Glaze leaching - does it matter?

[JeffS73]

As the thread title, does glaze leaching matter for bonsai pots? I'm thinking about the outside of the pot, for example a copper glaze like Koyo's crystal, could it fade or discolour over time?

I don't know if the pH of rain is acidic enough to have an effect over time, should I be testing my glazes or avoiding recipes that leach? Most metallic/crystal ones seem to.

Thanks potters!

 

[Bonsai Nut]

could it fade or discolour over time?

It is interesting that no one responded to this thread. Though I am not a potter yet, I have been reading up a lot on the subject. Without a doubt some glazes are more susceptible to leaching than others, and almost all glazes with high metal oxide content will leach to some extent.

There are really two questions here - to what extent will presence of leaching impact the APPEARANCE of the glaze, and to what extent will leaching impact the HEALTH of the tree and/or chemical composition of the soil. So much depends on the glazes used, and the water / rain composition. Perhaps it is one reason why the inside of bonsai containers shouldn't be glazed (?) I can't answer the question "should I be testing my glazes" - and quick Internet searches do not turn up anything related directly to plant health. The closest related material I could find dealt with making glazes inert for the purpose of food grade containers.

 

[JeffS73]

Well, the inside isn't glazed to any great extent, so I wasn't really concerned that tree health might suffer. I was thinking more about whether acidic rainwater would change the appearance over time. The usual test seems to be 24hrs exposure to lemon juice to see if there's a change in appearance, so I guess that's an option.

The conclusion I've come to is that Chinese and Japanese glazes were tried and tested, so it wasn't a concern then. They used Barium, Manganese and Copper plenty, and lots of glazes are matte, which are more prone to leaching. It just can't be too much of a concern, I was probably over thinking

 

[JeffS73]

Anyhow, here's a random glaze test pic to make this thread more interesting

 

[sorce]

I was fittin' to wait and see what other people thought. No one else thought!?

I don't think about it much either except to only use known safe liners for inside drinking vessels.

I reckon you'd never be able to hurt a plant, unless you were doing things to otherwise kill it, like watering with vinegar.

Sorce

 

[rockm]

I'm not a potter, but I've got a hundred or more bonsai pots, from 100-year old Nakawatari, to two-year old American pots. I've used many over the years. I can say I've never seen glazes leaching off of pots. I have seen glazes change over the years, staining, fading and weathering. I have also had glazes on some Chinese pots delaminate from the clay body and crumble over time. I have also heard second hand that some glazes used by some notable American potters has broken down (as in fell off) in very hot climates when exposed to full sun.

I have steered away from crystalline glazes not only because they're not really useable with tree and a bit faddish (to me), but because I suspect they're not going to hold up over decades of use. Same reason not to use Raku for bonsai pots...

An actual potter will know better though.

 

[JeffS73]

I'm not a potter, but I've got a hundred or more bonsai pots, from 100-year old Nakawatari, to two-year old American pots. I've used many over the years. I can say I've never seen glazes leaching off of pots. I have seen glazes change over the years, staining, fading and weathering. I have also had glazes on some Chinese pots delaminate from the clay body and crumble over time. I have also heard second hand that some glazes used by some notable American potters has broken down (as in fell off) in very hot climates when exposed to full sun.

I have steered away from crystalline glazes not only because they're not really useable with tree and a bit faddish (to me), but because I suspect they're not going to hold up over decades of use. Same reason not to use Raku for bonsai pots...

An actual potter will know better though.

Potters may know, but anecdotal reports are definitely of interest! If you're happy to share more info, please feel free to pm me, pictures say a lot.
I know Chinese potters went through a phase of copying old nakawatari but applied glaze thinner and fired at lower temps, to save on cost. I didn't know they would delaminate, or shiver as potters call it.
It's also interesting that modern artisan pots have this issue too. Gutting for everyone, but sharing failures is a good way to avoid it in future.
Crystalline glazes are as safe as any other to the best of my knowledge - oxides want to return to a crystalline state, meaning lots of glazes, like mattes for example, are all crystalline. It's a stable state for the oxide, potters just manipulate it for effect. Some crystal glaze potters fire a pot many times for an effect, but this can weaken a pot considerably. I get that aesthetically it can be distracting.
Raku pots are (normally) inherently weak - the groggy clay is strong but not normally vitreous, and crackle glazes can weaken a pot considerably.
Think I've used enough caveats for a noob now.

 

[rockm]

Potters may know, but anecdotal reports are definitely of interest! If you're happy to share more info, please feel free to pm me, pictures say a lot.
I know Chinese potters went through a phase of copying old nakawatari but applied glaze thinner and fired at lower temps, to save on cost. I didn't know they would delaminate, or shiver as potters call it.
It's also interesting that modern artisan pots have this issue too. Gutting for everyone, but sharing failures is a good way to avoid it in future.
Crystalline glazes are as safe as any other to the best of my knowledge - oxides want to return to a crystalline state, meaning lots of glazes, like mattes for example, are all crystalline. It's a stable state for the oxide, potters just manipulate it for effect. Some crystal glaze potters fire a pot many times for an effect, but this can weaken a pot considerably. I get that aesthetically it can be distracting.
Raku pots are (normally) inherently weak - the groggy clay is strong but not normally vitreous, and crackle glazes can weaken a pot considerably.
Think I've used enough caveats for a noob now.

Sorry for the confusion, the Nakawatari pots didn't do that. I've had new Chinese pots do it.

 

[JeffS73]

Some recent glaze tests to balance out all the words above. Glaze testing is very addictive.

A failed currie grid - I used the wrong Potash, its massively overfired at cone 10. Best thing about it is this weird photo.


Manganese. Running, wrong Potash again, but look at the colours.


A nice green / blue breaking black. Running again

Did I mention its addictive and not as easy as one might presume

 

[NaoTK]

I'm happy to see you testing glazes.

About leaching...

If the silica content is high enough and mature then the glaze should not leach* But if the oxide content is super saturated the oxide will precipitate on the surface as a sheen. That sheen will leach off.

Some glazes rely on this precipitate effect, but it matters what the precipitate is. A zinc-based crystalline glaze will leach (dunking in muriatic acid brings out the crystals) but a pyroxene crystal is less likely to leach. So your koyo is safe.

All this goes out the window if the glaze is underfired or outside normal limits.

 

[JeffS73]

Thanks Nao, very useful info. Now I've got some clay that vitrifies fully and looks good with it I can make unglazed pots while I work out a few glazes.
Since starting making I've gained a lot more respect and insight into what the Japanese potters have achieved.

 

[BrianBay9]

I'd be more concerned about the health of the potter than the health of the tree. I assume you use reasonable personal protective equipment to avoid exposure through the skin or by inhalation while applying or firing the glazes.

 

[NaoTK]

I'd be more concerned about the health of the potter than the health of the tree. I assume you use reasonable personal protective equipment to avoid exposure through the skin or by inhalation while applying or firing the glazes.

The clay itself is most dangerous to health...silicosis from clay dust. So having a clean studio floor is important.

Bioaccumulation is a key point, most materials don't bioaccumulate and are not absorbed through the skin at acute levels. Metal oxides in the air require a respirator. Lead is not used much these days.