Thanks for the invite PM, William. As you apparently know... I teach this stuff at the college level.
What you are looking at on that first image above in the start of this thread (the small black-ish areas) are XXXXXXXX-silicate crystalline formations stained with an excess of copper compounds (black copper oxide) that precipitated out of the molten glass during the cooling phase of the firing in places where the glaze layer was very thick and the cooling was very slow. (Exact composition of those silicates dependent upon the formulation of the oxides that went into the glaze batch to start with.) They are sitting mainly on the surface of the glaze but do "penetrate" vertically downward a tiny bit into the glassy matrix.
It is well known through extensive testing work on many types of glaze surfaces, that
in general materials that are precipitated out onto the surfaces of glazes are more prone to leaching than materials tied into the glassy phase. That being said........ as has been mentioned already...... only lab testing can tell the exact amount that will leach into acids OR bases.
The main color producing material for that blue-ish glaze is copper compounds. (From the visual, I'm guessing there is some titanium trace in that glaze also. And iron too. Possibly a tiny bit of cobalt.) Contrary to popular (studio potter) opinion (hysteria?) ...... copper is not all that toxic upon ingestion... although people with Wilson's Disease (rare) are very susceptible to copper.
A bit technical and not
fully useful without the actual verbal presentation .... here's something to peruse. I've posted this link on this forum before in the past:
http://www.slideshare.net/nceca/06-315- ... a2014final
It is a bit instructive on the overall toxicity subject for those that are interested. William Carty is one of THE people who know what glazes are all about.
As to the level of risk in using such a piece........ negligible. It is statistically there if you measure finely enough (lots of zeros to the right of the decimal place)....... but so small that you should be worrying about getting hit by lightening
.
The comments here in this thread on OLD Japanese raku bowls are a bit misleading. Yes, both old aka raku and kuro raku are covered with a lead based glaze. Guess what? NEW contemporary Japanese raku bowls are very, very frequently also still covered with lead based glazes. (If you own a new one...... hit it with a hardware store lead test kit.) Because ceramics is so important in Japanese culture, certain things are excluded from various laws. Raku pieces and overglaze enamel pieces are exceptions that are allowed...
because the art is important. Some effects in aka raku are hard to get without lead.
The biggest hazard from lead glazed anything is not to the user. It is to the potter. The handling of the lead compounds in the studio/workshop is a real concern for them.
I sometimes use lead glazed raku bowls for Chanoyu. Before the ceremony the bowl is washed thoroughly in preparation for the event (cleanliness is next to godliness). In the beginning of the ceremony event, before tea hits the bowl, the bowl is again ritually washed inside and at the lip. The serving of tea and water is in that bowl for a really short time before being drunk. The level of leaching possible given the ph of the mixture and the amount of unbound lead directly near the surface of the glaze..... and adding in the "time in contact factor"......... is such that again........ worry about that pesky lightening
.
Don't store you orange juice in the fridge in a raku bowl for a week.
Also note that the more material that IS leached out of a glaze surface... the lower the next subsequent level of leaching will be. So the more a piece is cleaned / used... the lower the levels of material it will leach. In an infinite time period...... with unlimited cleanings/usings..... statistically the levels should be heading toward zero but never quite reach that point.
Stability in glazes as to the leaching of constituents in the glaze out into "foodstuff" is a very complex subject. Generalizations are always wrong. I tell my students all the time, and have posted here before (and someone already mentioned this also) that the correct answer to ANY question about ceramics is always "
It depends". There are SO many variables it is amazing. Many that as potters we know and attempt to control... and many that most potters are not even aware exist.
Where things get a bit "concern raising" is where high concentrations of potentially toxic substances are involved in clay bodies, slips, and glazes. Or where the glassy phase of the glaze is "interrupted" by microcrystalline precipitations on the surface of the entire glaze. Things that tend to 'get my attention' are stuff like extensive gunmetal black semi-matt surfaces at any temperature firing range, any blood red coloration at lower firings temperatures, low temperature (soft) high boron glazes with colorants that are potentially toxic, low temperature lead glazes in certain food usage situations, and so on.
I'm a bit more concerned about the stuff from PLASTIC containers that gets into foodstuffs.
Generally....... fired ceramic is pretty "safe" stuff. I can carry $2 mil of product liability insurance on my pottery production for well less than $1000 a year; that tells you something about the real risks.
best,
....................john
Cheers
(sent him a PM).
