This piece was bought in Sinaloa in 2020. The merchant said it was made in Puebla. The lead test procedure involves leaving white vinegar in the piece overnight, pouring some of that into a test tube, dipping a cotton swab into a reagent solution and then stirring the vinegar. The color indicates lead content. As you can see, it has turned black, indicating heavy concentration of lead. This pottery is a tradition in Mexican culture and is used for food and liquid surfaces everywhere. There are manufacturers trying to make stoneware that retains the traditional appearance, but few people use it. Further refiring tests demonstrated that these are fired around 1700-1750F, a temperature sufficient to melt pure lead bisilicate into a glossy transparent but the body is still very soft and porous.
This demonstrates the amazing melt behaviour of lead-as-a-flux for ceramic glazes. Not only does it melt early, but it softens slowly over a 300F range of temperatures before it goes off the end of the runway on this GLFL test. Then, when fired 200F hotter than that, it remains a stable, clear and uncrazed glass. Beginning around 1750F, this becomes a transparent glaze, by itself.