Clay, glazes and food safety for functional ceramics

[Olena]

Hello everyone!

The moment of glazing my bisques is approaching and as that would mainly be mugs, plates I would like to do it so it is food safe. I am using stoneware clay that is to be fired at 1200 •C and I am surprised to find that there are actually not so many mid/high temperature commercial glazes available. I see many functional ceramics on the internet that were glazed in layers getting some incredible results. However, i have read that layering glazes might not be safe because the glazes can be food safe separately but if you combine them it can have a different reaction. Another situation if I paint with with a food safe underglaze and then cover it with a food safe clear glaze, can that potentially have a poisonous reaction too or is it thinking too much? And like people who create they own glaze and use it for functional ceramics how can they/do they make sure it is food safe? So the thing is that I would really love to experiment with layering glazes and using underglazes that are to be fired at cone6 or higher temp but I want to know that the final result is safe to be used by me or sold to customers. I guess it is expensive to do the lab tests every time one uses new glaze combination so how do you normally test that the functional ceramics you make is food safe? I read about some tests with lemon and vinegar but is it enough?

Hopefully I don’t sound  too paranoid about that Thanks! Olena

[Min]

Hi Olena and welcome to the forum.

10 hours ago, Olena said:

so it is food safe

I've never come across a definitive definition of "food safe". For glazes I think it's much more realistic to look at glaze durability for functional ware. For glazes that will have food contact you want a surface that won't leach or cutlery mark. To be safe many potters use a liner glaze that doesn't contain any harmful materials so even if the base glaze does leach there will be nothing to be of concern leaching out. These liner glazes are usually clear or white as it's the colouring transition metals (sometimes called heavy metals) that generally cause the most concern.  

The lemon or vinegar test will show which glazes leach when exposed to acids and keeping a test piece in the dishwasher for a couple months of daily use will show alkaline/basic leaching glazes. These tests rule out glazes that are obviously leaching but they don't rule in a glaze as being 100% safe, the only way to do that is to get it lab tested which I don't think many potters do.

There are many commercial glazes out there that are sold as "food safe" or "dinnerware safe". Testing standards for these glazes might just be for lead or cadmium and not all the potentially leaching ingredients in the glaze are likely tested for release. 

 

[Olena]

On 4/17/2021 at 7:45 PM, Min said:

Hi Olena and welcome to the forum.

I've never come across a definitive definition of "food safe". For glazes I think it's much more realistic to look at glaze durability for functional ware. For glazes that will have food contact you want a surface that won't leach or cutlery mark. To be safe many potters use a liner glaze that doesn't contain any harmful materials so even if the base glaze does leach there will be nothing to be of concern leaching out. These liner glazes are usually clear or white as it's the colouring transition metals (sometimes called heavy metals) that generally cause the most concern.  

The lemon or vinegar test will show which glazes leach when exposed to acids and keeping a test piece in the dishwasher for a couple months of daily use will show alkaline/basic leaching glazes. These tests rule out glazes that are obviously leaching but they don't rule in a glaze as being 100% safe, the only way to do that is to get it lab tested which I don't think many potters do.

There are many commercial glazes out there that are sold as "food safe" or "dinnerware safe". Testing standards for these glazes might just be for lead or cadmium and not all the potentially leaching ingredients in the glaze are likely tested for release. 

 

Thanks, that’s helpful. When it comes to underglazes would a layer of an appropriate clear glaze be sufficient? Or you would recommend running a lab test in this case as well? To avoid leaching, I mean

[Min]

5 hours ago, Olena said:

When it comes to underglazes would a layer of an appropriate clear glaze be sufficient?

I don't think that question can be answered with 100% certainty. If the underglaze doesn't contain a cadmium inclusion stain then it's probably okay if the glaze is a well balanced and durable one. If the underglaze contains a cadmium inclusion stain I'm less confident in saying that.  It seems there are more and more stains, underglazes and glazes that include cadmium inclusion stains but not a lot of literature about potters getting them tested. 

To be 100 % safe you could use a liner glaze without underglaze beneath on food surfaces and keep the underglazes to the outsides of the pots.

[Olena]

Yeah, liner glazes would be a solution. Any in particular that you would recommend? I found just the Coyote line. Hopefully, these are not so limited on the European market. That’s interesting that before when I read “food safe” glaze I assumed no further testing would be required because you would expect them to test for leaching as well in order to tag it as a food safe glaze. 
 

[Min]

@Olena, maybe start another thread asking about the commercial glazes you have available to you and ask which ones forum members have used with success. (I don't use commercial glazes so can't help with that)

[Callie Beller Diesel]

Glaze manufacturers can make a claim about a glaze being food safe only if it’s fired under the same conditions that the manufacturer suggests. And even then, it’s kinda fuzzy because of the lack of concrete rules about what constitutes food safety.  (See Min’s first comment.) As soon as you start mixing or layering glazes, you create a new substance that may or may not have the same properties as the separate ones you’re combining. That doesn’t mean you’re going to suddenly create a super noxious poison lying in wait for unsuspecting pottery users though. Layering glazes might make it less durable if it takes the formulas out of balance, or it might not. It depends on too many things to make a statement that covers every situation. It’s easier to make clear statements if the parameters are narrowed down. 

If you aren’t using materials like lead or cadmium in your glazes, the hazards of glaze mixing will *mostly* apply to your exposure to the raw materials, not the end user. This is a huge simplification of course, and that’s not to say you shouldn’t do due diligence in your testing though. You want your ware to hold up, and the better the chemistry, the more durable the pottery.

[Min]

Here is an example of a "food safe" glaze from Amaco.

Amaco Saturation Metallic.  

 

It shows the "food safe" icon but there is no way I would use that for anything that comes in contact with food. It probably doesn't contain lead or cadmium so it gets their "food safe" label. To get that colour the glaze will be really loaded with transition (heavy) colouring oxides plus the matte finish would rule it out for me.  Notice also that many glazes comes with the caveat "Tableware producers must test all finished ware to establish dinnerware status, due to possible variations in firing temperature and contamination."

[neilestrick]

4 hours ago, Min said:

Here is an example of a "food safe" glaze from Amaco.

Yeah, there's no way I would eat off that. When I worked for A.R.T. Clay and had to do all their safety labeling, we decided to be very cautious with out labeling, and went as far as labeling some of our matte glazes as 'not food safe' purely due to hygiene concerns.