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BlackDogPottery

Zinc in glazes

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I was going through more glaze research and came across an interesting note on zinc being easily soluble in glazes when strong acids are present. I've seen a few glazes with zinc and one I use myself.  From what understand zinc is a flux. Is it uncommon or would it be practical to replace zinc with some kind of frit or borate if its so easily to dissolve in a glaze?

Edited by BlackDogPottery

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First, can you identify the strong acids in your glaze recipe? If not, then the problem of zinc solubility in the presence of strong acids isn't a problem. I don't know about your glazes, but mine tend not to have strong acidic properties. The bigger problem with zinc glazes is the unattractive way it interacts with chrome as a colorant. If you are in total control of the oxide colorants in your glazes, just keep chrome away from a zinc-bearing glaze. If you use stains, either in the glaze or in an underlying slip, you need to learn what oxides were used by the manufacturer to achieve that color. Mason publishes a reference guide, but other sources are a mystery.

As for replacing zinc, that is a challenge. Zinc performs as one of the RO fluxes. But it also imparts other qualities to the glaze (opacity, surface durability) which are not easily replicated by swapping with other fluxes.

Rae Reich and BlackDogPottery like this

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I wouldn't worry about zinc solubility.  As far as I know, the only strong acid commonly used in glazes is muriatic acid as a flocculant.  I've never met anyone who's actuallt still does this.

Glazes are generally mildly basic (or strongly if you like unwashed wood ash).  Nothing really to worry about.

BlackDogPottery, Joseph F and Pres like this

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1 hour ago, oldlady said:

i read this question differently, that the worry is to the end user once the piece is finished.  does zinc somehow work its way out of the ceramic into the human?  

Yes,  that's it. It mentions zinc can leach but it's not a hazard unlike copper or cobalt. But it made me wonder why use something that could leach when there's other materials like borate that could do the same thing? Is there pro's and con's with using zinc in a glaze besides being an active flux?

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We had a Potters Quiz of the Week question not too long ago about zinc oxide. It is an interesting material in glazes, as it can act as a flux and as an opacifier to glazes. It seems that unlike most fluxes it retains its crystal identity, and some of its opacity. Also, depending on the flux used with it, It will also have an effect on some coloring oxides that may not be desirable, turning some greens to more brown, and changing some other colors. Soda, potach and calcia colors with cobalt and copper seem to remain clean and pleasant.  It works well with iron colorants, cobalt, and some others, but testing is needed.  As there are advantages, there is the possibility that it will promote crawling. Another disadvantage is the price. 

 

best,

Pres

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1 hour ago, Pres said:

We had a Potters Quiz of the Week question not too long ago about zinc oxide. It is an interesting material in glazes, as it can act as a flux and as an opacifier to glazes. It seems that unlike most fluxes it retains its crystal identity, and some of its opacity. Also, depending on the flux used with it, It will also have an effect on some coloring oxides that may not be desirable, turning some greens to more brown, and changing some other colors. Soda, potach and calcia colors with cobalt and copper seem to remain clean and pleasant.  It works well with iron colorants, cobalt, and some others, but testing is needed.  As there are advantages, there is the possibility that it will promote crawling. Another disadvantage is the price. 

 

best,

Pres

It's definitely a good opacifier. I ran a test substituting the zinc with borate plus another 10% borate. The difference was night and day. The zinc version was very opaque with a nice smooth surface, off white. The gerstley borate trial was clear as it could be but crazed all over and cracked the piece in half from thermal expansion I believe, the surface however was smooth but felt cheap like imported earthenware.

Edited by BlackDogPottery

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Because of its solubility, don't drain the water off the top of our glaze bucket as a way of thickening Zinc containing glazes. Not a good practice anyway, I guess.  But as said above, not likely to have strong acid Glazes anyway.

BlackDogPottery likes this

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38 minutes ago, Babs said:

Because of its solubility, don't drain the water off the top of our glaze bucket as a way of thickening Zinc containing glazes. Not a good practice anyway, I guess.  But as said above, not likely to have strong acid Glazes anyway.

Sorry if my question missed the mark but I was meaning is the zinc leachable after firing.  As in a zinc glaze inside a coffee cup.

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2 hours ago, BlackDogPottery said:

Sorry if my question missed the mark but I was meaning is the zinc leachable after firing.  As in a zinc glaze inside a coffee cup.

Could you cite the source you got this from or a link to it?

 

 

Edited by Min
typo

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8 minutes ago, Min said:

Could you site the source you got this from or a link to it?

 

 


"The use of zinc in standard glazes is limited by its price, its hostility to the development of certain colors and its tendency to make glazes more leachable in acids (although zinc itself is not considered a hazardous substance)."  From Digitalfire  https://digitalfire.com/4sight/material/zinc_oxide_1718.html . I also have an old 70 ceramics book that more or less notes the same thing. Just curious.

Edited by BlackDogPottery

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38 minutes ago, BlackDogPottery said:


"The use of zinc in standard glazes is limited by its price, its hostility to the development of certain colors and its tendency to make glazes more leachable in acids (although zinc itself is not considered a hazardous substance)."  From Digitalfire  https://digitalfire.com/4sight/material/zinc_oxide_1718.html . I also have an old 70 ceramics book that more or less notes the same thing. Just curious.

Thanks for the link. I think this comes down to making good glass. With glazes used for all surfaces that could come in contact with food don't use Pb, Cd and Ba , fire the glazes to maturity, use gloss glazes and wash the pots before using them. Tyler linked some good studies on leaching in this thread. If in doubt it doesn't cost much to get a glaze lab tested.

https://static1.squarespace.com/static/527ac372e4b0d4e47bb0e554/t/560584f4e4b040325112ea61/1443202292258/GlazeSafety-Carty-2014.pdf

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