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Inside Mug Color Change


Jamie page

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Newly here

Have started drinking coffee from glossy black mug for a few days and the inside of mug has turned dark blue. Is this normal or is it a mistake from my glaze? The recipe came from ceramicartsdaily.org. I am new to the pottery world and would never want to harm anyone drinking from my mugs. Thanks for any replies

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Welcome to the forum!

Also, congratulations on discovering the leach test under your own power :). Not the most auspicious way to discover it, but better sooner than later.

A glaze that changes colour like that when exposed to a mild acid like coffee, tea or vinegar is soluble, and shouldn't be used on the inside of a functional vessel. This does happen with shiny glazes, not just matte surfaces.

The likelihood of you acutely poisoning anyone healthy from drinking out of such a mug is pretty low, but materials that dissolve out of a glaze might be detrimental to someone with a compromised immune system, or who is on certain medications. Probably not good for anyone in the long term.

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Great Chantay, I meant only that some details are obvious to some and not others, I was really glad when my eye was drawn to this aspect of my pots. In this forum, seems we are open to any hints so I passed it on! Hope no offense was taken :)

On the unglazed areas of some of my pots I recently tried the soda wash that Chris Campbell kindly described to us. it is worth a try , doesn't alter the colour but takes it from clay to a satinny feel which I kinda like.

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Really importatn that you know what the properties of the chemicals you are dealing with , both for your safety as a potter handling and firing chemicals, and for the eventual users of your pots.

Some one on this forum would be able to advise on a suitable source of info for you.

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Guest JBaymore

OK without doing ANY formal glaze calculation in Insight...... let's look at  that colorant supply. 

 

That's a total of 10.3 percent of metallic oxides.  There are few to no glazes that can hold that kind of saturation of such coloring oxides in solution as the glaze melt cools.  Just ain't gonna' happen.  So some of the coloring oxides are going to precipitate out on the surface of the glaze.  Any oxides that are in this condition are almost certainly subject to easy leaching into strong-ish acids and bases.

 

Then looking at WHICH oxides these are.  Copper is notoriously hard to keep in solution in glazes.  And at 4% that oxide alone is flirting with the top amount that can even closely be stable.  Then there is 4% of cobalt OXIDE.  That alone is enough to make the glaze almost black and be supersaturated.  The manganese is there just for good measure to totally make this absurdly oversupplied with colorants. 

 

Then take into account that this looks like a Cone 6 recipe.  At Cone 6 a significant amount of boric oxide glass is introduced to lower the melting point.  Boric oxide glass is softer than silica glass... so this is a less durable base than if it were at Cone 9-10 or higher.  SO the glass has a lower ability to hold the oxides that are still IN solution also.  (note... yes adding B2O3 in very precise proportions to the SiO2 can harden borosilicate glasses... but just looking at the recipe, I don't thik that is the case here.)  If this is getting fired atr cone 9-10....... worse... don't need that boron.

 

NOT a glaze for food bearing wares.  Period.

 

best,

 

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

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Guest JBaymore

Just ran it in Insight.  Also it is WAY oversupplied with KNaO for both cone 6 and cone 9.  (Other oxides are within limits).  Those are not helping with the leaching stability either.

 

best,

 

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

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Guest JBaymore

OK... venturing into "dangerous grounds" here................  going off the "politically correct" spectrum........

 

This discussion above cuts to the core of one of the things I always mention in my ceramic chemistry classes.  Which I think is VERY important for students to really understand.  It gets to core ideas of "how the world works".

 

In general, how someone gets to be a "famous ceramic artist"........ is from the visually apparent aesthetic qualities of their work, the technical execution of the pieces (from a forming standpoint), and is often combined with the way they present themselves in public settings (exuding a professional feeling and being nice people).  Maybe add in a dose of "right place at the right time" serendipity, and also a bit of a "who you know" factor.

 

What this "fame" does NOT necessarily tell you is their depth of understanding of the technical sides of the process.  For all anyone knows... they flunked or got a D in their ceramic materials / ceramic chemistry / kiln design-building classes (if they ever had that kind of training).

 

They may have mastered the aspects of the process to get the visual result THEY use...... but that might be the real limit of their technical expertise.  They might even have had training that was not all that accurate, and are sharing that stuff once again, spreading the misinformation.

 

So just because "Famous Ceramist" shares some glaze recipe or some ideas on the technical side of things........ that does not make it automatically "truth".  There are things I've seen printed in studio ceramics oriented books that fly in the face of basic science and engineering principles.

 

Sometimes the information presented by "Famous Ceramist" is taken out of context, and then transferred to others incorrectly.  For example, a glaze used solely for sculptural work gets shared... and then a participant in a workshop comes back and shares it with their functional potter table-ware producing friends.  Suddenly it shows up on tableware.  Then that tableware potter (maybe who has little technical knowledge) becomes "famous"... and that glaze then goes on to be shared as a tableware glaze.  And so on.

 

Vet your sources folks, vet your sources.  Sometime the emperor has no technical clothes.

 

best,

 

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

 

PS:  Luckily, most of what we do is not "death incarnate".  But having a good grasp of some basics is important.

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Preach on John!

 

I'll out myself as a propagator of false information, not knowingly of course.  For years, I told students that air pockets caused explosions.  Why?  Because that's what I was taught, and had that belief reaffirmed many times over.  

 

I also was always under the belief that glazed low fire wares were always safe for use.  I didn't know about crazing (other than it looked cool for Raku effects), and definitely hadn't heard of leaching.  Don't get me wrong, I wasn't promoting the use of "decorative" glazes for food use, like "Hey kids, that glaze looks cool, so go ahead an use it on the inside!"  The companies are nice enough to put some guidelines on the bottles.

 

Sure, my misinformation wasn't horribly detrimental to my students.  Removing air pockets, is still good practice, and I doubt any of them will be poisoned from food safe commercial glazes.

 

But my time on these boards have definitely changed the way I present information.  I only wish I could get a credit towards my license renewal for our discussions here.  Honestly, I've learned more, that I can apply to my teaching here, than I have in the classes that I actually can count...

 

The internet as a tool for sharing of information and ideas; who would have thought?

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Guest JBaymore

Jamie,

 

Insight Level II from Digitalfire.com.  I have been using that since god was a kid.  The program has all the capabilities you will EVER need.  And with a science type background, your husband will likely enjoy the subscription access database / website that comes with the Level II version.

 

best,

 

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

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Now my xmas wish is to be reborn at the age and financial enough to attend J Baymore's classes, just next door to where I live.. Oh I would travel to do this.

However, this forum is fantastic in the way the knowledge is freely shared.

Thanks.

Babs

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