Today's MatchaWan :D (new name, but tea's the same)


Discussion on virtually any teaware related item.

Re: Today's MatchaWan :D (new name, but tea's the same)

Postby William » May 5th, '14, 06:57

I read that raku, during the old days, had traces of lead. Modern production is safe to be used?
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Today's MatchaWan :D (new name, but tea's the same)

Postby blairswhitaker » May 5th, '14, 08:14

Modern production still has lead. I believe it's been debated on here before. I'm not going to be loosing any sleep over it.
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Re: Today's MatchaWan :D (new name, but tea's the same)

Postby William » May 5th, '14, 08:39

blairswhitaker wrote:Modern production still has lead. I believe it's been debated on here before. I'm not going to be loosing any sleep over it.


Yes, I already read old discussions about this matter. I was only interested to hear your opinion. :D
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Re: Today's MatchaWan :D (new name, but tea's the same)

Postby JBaymore » May 5th, '14, 10:06

Current Japanese raku typically still has lead in the glaze. As do Japanese overglaze enamels.

A chawan is well washed prior to the gathering. Then it is ritually cleaned again during the ceremony. The tea is in the bowl at most minutes.

Leaching issues in this setting would be minimal........ likely below detectable thresholds.

best,

.................john
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Re: Today's MatchaWan :D (new name, but tea's the same)

Postby JBaymore » May 5th, '14, 10:08

blairswhitaker wrote: Everything is reduced down to the calm black enso.


Nicely said. :D

best,

.............john
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Re: Today's MatchaWan :D (new name, but tea's the same)

Postby William » May 5th, '14, 11:21

JBaymore wrote:Current Japanese raku typically still has lead in the glaze. As do Japanese overglaze enamels.

A chawan is well washed prior to the gathering. Then it is ritually cleaned again during the ceremony. The tea is in the bowl at most minutes.

Leaching issues in this setting would be minimal........ likely below detectable thresholds.

best,

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


So this lead comes from the glaze itself rather than the clay used, right?
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Re: Today's MatchaWan :D (new name, but tea's the same)

Postby JBaymore » May 5th, '14, 11:54

Yes. Lead oxide (PbO) is a flux (melting temperature lowering agent) on the main glass former in glazes, which is silica (SiO2). Raku is fired at low temperatures, so the melting point of the glaze must be lowered a lot from the melting point of pure silica. Lead oxide is very good at doing this in the raku temperature range. Lead oxide has a lot of advantages as a ceramic flux.... and just this one teensy, weensie little problem :wink: .

In fact, lead oxide is STILL in a LOT of commercial dinnerware glazes for stuff you might buy today. When properly formulated, lead does not HAVE to leach out of a glaze. But this requires industrial levels of "controls" on raw materials sourcing, forming and glazing processes, firing process, and technical level monitoring of the stability of the glazes.

Lead is FINE being in a glaze..... as long as it does not leach out into food stuff. THAT'S the trick.

best,

................john
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Re: Today's MatchaWan :D (new name, but tea's the same)

Postby William » May 5th, '14, 12:21

JBaymore wrote:Yes. Lead oxide (PbO) is a flux (melting temperature lowering agent) on the main glass former in glazes, which is silica (SiO2). Raku is fired at low temperatures, so the melting point of the glaze must be lowered a lot from the melting point of pure silica. Lead oxide is very good at doing this in the raku temperature range. Lead oxide has a lot of advantages as a ceramic flux.... and just this one teensy, weensie little problem :wink: .

In fact, lead oxide is STILL in a LOT of commercial dinnerware glazes for stuff you might buy today. When properly formulated, lead does not HAVE to leach out of a glaze. But this requires industrial levels of "controls" on raw materials sourcing, forming and glazing processes, firing process, and technical level monitoring of the stability of the glazes.

Lead is FINE being in a glaze..... as long as it does not leach out into food stuff. THAT'S the trick.

best,

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


Thanks for the explanation, John!
So, a raku ware fired at higher temperatures, if SiO2 reaches its melting point, it will cover PbO?
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Today's MatchaWan :D (new name, but tea's the same)

Postby debunix » May 5th, '14, 21:27

Those raki wares are quite seductively gorgeous, but with my habits of slow drinking teas from one cup over several hours (not matcha!), the lead risk does not make sense. But for occasional brief use, I can certainly understand the attraction.
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Today's MatchaWan :D (new name, but tea's the same)

Postby blairswhitaker » May 6th, '14, 04:43

Thanks for providing the science john.
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Today's MatchaWan :D (new name, but tea's the same)

Postby blairswhitaker » May 6th, '14, 05:57

Here are some photos from my session today, it's very hard to get "quality" pics, but maybe these pictures relate the story of this bowl more, why I'm attracted to it.

Image

Image

Image

Image

Image
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Today's MatchaWan :D (new name, but tea's the same)

Postby blairswhitaker » May 7th, '14, 18:17

First machawan if the day, chabako wakei. Having some ryuouen.
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Re: Today's MatchaWan :D (new name, but tea's the same)

Postby Cdragon » May 8th, '14, 10:28

William wrote:
JBaymore wrote:Yes. Lead oxide (PbO) is a flux (melting temperature lowering agent) on the main glass former in glazes, which is silica (SiO2). Raku is fired at low temperatures, so the melting point of the glaze must be lowered a lot from the melting point of pure silica. Lead oxide is very good at doing this in the raku temperature range. Lead oxide has a lot of advantages as a ceramic flux.... and just this one teensy, weensie little problem :wink: .

In fact, lead oxide is STILL in a LOT of commercial dinnerware glazes for stuff you might buy today. When properly formulated, lead does not HAVE to leach out of a glaze. But this requires industrial levels of "controls" on raw materials sourcing, forming and glazing processes, firing process, and technical level monitoring of the stability of the glazes.

Lead is FINE being in a glaze..... as long as it does not leach out into food stuff. THAT'S the trick.

best,

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


Thanks for the explanation, John!
So, a raku ware fired at higher temperatures, if SiO2 reaches its melting point, it will cover PbO?


The glass SIO2 does melt at that lower temperature because of the lead. Higher fired ware doesn't use lead to flux the glass. We use other safer but less effective fluxes like sodium, potassium, and lithium to bring the melting temperature for glass down. This safe glaze is bought at a higher cost in fuel and time to get to that Silica melt temperature. Still way below where Si02 melts on it's own. Lead isn't needed in those higher fired finished glazes and shouldn't be present.

Don't think of the lead as being covered. It is a jar of mixed nuts. Like brazil nuts some of the lead is on top at the surface and some is in the middle and some is on the bottom interfaced to the clay. The silica/glass, lets say the peanuts, are everywhere as well. Just like glass, they have no defined crystal structure. Anything on the surface, top of the jar, is fair game to be leached out.

Too much of a colorant let's say cashews as copper carbonate, in any glaze can soften the glass. Spreading out the peanuts too much. Softer glass/glaze is easier for acids to leach out heavy metals like lead, copper, nickel, and so forth. One down and dirty way to check for leaching is placing a slice of lemon on the glaze surface. Don't try this on anything you like or want to keep, because in an hour or a day the citric acid might have created a lemon sliced sized patch of a different color on your piece and ruined it. But that shows the glaze isn't fit for long term contact with acids.
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Re: Today's MatchaWan :D (new name, but tea's the same)

Postby William » May 8th, '14, 10:56

Cdragon wrote:
William wrote:
JBaymore wrote:Yes. Lead oxide (PbO) is a flux (melting temperature lowering agent) on the main glass former in glazes, which is silica (SiO2). Raku is fired at low temperatures, so the melting point of the glaze must be lowered a lot from the melting point of pure silica. Lead oxide is very good at doing this in the raku temperature range. Lead oxide has a lot of advantages as a ceramic flux.... and just this one teensy, weensie little problem :wink: .

In fact, lead oxide is STILL in a LOT of commercial dinnerware glazes for stuff you might buy today. When properly formulated, lead does not HAVE to leach out of a glaze. But this requires industrial levels of "controls" on raw materials sourcing, forming and glazing processes, firing process, and technical level monitoring of the stability of the glazes.

Lead is FINE being in a glaze..... as long as it does not leach out into food stuff. THAT'S the trick.

best,

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


Thanks for the explanation, John!
So, a raku ware fired at higher temperatures, if SiO2 reaches its melting point, it will cover PbO?


The glass SIO2 does melt at that lower temperature because of the lead. Higher fired ware doesn't use lead to flux the glass. We use other safer but less effective fluxes like sodium, potassium, and lithium to bring the melting temperature for glass down. This safe glaze is bought at a higher cost in fuel and time to get to that Silica melt temperature. Still way below where Si02 melts on it's own. Lead isn't needed in those higher fired finished glazes and shouldn't be present.

Don't think of the lead as being covered. It is a jar of mixed nuts. Like brazil nuts some of the lead is on top at the surface and some is in the middle and some is on the bottom interfaced to the clay. The silica/glass, lets say the peanuts, are everywhere as well. Just like glass, they have no defined crystal structure. Anything on the surface, top of the jar, is fair game to be leached out.

Too much of a colorant let's say cashews as copper carbonate, in any glaze can soften the glass. Spreading out the peanuts too much. Softer glass/glaze is easier for acids to leach out heavy metals like lead, copper, nickel, and so forth. One down and dirty way to check for leaching is placing a slice of lemon on the glaze surface. Don't try this on anything you like or want to keep, because in an hour or a day the citric acid might have created a lemon sliced sized patch of a different color on your piece and ruined it. But that shows the glaze isn't fit for long term contact with acids.


Thanks for the exhaustive explanation, Cdragon! :D
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Today's MatchaWan :D (new name, but tea's the same)

Postby blairswhitaker » May 14th, '14, 09:11

Today's matchawan was a little unusual, I ground my own tencha by hand then drank the results.

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Can't remember the name of the tea off the top of my head (this was the fifth or sixth different tea of my day)

More photos from my trip to uji on the travel threads.
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