Chrome & Zinc - toxic?

[Beccap]

I started making a new glaze using a Stephen Murfitt satin matt base recipe that contains 3% zinc oxide. I understand zinc oxide is not the problem but that it may contain traces of lead? But not in levels I should be concerned about? That’s at least the conclusion I’ve come to but please correct me. 
 

The glaze makes a lovely deep green, but it contains 1% chrome, 2% cobalt, 1% RIO and now I’m worried that it is not food safe? I haven’t experienced any leaching. 
 

I’ve read the type of chrome potters use is not toxic and should be ok? But in the presence of zinc is it still ok? And also is 2% cobalt safe?

 

feeling lost and very confused …..

Edited April 7 by Beccap

[Babs]

23 minutes ago, Beccap said:

I started making a new glaze using a Stephen Murfitt satin matt base recipe that contains 3% zinc oxide. I understand zinc oxide is not the problem but that it may contain traces of lead? But not in levels I should be concerned about? That’s at least the conclusion I’ve come to but please correct me. 
 

The glaze makes a lovely deep green, but it contains 1% chrome, 2% cobalt, 1% RIO and now I’m worried that it is not food safe? I haven’t experienced any leaching. 
 

I’ve read the type of chrome potters use is not toxic and should be ok? But in the presence of zinc is it still ok? And also is 2% cobalt safe?

 

feeling lost and very confused …..

Covalt oxide or cobalt csrbonate?

Photo of glaze and recipe will help folk answer.

I would not use chrome on inside of functional ware. Lots of folk use liner glazes because of ingredients.

Havent heard of folk worrying about Zinc Oxide.

Wearing gloves and proper mask would protect you when mixing glazes and applying glazes.

[Beccap]

I’ve read that chrome & zinc can make zinc chromate ? And that’s a carcinogen? Now I’m worried….. does that happen in the kiln?

 

I read the type of chrome potters use the green one, is actually low in toxicity but also may be wrong. What about chrome pink seems to be food safe?
 

I’ve used 2% cobalt oxide. 

I will try get a picture but it’s just a lovely green….

 

[Denice]

I don't use any Chromium in my studio,  I considered it  dangerous to my health.   One of the first things my glaze professor told us was to wear mask and gloves and don't  use chromium if you have a open cut,  it can kill you.   I decided that there was different ways to make green glazes,  I can live without chromium green.

[Min]

Let's not throw the baby out with the bathwater here. The form of chrome used makes all the difference in the world in regards to toxicity.

Potters use chrome oxide, it is insoluble in water and gets bound in the glaze matrix in a balance glaze. It is volatile in the kiln so it can cause a pink blush on tin white glazes. Wear gloves if you get contact dermatitis with it. (it's a good idea to wear gloves whenever handling raw materials and glaze slurries to avoid skin reactions)

We do not use the exceedingly dangerous hexavalent chromium in ceramics. Iron chromate is toxic, stay away from using it.

Zinc oxide fumes from kiln firing shouldn't be breathed in, zinc can become volatile above around 850C/1560F and like all the materials we use in glazes practice good studio hygiene and wear a respirator while handling dry materials. In a glaze it isn't a problem. 

Chrome + tin pinks and reds are perfectly safe, whether in a stain or from the oxides.

 

 

 

[Dick White]

As others have already noted, neither zinc oxide nor chrome oxide used in reasonable amounts in a stable glaze are toxic. A different issue arises when both are used together in the same glaze - ugliness abounds. Chrome is supposed to be green when in a glaze that does not contain tin. A combination with tin in the right amounts creates a variety of pinks and reds. Zinc with chrome, however, turns assorted shades of brown, when copious amounts are used. It won't hurt you, but you probably don't want to look at it. Perhaps your glaze recipe doesn't have enough for the adverse color reactions.

Edited April 7 by Dick White
typo

[Beccap]

Yes I think when I started using 1% chrome I didn’t realise the dangers associated with it, and then looked it up and read the type that potters use is not the super scary harmful one. My over thinking brain has maybe thought in the presence of zinc, if chrome turns brown - is that chemically altering it to something that is toxic? And I can’t see it because there is also cobalt & rio so the colour change is not there? I think I am possibly overthinking the combination of zinc and the use of chromium oxide….

Edited April 7 by Beccap

[Babs]

Cobalt being v expensive I wonder if your glaze could handle less. 2% seems a high amount of cobalt oxide.

 

[Rae Reich]

Oh, that is a lovely green! It looks enough matte, though, to use a liner glaze with it on food surfaces. A white with tin in it could blush nicely at the lip of a vessel. (Use a liner because a matte glaze doesn’t clean as well as a gloss and can harbor bacteria over time, not to save the user from leaching-chrome exposure.)

I think you’re right that the cobalt, besides modifying the chrome, also helps to keep it from ‘browning’ or to rescue a brown by sending it toward blue - like the  little-bit-of-cobalt “cheat” in copper reds that rescues an uneven reduction red from ‘snot green or bleached white to soft blue (I learned this from a Tom Coleman student).

[Beccap]

Yes it is a lovely green! I think the feedback is good about using a liner glaze for this one. But relieved to understand that it is the Chromium III that potters use and that isn’t the highly toxic one. Here is also where I read a bit more on the topic 
https://glazy.org/posts/325743 

Would anyone know a situation in which the chrome in the glaze would convert to the more toxic one? I’ve read it can happen in lead glazes but also non lead ones too.  I’ve read that’s possible and I suppose that’s why I got scared if zinc changes its colour, maybe it is changing it chemically? I have no idea what zinc chromate is or if it is a thing in pottery or if my glaze would do it but equally wondering if after firing there’s a possibility of the chrome changing. 
 

I did message Linda Bloomfield also and her response was that the oxides would be locked in a stable glaze and only amounts above 4% would you begin to question the potential of leaching. It is a recipe that is very similar one she has for a satin matte glaze, and Stephen murfitt has it also. I don’t have the recipe to hand right now. 
 

I think the glaze is actually fine. It hadn’t leached and is a satin base I use a lot. But wanting to educate myself as best I can when using these ingredients. 

[Beccap]

Oh ive remembered the glaze recipe!

 

Edited April 12 by Beccap

[Roberta12]

@BeccapLovely glaze!   cone 5/6?    Lovely mug, too!

[Min]

1 hour ago, Beccap said:

Would anyone know a situation in which the chrome in the glaze would convert to the more toxic one?

Nope, trivalent chrome does not convert to hexavalent chrome in glazes.Re zinc chromate, according to Hansen "Chrome in zinc glazes tends to form the stable crystalline compound, zinc chromate (ZnCr2O4). which is brown." from this link. 

I plunked your glaze recipe into the glaze calc program I use but I'm not confident in the accuracy of it since specific types of potash feldspar, talc and china clay aren't mentioned in your recipe. Using generic analysis the formula is on the low side for silica though. I would do a lemon slice test on it and see if the colour changes., also keep a test piece in the dishwasher, if you use one, for a few months then compare that to an unwashed piece.What is really important is to have a stable base to prevent leaching. With colouring oxides, aka transition metals, less is better.

 

[neilestrick]

2% cobalt oxide is a lot. Even 2% cobalt carbonate is a lot for most glazes. I would definitely test lower amounts of cobalt and see if you can get away with less.

Most chrome-tin pinks (reds) use about 0.2% chrome, which is very little. I just tested a light pink that only uses 1/10 of that! Vey difficult to weight out for testing!

[Beccap]

Right yes - I’m slightly scared of that word zinc chromate - I think that’s where I read it actually was the Hanson page. If you google zinc chromate it says highly toxic and contains the bad version of chrome. But does that apply to pottery? And there is iron in the glaze - can that combine to make iron chromate? Or am I not understanding how that actually works aka overthinking putting ingredients together makes a/b/c….
 

I did chat more with Linda Bloomfield and she said the base I use is one she uses and it’s a fairly fine one. I source all my materials from bath potters so all ingredients literally just say ‘potash feldspar’ ‘china clay’ etc. 

Re: cobalt, I have tried it with 1% cobalt and it works so will go back to that. 

 

[Min]

The zinc and chome are tied up in the glass matrix, if the glaze is stable.

If you still have concerns there is also the option to supply the chrome with a stain. This type of stain is a spinel stain, the chrome is bound with silica and is very stable. Stains are suspended in the glaze matrix, they don't dissolve.  I don't know what is available on your side of the pond, over here there is Mason  #6209. Then the glaze would have the zinc + iron which isn't an issue either. 

44 minutes ago, Beccap said:

Re: cobalt, I have tried it with 1% cobalt and it works so will go back to that.

Might be okay with even less than that, same with the chrome. A couple progression tests would show you how much you could reduce both colourants.