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. 
 

Oh ive remembered the glaze recipe!
 

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. 

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….

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.
 
 
 

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.

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

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).

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….