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Does Fading Always Mean A Glaze Isn't Food Safe?


tb001

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We purchased several mugs from a very well known ceramics studio a few years ago, known for their functional work. Their wares are used in most of the high end restaurants in our area. They produce just extraordinary matte glazes, which I'm trying to emulate in my own work.

 

After a few years of use for morning coffee (1-2x/week), the insides (brown) are faded and almost have a dusty look. I've noticed that though they use some of their lighter colors on dinner ware (plates, bowls, etc...), when used on coffee mugs, the insides are always glazed with a dark color, which makes me wonder if there's an issue with staining/the glaze being too porous.

 

Would love to hear what people think. I love their work, and they have such a great reputation that I'd hate to think they weren't selling a good product, but any glaze that did that wouldn't pass my own at home quality tests...

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From your description, the interior matte surface does not appear durable. Could be from the acidity of coffee or the harshness of dishwasher detergent, or both. The colorants used for brown could also be leeching. Not food safe in my book. Liner glazes for functional ware are usually glossy -- the higher melt of the glaze materials makes them more durable.

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my PERSONAL OPINION is that using matte glaze on the interior of a mug is simply stupid.  

 

i have one made by a true technical expert and it is a soft green that goes darker when i use it for tea.  i occasionally rinse it with clorox and the color returns immediately.

 

i know a potter who has used matte glazes for years and have heard from others that her mugs leak.

 

i know a third who thinks it is ok because it is pretty.

 

i now use Corelle teacups every day.

 

 

 

(i have no ceramic degree, no degree at all, all i go by is my own experience and observation and that tells me that matte glaze interiors on functional work is stupid.)  

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You'll see a lot of matte glazes out there in the world that are simply under-fired glossy glazes. It's not a good way to make a matte, because it means you are not getting good glass formation because it's not going hot enough. These types of matte glazes are not usually very durable, and can leach because the materials are not locked up in the glass.

 

The better way to make a matte glaze is through the use of materials that promote matteness, such as magnesium. The glaze can be fully matured (runny even), with good glass formation, and still be matte. This type of glaze is much more durable, and more likely to be food safe.

 

The big problem is that there are a lot of folks out there who don't care to do their homework before using a glaze. They assume that if someone else was using the glaze, then it's good to go. A good number of the glaze recipes I find online I would never use without doing some reformulation. It's easy to make a pretty glaze. It take a little more work to make a durable glaze.

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In my personal opinion, any time a glaze surface changes when used in contact with food, then it should be considered unsafe. One would question glazes that have no poisonous oxides, or glazes that are white or transparent. Lack of poisons may be true. However, I consider the surface to be flawed in that it can sustain bacteria or other organics in the small pits, So for me this would be considered unsafe. I may be going extreme here, but at least I don't wash my hands in antibacterial every-time I touch door handle. :wacko:

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Your discription does not mention if the inside glaze is matte or shiny???

No comments really valid until we know-matts fine for outside but insides need a smooth liner glaze thats easy to clean.

It matters not how well they are known -what matters is does it work over time. 

Sounds like it does not.

 

Mark

 

PS I had a customer who used my white liner mugs for years say the inside was stained from tea-turns out she just never scrubbed out the insides . They cleaned right up with a good cleaning.

Not sure if my story applies but scrub them out to see.

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Guest JBaymore

If the glaze is changing like you mention ... it is deteriorating and losing SOMETHING.  What that material is, is the question.  At one level it is possibly simply what you might call "poor product longevity" and is mainly a "repeat customer" issue.  At another level it might be a health concern for the user if the materials get into foodstuff and are ALSO toxic (see link below).

 

"Food Safe" is a term that has absolutely NO legal meaning as a term.  In the USA, for example, ASTM does not have a test for "Food Safe" for general consumer food serving goods.  The US FDA only regulates (and specifies procedures for using) lead and cadmium in any ceramic wares.

 

At the last NCECA it was stated by William Carty (you cannot get a better technical source than this guy!) that all matt glazes are not suitable for food use and that food use glazes should be fired above Orton Cone 1.  I was there... heard it first hand. 

 

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

 

best,

 

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

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Interesting responses. Glad to hear I'm not nuts! And yes, as far as I remember, all their wares are matte inside and out. I just find it so odd--this is by no means a hobby ceramicist. They are a huge production studio well known for their high end wares. Would have expected more, but perhaps it's an issue of the products being "FDA safe" as John mentioned. Like I mentioned, they would have failed my backyard hobbyist's standards!

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ok im going to take this a different direction

 

if glaze has nothing dangerous in it  and it leaches.   does it still cause any harm

 

if a tree falls in the forrest ..........

 

look if all the ingredients are non poisonous  and it leaches....    to me thats a safe pot.

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ok im going to take this a different direction

 

if glaze has nothing dangerous in it  and it leaches.   does it still cause any harm

 

if a tree falls in the forrest ..........

 

look if all the ingredients are non poisonous  and it leaches....    to me thats a safe pot.

 

The original post was about the deterioration of the glaze surface . . . which raised the potential for leaching.  And leaching could be of a harmful or toxic ingredient, e.g., manganese, or it could be something benign as far as we know.  Deterioration of the glaze surface also raises the potential for trapping harmful bacteria.  Not a guarantee, but potential.

 

And, with folks having allergic reactions to so many different chemicals and things now days, you never really know what will cause a reaction -- I love peanut butter; others can't have it in the same room.  Peanut butter is not toxic or poisonous -- unless you are allergic to it.  Some people might have a high tolerance to manganese; others, very little. 

 

There are no agreed upon standards for leaching.  Hesselberth and Roy used leaching for drinking water as their guide.  Others use higher limits than those gentlemen.  Oftentimes a single exposure is not fatal, but a build up of doses leads to fatality.  Durability in glazes is the way to remove leaching from happening -- whether it is leaching of bad things or benign things.  Matte glazes, unless specifically made to be matte -- as noted by Neil -- tend to lack durability because they don't melt properly.  And I love matte glazes, too. 

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Guest JBaymore

ok im going to take this a different direction

 

if glaze has nothing dangerous in it  and it leaches.   does it still cause any harm

 

if a tree falls in the forrest ..........

 

look if all the ingredients are non poisonous  and it leaches....    to me thats a safe pot.

 

Not in a different direction... I already made that point :

 

 

If the glaze is changing like you mention ... it is deteriorating and losing SOMETHING.  What that material is, is the question.  At one level it is possibly simply what you might call "poor product longevity" and is mainly a "repeat customer" issue.  At another level it might be a health concern for the user if the materials get into foodstuff and are ALSO toxic (see link below).

 

"Food Safe" is a term that has absolutely NO legal meaning as a term.  In the USA, for example, ASTM does not have a test for "Food Safe" for general consumer food serving goods.  The US FDA only regulates (and specifies procedures for using) lead and cadmium in any ceramic wares.

 

At the last NCECA it was stated by William Carty (you cannot get a better technical source than this guy!) that all matt glazes are not suitable for food use and that food use glazes should be fired above Orton Cone 1.  I was there... heard it first hand. 

 

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

 

best,

 

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

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Guest JBaymore

Hesselberth and Roy used leaching for drinking water as their guide. 

 

 

I think it is important to point out here that Mononna Rossol (ACTS NYC) was using the drinking water standards for comparisons to glaze leaching as a guideline before that book was ever published.  That is a GREAT book and Ron and I have presented together on glazes at NCECA in the past........ not slighting their work........ but giving credit where credit is due.

 

best,

 

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

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