The Official "Ask the Artisan" Community Topic

[Seeker]

In the end, I don't think it's all that important, but it's an interesting question

Thanks 3B!
Beautiful "glass" bottom!
Great info too.

Don't you think the possibility of the presence of lead (from recycled glass or from marbles) is important?
edit/add - I notice (see my last past - I think I was writing it as you posted) that there is no "murky" border around your "glass" bottom. Is this indicative of silica fritt then (no weird border)?

[bearsbearsbears]

Thanks 3B! Beautiful "glass" bottom! Great info too.

Don't you think the possibility of the presence of lead (from recycled glass or from marbles) is important?

Thanks for the kind words

Most bottles are used to hold foodstuffs. I would assume they contain little or no lead, as long as they're not too old (1980s or later?). Most people at my studio use new water/beer/wine/liquor bottles if they're fusing glass to their pieces.

Not sure about the lead content of marbles!

I notice (see my last past - I think I was writing it as you posted) that there is no "murky" border around your "glass" bottom. Is this indicative of silica fritt then (no weird border)?

I didn't use a silica frit by itself there; the crackling glass bottom is just what my glaze looks like when it's thick. The glaze contains some frit, as do many, but that is only 1 ingredient of 6 in the glaze.

Any overlap of two glazes, or glaze + frit, or glaze + recycled glass would yield some sort of border. No border could be some evidence that only one glaze was used?

[elmstudios]

I have a question that I thought I'd pose here for our distinguished artisans.
This is inspired by a most recent couple of posts on the artisans topic under the Ericka O'Rourke (Elm Studios) thread.
Here's the question:
How can one tell -or- can one tell, if the beautiful "glass pool" chadamari or bottom of your yunomi (or whatever type vessel) is formed from ash as a part of the firing process versus pieces of glass or marbles added to the piece and then fired?
Also, what percentage of 'recycled glass' that might be used in such a way, say old bottles, etc might contain lead (or other potentially harmful stuff that could leach into tea/food)?

IMHO this would be very hard to tell simply based on a visual assessment. The obvious would be color of course...wood ash tends to be browns, clear and other neutral colors, and obviously if you have blue, you know it is something else. But I think otherwise a strictly visual based guess would be nothing more than opinion.

As far as lead glass goes, I would hope just as artist stay away from using lead in their glazes they would be responsible enough to not use antique glass of unknown origin.

[elmstudios]

I was kind of hoping that other people would have questions to ask the artisans also. With all the discussions about glazes in other threads, it seems that quite a bit of chemistry is involved.

So is the knowledge about ceramic properties, and chemical properties of glazes and how to achieve different colors come from intensive studying, experimentation, or piecemeal where a wealth of knowledge was acquired bit by bit over your pottery career?

I hope this isn't a loaded question, but being a student of math I've acquired knowledge in each of these fashions, and I feel the later two tend to lead to better retention.

LOL...I was pretty much an art-geek- throughout my Pre-college years and remember how much I absolutely LOATHED chemistry and math. I really thought it was one of those things that I would never have a use for...And I find it hysterical that my first major job as a designer in NYC had me working with Picas and fractions (which I was horrible at) and now, I know more about chemicals and so forth than I ever did...It is so funny the way things come around. I feel so old when I explain this to my 10 year old son!

[Chip]

Thanks Ericka for sharing your insights on the subject!

Regarding the soda ash versus glass added, seems every soda ash piece has "glaze" on other parts of the piece, and it forms pools on the bottom or sides as the artisan "plans" such as Cory's and ShyRabbits. You can see the "glaze" was formed in the firing process. Wish I could articulate this better.

The glass is simply glass sitting in one place. It was fairly obviously placed there in solid form prior to firing.

[Seeker]

Thanks to everyone for insightful and educational commentary!

[timseeclay]

IMHO this would be very hard to tell simply based on a visual assessment. The obvious would be color of course...wood ash tends to be browns, clear and other neutral colors, and obviously if you have blue, you know it is something else. But I think otherwise a strictly visual based guess would be nothing more than opinion.
.

Color can be a pretty fickle way to tell if the pool is from melted glass or from collected ash. Different woods give different color for example our cedar gives a dark green to turquoise.
The glass itself is not the only part one can read when trying to figure if the glass is formed from melted wood ash or soda or was added by melting a glass object. A wood fired piece that has a large enough collection of ash on the inside to create a pool, will also have a great deal of ash on the outside. the ash on the outside will be on one side, the side facing the flame. Likewise the ash on the inside will be built up more on the trailing side of the pot. As for a build up soda in a soda firing to the point at which it pools seems unlikely because that amount of soda ash would also be on the shelves and in the kiln. You can put soda ash or other glass forming materials in a teabowl or on a pot to form a pool, or have an abundance of glaze.

What initially caused my concern was a small yunomi I bought from a Canadian vendor. When I touched the bottom I could feel a crack in the glass. That really freaked me out and I have never used the cup. The thought that a tiny sliver might come loose ...

I think you have a very valid concern. I have had large drips of wood ash break off and fall off on their own.Those were not in the bottom of a vessel but they are a fragile matrix of cracked glass and changes in temperature are going to cause stresses that may indeed separate a sliver of glass. I think a very deep pool of glass, glaze, wood ash, or any other melted substance that has crazing (cracks) would give me concern about slivers. I would not sell a object as food safe if the crazing was able to move parallel to the surface of the glaze perpendicular is not as scary to me. I have a piece here that was filled with soda ash and fired it has a wonderful green glass pool in it I will freeze and heat it a few times and look at it with some magnification to see if I can dislodge something. I will report back.
Tim

[Chip]

What initially caused my concern was a small yunomi I bought from a Canadian vendor. When I touched the bottom I could feel a crack in the glass. That really freaked me out and I have never used the cup. The thought that a tiny sliver might come loose ...

I think you have a very valid concern. I have had large drips of wood ash break off and fall off on their own.Those were not in the bottom of a vessel but they are a fragile matrix of cracked glass and changes in temperature are going to cause stresses that may indeed separate a sliver of glass. I think a very deep pool of glass, glaze, wood ash, or any other melted substance that has crazing (cracks) would give me concern about slivers. I would not sell a object as food safe if the crazing was able to move parallel to the surface of the glaze perpendicular is not as scary to me. I have a piece here that was filled with soda ash and fired it has a wonderful green glass pool in it I will freeze and heat it a few times and look at it with some magnification to see if I can dislodge something. I will report back.
Tim

Hi Tim, thanks for posting. Can you clarify for me please, so any thick potentially pooling glaze that displays crazing/crackling including for instance thick glazed Hagi concerns you as well which has crackles often all over including the bottom?

Also, is a test going repeatedly from freezing to heating/boiling a realistic test? Conducting such an extreme test would not really show what would occur under normal use for these pieces.

These pieces would never in normal use be exposed to such extremes even once. And just about any pottery or glass would suffer dire consequences under such abusive and atypical testing. It seems the works should be tested closer to actual usage parameters.

And of course if there is moisture in the clay, that alone could cause failure when freezing due to expansion.

I am not trying to sound like a wize guy. But I do feel any test should more realistic.

[debunix]

This is an interesting topic, and I've already learned something new: that some of the beautiful clear pools of color on the bottom of various wares are sometimes made from melted glass rather than glaze turning into glass in the firing. Very interesting.

[timseeclay]

These pieces would never in normal use be exposed to such extremes even once. And just about any pottery or glass would suffer dire consequences under such abusive and atypical testing. It seems the works should be tested closer to actual usage parameters.

And of course if there is moisture in the clay, that alone could cause failure when freezing due to expansion.

No Chip I wouldn't think of you as a wise guy, those are good thoughts. Pushing the test to the extremes in any application makes me much more comfortable with the results. If I was to say it is safe to drive 100 mph I would want to be sure it was safe to drive 120mph. It has been a while but I remember reading that industry will make a claim 20% lower than the tests to ensure that their claim will be substantiated even if there are small variations with a test or a product. The first freeze thaw cycle went fine nothing that I see has changed in the test subject. But I want to do it again with a camera and a macro lens, So I can share what I see.
As for the freezing and moisture in the clay, I am using wood fired work that has a percentage of absorption that I do not have a test accurate enough to measure. It is less that .05% but I will take pictures before and after the freezer to get more information on how freezing might impact the structure.

Hi Tim, thanks for posting. Can you clarify for me please, so any thick potentially pooling glaze that displays crazing/crackling including for instance thick glazed Hagi concerns you as well which has crackles often all over including the bottom?

There is a difference with a glaze that crazes and one that is thick enough to craze in a way that a crack develops between the surface of the glass/glaze and the clay. If the crazing is only cracking in 2D then the glass/glaze has an attachment to the clay it has interfaced. A 3D pattern would allow for a chunk or sliver to become dislodged. My brain can see it but I don't know if I am explaining it right. I am off to look up Hagi now.
Tim

[Chip]

I am off to look up Hagi now.
Tim

Hagi has a bunch of topics on the forum, this is the "official" one. http://www.teachat.com/viewtopic.php?f=36&t=8562

Thank you for your courteous and informative reply! The forum is all about learning and sharing, and share and learn we do.

[timseeclay]

Ok so I checked out Hagi and didn't see any pooling that was great enough to concern me. For the most part it seems that the glazes are nice thick shinos with just enough COE (coefficient of expansion) to cause decorative crazing. I could not say for all glazes used on Hagi but for a shino to be nice and thick it needs to be stable and not run. It's the running and the pooling that would cause deep glaze pools. I do not have my camera at home but I have a piece that has not been abused yet and it is definitely missing a sliver of glass. The piece was a happy accident. we were introducing soda ash to the kiln environment by putting it in little pinch pots. It did not all evaporate and what was left made this pretty pools of green in the one I have here there is a small chip missing. I will attempt the picture thing on Monday.
Tim

[timseeclay]

Well I did a test on the pieces that have a soda glass formed in the bottom of them. In regards to glass shards coming off from normal use I would say (in myth busters terms) its plausible. I subjected one test piece (test a) that had no missing surface at all to hot water only, to a maximum temperature change of 120 degrees. After to cycles of hot to room temperature the piece lost glass from two spots both spots being smaller than a grain of sand. After 15 cycles it lost 5 more pieces that were as small as the first and one that was the size of about 4 grains of sand I did not catch the piece in the coffee strainer. Test b had already shown signs of glass loss and I abused that piece more. Starting it in the freezer and dropping it into boiling water for a maximum recorded (could have been greater in some instances) temperature change of 210 degrees. Test b lost glass at nearly every intersection of cracks some the size of 4 or more grains of sand and in the end I froze it in a cup full of water and it failed structurally losing one side to the pressures of ice. Photographing the results proved to difficult with the reflection but I tried to highlight in the one photo the large chip from test a in the glare, and on of the glass faults from test b before the abuse. If I were pressed for an opinion on what I found, I would say that drinking from deeply crazed/cracked glass pools has the potential for hazards. I would not say every crazed pot will release glass slivers, and I will test other forms of pooling when the subjects are found or made.
Tim

[elmstudios]

I just took one of my (that I created) glass bottomed tea bowls and boiled water inside of the microwave in it for over three minutes repeatedly. Something I would never normally do with one of my tea bowls other than for testing purposes. My glass did remain completely intact and smooth. I then scratched it and banged it hard with a metal spoon afterwords to see if I could dislodge anything. Nothing happened. I will upload the photos of the entire process to my flickr. The only negative I would say occurred, was that I used a kitchen towel to remove the tea bowl from the microwave because it was warm to touch. I do this whenever I heat up food in the microwave whether it is on a store bought ceramic plate or Pyrex glass food bowl. Unfortunately running these tests will never be conclusive for all ceramics though. Every ceramicist has a completely different firing process and creation process using different claybodies, raw materials, and glazes. Each claybody has its own set of characteristics that are relative depending on the vitrification reached, glazes, firing temperature, firing cycle, and firing method used. Unless a piece was created in the exact same way and using the same base materials, and fired the same, than the controls of any experiment would be inaccurate. And therefor the results would not be dependable.

[Seeker]

Wow, very cool discussion here - very interesting.
I noticed I kept thinking how extreme the tests were as I read and thought about my use habits (and of course the tests are intense, they are tests after all).
And I also hoped that my glass bottom vessels will remain okay because I don't put them to such extreme temperature shifts. These days, thanks to posts like those above, and from others, like Victoria and GeekGirl regarding crackling, I warm my vessels, stage 1, with hot tap water, then, stage 2, I warm some more with water from the kettle at the temp I will use for the tea.
I'm hopeful this is sufficient care to maintain the integrity of the vessels, as well as to prevent any cracking/crazing (vs crackling).
I remain hopeful, and I suppose I will be paying more attention to my glass bottoms from now on.