt finish then it would not be suitable for food due to leaching? I would think this would also depend on the glaze ingredients. Can anyone add to this please

[Old Mike]

I was once told that if a glaze that should fire to a gloss finish has any amount of matt finish then it would not be suitable for food due to leaching? I would think this would also depend on the glaze ingredients. Can anyone add to this please

 

Linnet,

A simple test would be to lay a slice of lemon on the piece overnight and see if it does anything to the glaze. That covers the acids which covers most food items. You could also run it through the dishwasher a few times and see if it fades as that would cover the bases.

Mike

[Carl]

One of the 'old codger' potters told me that he does the lemon test as Mike suggested in the previous post, but also pours vinegar into/onto the ware to check to see if anything leached out into the vinegar overnight. He also said to check the lemon itself to see if it showed any signs of discoloration due to leaching. Not sure if that is enough, but he seemed to think that was appropriate for items used for food.

[Linnet]

Great, will do both, thanks for that

[Old Mike]

Linnet,

The testing methods I mentioned are certainly not like sending it to a lab as a later post mentioned doing. That would be expensive though and your money might be better spent in buying Mastering Cone 6 Glazes and a glazing computer program. The book would teach you what makes a stable glaze and the glaze program would let you see if you have one. On the pieces you already have that you are wondering about, I'm more curious about why they didn't come out glossy. The first two things that come to mind are a mistake in making the glaze especially if you are mixing your own from raw materials (suppliers of glazes can make a mistake on a batch too though that would be less likely or that they were underfired. Underfired would be my first choice and if that is the case you could simply refire them and make sure you reach the correct temperature to mature them. We once had a glaze that should have been gloss turn matte because when it was extended from the 100g. formula to 2000g a mistake was made in one extension and it had 40g too much Custer Feldspar in it. If it had been wonderful it might have been worth the price of testing but it wasn't so it was discarded. If a mistake was made and you can't identify it, you won't be able to reproduce it anyway.

Mike

[Matt Katz]

I was once told that if a glaze that should fire to a gloss finish has any amount of matt finish then it would not be suitable for food due to leaching? I would think this would also depend on the glaze ingredients. Can anyone add to this please

 

 

There is no corelation between Matte glazes and food saftey. A properly formed Matte Glaze is a gloss glaze that crystals form in upon cooling. This is a result of composition and an abundance of Alumina and Calcium (or other Alkaline Earths) in your glaze matrix. Because of this, a properly composed matte is food safe.

Improperly formulated mattes are under fired and can be non-food safe

[dee kat]

I was once told that if a glaze that should fire to a gloss finish has any amount of matt finish then it would not be suitable for food due to leaching? I would think this would also depend on the glaze ingredients. Can anyone add to this please

 

 

 

I recommend you read Mastering Cone 6 glazes and what is on this site.

http://www.frogpondpottery.com/glazestability/stableglazes.html

 

You can have a glaze that is matt because it is underfired (and therefore not good to put liquids in or on that are to be consumed) or ones that are matt from crystal formation, which is what you get from the matt recipes in the MC6 book. My favorite glaze is Bone which does wonderful things with just about every glaze I have tried it with. I once casually used the term food safe (meaning a stable glaze) when referring to a MC6 glaze and was called on it by one of the authors. I will never use that term again. However I would say that any glaze that is only using iron or tin for its colorant or opacifier (and doesn't have lead or barium as a flux) isn't going to be much different than what we cook food in (glass casseroles and metal pans). IMO we have gone a little nuts on the 'food safe' issue. You want a glaze that is not going to change with normal use. A cup should not change color because you put it in the dishwasher or used it for orange juice.

[Guest JBaymore]

Having a glaze tested for the release of oxides in not that expensive. For testing for one oxide it will typically cost you under $75, plus shipping. Add about $30-35 for each additional oxide. (If you use lead and/or cadmium compounds on functional wares, if you are NOT doing this you are in violation of FDA regulations.)

 

First of all, in order for you to be really concerned there has to be an oxide present that has some toxic properties upon ingestion. These form a small subset of the oxides used in glazes. For example you don't really have to test for Al^₂O^₃ release since it is basically benign. Additionally, it has to be present in a molecular equivalent level that is significant enough to potential pose a hazard should there be release. This further narrows things down. And the amount of surface covered by the questionable glaze has to be a high enough percentage of the total area to have enough materials there from which to actually leach material. It is a complex subject.

 

The lemon test and the place in vinegar test and many other "home remedies" are based upon the "Mark 1 Eyeball" gauge.......... which is the main test that is typically used by most potters. It is not telling you the story when it comes to a lot of factors. You can't easily see low levels of leaching or degradation. If you can easily see it with the naked eye..... that is just telling you it is pretty darn bad. If it "looks like a good glaze, smells like a good glaze, and walks like a good glaze" most people think it is just fine. This is not necessarily the case. Testing for "food safety" this way is not a very good nor thorough approach. If there is something of concern in there...... you likely really ought to be doing more "due dilligence".

 

Conversely, just because a glaze is matt does not mean that it leaches. It depends on the chemistry of that glaze and exactly what is percipitating onto the surface to form the matting effect. Understanding glaze calc helps you understand the potential for leaching in this regard. Popping the recipe into something like Insight software and looking at the formula can tell you a lot.

 

If you don't have the technical background to get into all of this stuff... then one good approach is to just limit your glaze recipes to supplying oxides that have little to no toxic properties. That means even if the glaze is not "hard" .... what might leach out is not a health hazard to the user. (This says nothing about the potential visual changes the ware might exhibiot over time.)

 

best,

 

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