Metallic Taste To Glaze

[ronfire]

I made a couple of mugs and glazed them with Spectrum gold #1112, fired to ^6. I find there is a metallic taste when used. This glaze is listed as food safe, why do I get a flavour. 

I was thinking I might have to put  a clear glaze overtop.

 

[Tyler Miller]

"Dinnerware" safe doesn't really mean anything on a glaze jar.  It just means there's either no lead/cadmium in the glaze or that the lead/cadmium levels are such that it passes their leach test when they tested it.  Since firing conditions vary, your results can and will differ.  

 

Also, it's no guarantee that it's not leaching something else.  Copper is especially evil when it comes to leaching, but it's never noted as a hazard because it's not regulated.  High levels of manganese are often a part of metallic finish glazes--so high that it can't be considered a food-safe glaze.

 

I don't know what a clear glaze will do to the look of the original, but I definitely wouldn't use these mugs for food.

[Joseph Fireborn]

Dinnerware safe. Contains some heavy metals and/or cadmium compounds but passes test for lead and cadmium release.

 

http://www.axner.com/spectrum1112metallicgoldglaze1pint.aspx

 

This glaze shouldn't be considered dinnerware safe imo. Like Tyler said, the laws/rules on glazes are pretty lax. Anything that looks metallic or has a surface that is metallic sheen is going to have some pretty leachable toxic ingredients. The only way to use this glaze would to be fire it to the perfect temp and have the cup tested for leaching. If your physically tasting metallic in your food its obvious it is leaching stuff into your drink right away, and that isn't something you want to be drinking.

 

Also just coating it with a clear isn't going to be a real solution to the problem, as when two glazes interact all sorts of things can happen. The best thing would be to stop using this glaze on surfaces that touch food or your lips.

[Guest JBaymore]

I'd say if the glaze is giving you an obvious  'metallic taste'... that is a big "red flag" right there.

 

Note that suppliers of commercial glazes will not indemnify the user of their products.  They will not defend you in the case of a product liability suit for what you make with their products.  In fact they will almost 100% for certain work hard to defend themselves from the potter. That means that it is the potter's responsibility as a "manufacturer" (yes... you are one) to assess if your products are appropriate to the use you are marketing them to fulfill. Any product literature they might provide is basically 'marketing materials'.  And the info about stuff like "not food safe" are likely mainly to help protect themselves from potential issues...... not to protect you.

 

Also note that the ASTM non-Toxic label on many art supplies products is based on the hazard to the user of that product, not the buyer of products produced USING those products, and speaks only to the hazard in the state that the materials are supplied (as in wet clay, .... not dry clay).

 

This following is a little "techy"... but it comes from THE 'horses mouth' of glaze gurus at the moment, William Carty, from Alfred's ceramic engineering school.  (He and Michael Katz (also Alfred) will be lecturing at NCECA in Kansas City this coming spring.)  It was Carty's lecture at a past NCECA conference, and was sparked by the information (and mis-information) circulating in the studio ceramic community about glaze safety and leaching and the like.  Obviously to get everything out of it... you had to be there (I was).  But there is good info still to be had from his slides. 

 

http://www.slideshare.net/nceca/06-315-carty-glaze-safetynceca2014final

 

Hope it is useful for those reading the thread that are concerned about the various "food safe" issues.

 

best,

 

.................john

[Mark C.]

You need to use a safe liner glaze in side drinking vessels

Keep those metallics on the outside and away from the lip.

It does not matter what the label says on commercial glazes

[Min]

John, did Carty have examples of washed and unwashed leaching tests on copper bearing glazes? Be interesting to see the figures. Was the 4X across the board with gloss or mat?

 

Ron, have you read this http://digitalfire.com/4sight/education/are_your_glazes_food_safe_or_are_they_leachable_12.html, kind of a primer to what to look for re glazes for food safety. 

[Guest JBaymore]

Min,

 

Yes... he said each progressive washing dropped the leachate amounts.  First washing was a BIG BIG drop.  Second was large.  After that..... there was actually little leaching evident.  But the only slides he used is what is shown here, so no other quotable data points. Sorry.   No differentiation about copper in mattes.

 

See your PMs.

 

best,

 

....................john

[Tim T]

Matt glazes are not well formed glasses, and so will be more prone to leaching. Also, as their leachability is a result of their structure, I don't think it will drop off as quickly with washing as on a more glassy glaze, as the leachate will be released from the glaze, and isn't just taken from the surface. But if there is nothing toxic in the glaze, this isn't a health hazard. Copper per se isn't toxic; only if its presence causes other, toxic, materials to be leached from the glaze is there a problem, e.g. copper causing lead to be leached out of lead glazes.

[Guest JBaymore]

 But if there is nothing toxic in the glaze, this isn't a health hazard.

 

Right there is the "key" to approaching this subject when you don't have a large technical background, or you don't want to get involved in the testing work to ascertain if the work is suitable for food use.

 

Does not say anything about the "archival quality" of the surfaces of your work..... (may deteriorate over time/ use) ... but it solves the "don't want to poison anyone" issue and the potential "I'm gonna' sue you" liability.

 

best,

 

....................john