Glaze ingredients to avoid for food safety

[Hailey]

Okay I know that the only way to have something truly guaranteed food safe is to have it tested by a lab. But I need some *less official* advice. 
I’m wanting to get into making my own glazes but I’m scared about food safety. I understand the whole no matte, crystalline, or crackle glazes for the bacteria thing, but what I’m worried about is literally poisoning people.

So my question is: which glaze ingredients should never ever be used on the inside of a functional piece?

lead and cadmium are a given, are there any others? I thought cobalt was one but I’ve seen Joe with Old Forge Creations using a cobalt floating blue on the inside of his mugs. I also thought maybe copper? 
 

like I said, my concerns about food safety are not about bacteria or anything, but about toxic chemicals. Since no one has given me a straight answer, I don’t even know if my basic clear and white liner glazes are considered food safe. If they fit my clay and have no toxic ingredients, they are safe right? (Again, unofficially since not tested by a lab) Like is it even possible for a white/clear basic glaze to be unsafe without the colorant oxides and stuff?

[Min]

First off: defining the terms. “Food safe” as it applies to Canada (and the US plus many other countries). In Canada only two oxides are regulated, these being lead and cadmium. This doesn’t mean glazes can’t contain these oxides but the amount of said oxides mustn’t exceed Federally regulated amounts when leach tested by a lab. More info on that here if you want it. For practical purposes potters don't use any lead and the cadmium we use is in the forum of inclusion stains which in theory render the cadmium locked into a structure that is stable. (I do know of one person who uses a cadmium underglaze inside her pots and had lab testing done, results came back with the cadmium well under allowable levels)

Example of what glaze companies call “food safe” here.  This is just ridiculous, the glaze obviously contains high amounts of colouring oxides and yet since it doesn’t contain lead nor cadmium they can put their “food safe” icon on it.

I feel a more appropriate term than “food safe” is durable. If glazes can withstand heavy usage without showing degradation of colour or gloss level, the glaze is fully melted (this is super important, excludes matte glazes that are matte simply because they are a higher fire gloss glaze that are underfired versus a properly formulated matte), glaze doesn’t contain out of proportion amounts of transition oxides (heavy metal colourants), has as adequate levels of both silica and alumina and the flux ratios fall within reasonable levels, ideal flux ratio being 0.3 R2O:0.7 RO. Meet these requirements and the glaze is likely to be durable.

This is where looking at the glaze chemistry on a molecular level using the Unity Molecular Formula (UMF) is very useful. Since you mentioned Old Forge Creations work we are talking cone 6 glazes here. For those the alumina and silica levels of a UMF should be at least 2.5 and alumina at least 0.275  Silica and alumina can both be higher than these minimum levels as long as there are enough fluxes to melt them. 

One other area that is worth mentioning are crazed glazes. One school of thought is a crazed glaze harbours  bacteria and therefore is a health hazard. Have a read through of Ryan Coppage testing of crazed glazes here. The article is titled “Dirty Dishes” and is about 1/2 way down that link. In a nutshell what it demonstrates is that if a clay body is vitrified and the dishes cleaned well after usage, preferably in a dishwasher, then there is nil to minimal risk of bacterial growth. Crazed glazes to reduce the strength of a pot but that’s another issue.

So, onto how one tests for leaching oxides, defining the terms of what is an acceptable level of leaching. Since only lead and cadmium are regulated (for now, see below for the EU) then a potters next best thing to use as a guideline are drinking water levels of the other oxides. This in itself is not very helpful unless you actually know by lab testing how much leachate is coming from your glazes. So, that leaves one with the option of getting a questionable glaze lab tested. Brandywine Science Center. Cost is $30 (US) for most oxides tested for. (a few oxides are 60-) Again, I don’t know many people who actually do lab test their glazes. I have had a few tested, Brandywine sends you an email with results in about a week.

So if it isn’t financially logical to have all of ones glazes lab tested at at least $30 a pop that leads one back to common sense and home testing. 

Common sense part of the equation is not to overload the transition metals (aka colouring oxides) or use barium for glazes that come in contact with food. An example would be if you see a black or dark blue glaze with more than about 2% cobalt carb  I would start to question it. Lower the colouring oxides to more acceptable levels. Check the formula in a glaze calculation program if you are comfortable using them. Glazy is free and will give you a UMF where you can look at silica and alumina levels etc. It also plots your glaze on the Stull chart if you find that useful.

Don’t overload a cone 6 glaze with boron. Lots of the really cool looking visually textured glazes are high in boron. This is fine but be aware that excess boron will make the glaze “soft” and prone to leaching. For cone 6 durable and stable glazes usually a maximum of 0.15 on the UMF is all that’s needed. It’s okay going over this but I wouldn’t exceed 0.30  Even though coefficient of expansion numbers will decrease when you overload a glaze with boron and in theory reduce crazing it’s the opposite that actually happens. Glazes will craze more since boron expansion doesn’t follow a linear pattern after a point.

Home test for leaching; lemon slice on glaze surface or submerge a glaze sample in household white vinegar for a couple days then wash and dry the sample and compare gloss and colour to an untested same and look for gloss degradation or loss of colour. If there is any change then the glaze is leaching.

Lemon or vinegar tests checks for acid resistance. Dishwasher (or soda ash) tests for alkaline/base resistance. There are tests out there that recommend simmering 50 grams of soda ash in 1 litre of water and simmering sample piece for 6 hours then checking for gloss / colour change. I find this test too harsh and prefer just leaving a sample in the dishwasher for a few months and then comparing the sample to an untested one.

Wild and crazy glazes are wonderful and fine for the use on the outside of pots but for food surfaces if in doubt use a liner glaze. Clear or white gloss glaze fired to maturity for a liner. If there is nothing of harm to leach out of a glaze or make it foods taste funny (like copper) then there isn’t anything to worry about.

Really good article about Glaze Safety from Carty and Lee here. 

note: Since you mentioned cobalt, in the EU there are new proposed limits that include cobalt plus a number of other elements. It will be interesting to see how those high alumina matte glazes fare.

Exhale! Short answer to your question would be a clear or white gloss glaze that don’t contain barium fired to maturity should be fine and wash your pots in soapy water before using them.

 

[Roberta12]

@Min You're da bomb.  

[Min]

@Roberta12, thank you.

[Pres]

I would add to this that some chemicals should be either not present in the studio or kept very safe and only used with the strictest of safety measures. It is the health of the potter I think of here. One of those glaze materials used in some glazes is Barium, which I will not allow in my studio, nor in the HS that I taught in.

 

best,

Pres