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Starting clay in my classroom


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#21 Benzine

Benzine

    Socratic Potter

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Posted 22 January 2013 - 09:38 PM

Benzine;
"How is it the glazes used lead?"
Well, sit down, young man, and I will tell you the story of lead in glaze.
The source of lead is the mineral "galena", found all over the world as a surface rock. The galena can be used straight as a glaze, and was dusted on the pots through a bag or an old sock. This was a job for the potter's wife, and many died from lead poisoning. People did not know that lead was poisonous, or if they did, they did tell their wives.[Sick, I know. I apologize]
The great thing with lead was that it made a beautiful glaze and a very reliable flux in low fire glazes. Lead glazes were common in schools right up into the sixties. Art teachers were also doing copper enameling in their art rooms without proper ventilating.
Lead is still used in glazes in third world countries, as it is easily available in car batteries, which are taken apart and pounded into a glaze.Be careful when buying souvenir pottery from Mexico and other countries, and don't use them with food.
The opinions expressed here are my own.
TJR.


Good information TJR. I knew, why the lead was used. I just didn't know how safe the wares actually were, that used it. I definitely didn't know the bit about car batteries. That seems like a lot of work, for a potentially toxic glazing material.




Having read all of the above, it's making me worry about my clay / glaze now. I am using smooth white earthenware - and mostly make decorative pieces. However, if I want to make some mugs, what would be the best bisque firing temperature and same for glaze fire. I generally use a mid range transparent glaze, coloured with small qty of oxide. (Copper carbonate, cobalt carbonate, red/black iron oxide,)Are these 'safe'?
All advice greatly welcomed.

Celia


It is morally reprehensible for a potter to sell pots used to serve food without testing the glaze for leaching and the clay for leaking. Of course, most of us would rather have a mug that leaks than a mug that poisons us. If a glaze contains any metallic colorants other than iron then it should either not be used as a liner or it should be tested for leaching. You can do a simple lemon test to eliminate an obvious leacher but only a lab test will detect more subtle leaching.

Jim


The glazes I use, are marked as food safe, by the company. I'm assuming, that would indicate, that leaching isn't isn't an issue. Yes, I agree, that I prefer my mugs leaky, than poisony.

Some of my classroom glazes have soluble copper and cadmium. They are leftovers from a previous teacher, and I'm using them until they are gone, but I don't allow students to use them, for anything that might be used with food or drink.
How does the lemon test work?
Also, how is it, that glazes used lead for so many years?


Lead is not all that deadly and it is even possible to use very small amounts safely in a glaze that is carefully fired, so it has been used on pots for many years. But, it's for the best that we now consider it better to not use it at all for dishes. It is still available, usually in frits, for glazes. Pres answered your question about the lemon test. Since most potters aren't going to go to the trouble and expense of sending pots to a lab to be tested, the lemon/vinegar tests are better than no testing at all, but even better is to not use any glaze for a liner that has any metalic colorant in it other than iron unless it is tested in a lab.

Jim


No worries about the metallic colorants. As I said, I have some, but they are not used, with food/ drink wares. The students always ask, "What will happen if they use them to eat or drink out of anyway?" I tell them probably nothing, unless they eat or drink out of them everyday for a long time. But just to be safe, only use them for decorative items.
"Anything worth believing, is worth questioning"




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