Yellow mason stains leeching out of glaze matrix?

[Liz Wheeler]

Hello,

I am making some transparent glazes using a base glaze mixed with various mason stains or colorants such as cobalt, copper, and iron.  Are there any concerns with mason stains leeching out of the base glaze?  (Fired to cone 6; I don't have the recipe at hand but it's a fairly shiny, very transparent glaze).  Are any of the mason stains of particular concern with regard to food safety?  I am using vanadium yellow 6404 and encapsulated red 6088 at present in addition to the oxides/carbonates mentioned above.  Any help is appreciated!

 

[liambesaw]

Vanadium is extremely toxic

[Min]

Stains are basically fritted colourants. In the case of the 6404 Vanadium Yellow it's vanadium oxide plus tin oxide, for the 6088 Dark Red it's cadmium combined with selenium, zirconium and silicon oxide. The fritting process ties up the oxides to make the stains far safer to use than the raw colourants they are made from. They are much less toxic in both the raw and fired glaze.

If you post your base glaze recipe we can have a look at it and see if we think it would be a good candidate for a non leaching base glaze. The only way to know for sure your glaze is not leaching is to have it lab tested but you can do a couple home tests first to rule out any iffy glazes.

As far as cobalt, iron and copper are concerned, again the base glaze will play a significant role as to whether the glaze will leach. Cobalt is a strong colourant, try and keep this (and any heavy metal colourant) to as low a level as you can. 2% cobalt carbonate will give you a strong blue. Iron isn't really an issue unless it leaches and the person using the pot has hemochromatosis. Copper is one of the harder colouring oxides to keep bound up in the glaze matrix. Again, a durable base is important as in not overloading the glaze with it, under 5% should be okay but I would definitely do a home test to see if the copper leaches out. If you can keep the copper to under about 2.5% that would lessen the risk of it leaching out. If the user of the copper glazed pot(s) has Wilson's disease it can be an issue, copper leaching  will also make the food taste funny if nothing else.

[liambesaw]

I will point out that their vanadium stain touts trapping the vanadium inside of an SnO2 crystal, however SnO2 is soluble in hot acids and hot alkalis which anyone who uses a dishwasher will be exposing their stuff too.  Chances are only a tiny amount would make it out, but you have to use a heck of a lot of stain to make something yellow too.

[Bill Kielb]

Generally stains that have been encapsulated in zircon are said to be safe because the transition metal cannot escape. At least that’s the theory. The two types of zircon stains realistically are encapsulated and lattice. There is a third type of stain, a spinel stain if you will. Based on the molecule spinel  Mgo Al2O3 which is very stable at high temperature. From there the transition metals are just substituted in for the various colors.

All of these are theoretically very stable. How can one tell? I think ya have to rely on the manufactures representation so I would guess if the documentation didn’t explicitly say it, I think it would be bad to assume it was one of these three or a lack of an explicit declaration means it must be ok.

I looked  up the Vanadium and of course there is an ominous exculpatory paragraph on the page so I don’t know what to think after reading it. I guess my best thought is always strive to make durable glazes for food service and  try to make sure the manufacture says it’s ok for the use.

 

[Liz Wheeler]

Thank you so much for taking the time to reply!   Here's the base glaze recipe:  Minspar 46, Gerstley borate 30, EPK 13, Silica 11.  Would love to hear your opinions about how well sealed it might be at a high cone 5.   Liambesaw, would you be willing to link to your sources describing vanadium's toxicity?  In terms of oral ingestion this source https://www.atsdr.cdc.gov/PHS/PHS.asp?id=274&tid=50#bookmark07. indicates only that it can cause stomach cramps at 13mg/day.  If one assumed the worst case scenario -  that a person put hot, acidic liquid in a bowl with a leechy glaze suffused with vanadium - how much might one be concerned about entering that person's food?  (I'm curious about this more generally not just about vanadium mason stains.).

[liambesaw]

24 minutes ago, Liz Wheeler said:

Thank you so much for taking the time to reply!   Here's the base glaze recipe:  Minspar 46, Gerstley borate 30, EPK 13, Silica 11.  Would love to hear your opinions about how well sealed it might be at a high cone 5.   Liambesaw, would you be willing to link to your sources describing vanadium's toxicity?  In terms of oral ingestion this source https://www.atsdr.cdc.gov/PHS/PHS.asp?id=274&tid=50#bookmark07. indicates only that it can cause stomach cramps at 13mg/day.  If one assumed the worst case scenario -  that a person put hot, acidic liquid in a bowl with a leechy glaze suffused with vanadium - how much might one be concerned about entering that person's food?  (I'm curious about this more generally not just about vanadium mason stains.).

https://en.m.wikipedia.org/wiki/Vanadium(V)_oxide

On the safety square (fire diamond) it is rated a 4 which is the most severe health rating.  The powder and fumes are the main danger for you as a Potter.  Obviously it is not a good thing to eat either.  It is soluble in water, so it's kind of nasty stuff.  

Here's a link to the SDS: http://landrspecialties.com/MSDS/Vanadium_Pentoxide.pdf

[Min]

5 hours ago, Liz Wheeler said:

Minspar 46, Gerstley borate 30, EPK 13, Silica 11.  Would love to hear your opinions about how well sealed it might be at a high cone 5. 

That's a lot of minspar for a cone 5/6 glaze, the sodium is going to be quite high from it. Do you get crazing with this glaze? Boron is also higher than it needs to be (from the gerstley borate). Boron is found in most midrange glazes but too much of it makes for a "soft" glaze. Silica and alumina levels are good. I would be looking to either improve this recipe or look for another one if you are going to be using it with either of the stains you were asking about.

[Bill Kielb]

On 12/5/2019 at 11:47 AM, Liz Wheeler said:

Thank you so much for taking the time to reply!   Here's the base glaze recipe:  Minspar 46, Gerstley borate 30, EPK 13, Silica 11.  Would love to hear your opinions about how well sealed it might be at a high cone 5.   Liambesaw, would you be willing to link to your sources describing vanadium's toxicity?  In terms of oral ingestion this source https://www.atsdr.cdc.gov/PHS/PHS.asp?id=274&tid=50#bookmark07. indicates only that it can cause stomach cramps at 13mg/day.  If one assumed the worst case scenario -  that a person put hot, acidic liquid in a bowl with a leechy glaze suffused with vanadium - how much might one be concerned about entering that person's food?  (I'm curious about this more generally not just about vanadium mason stains.).

If you go through the public health statement it’s pretty complete about what is felt as known: allowable limits, exposure paths etc. from Niosh to OSHA the summary report although lengthy will likely provide you answers. The document is the downloadable pdf  which is about ten pages. The document will not necessarily tell you:

how much might dissolve out of a glaze that uses it as a colorant. That would take independent testing of each formulation with respect to : amount of vanadium, durability of the glaze, and solvent exposure.

In general many glaze kitchens omit it from their store of chemistry because there appears to be an elevated risk in handling, firing, etc.....

 

 

 

[Liz Wheeler]

Thank you so much for your replies!  I have some reading to do