Dark Brown Glazing Inside Chinese Pottery Cookware
Q: Does anyone know what is the dark brown glazing on the inside of (traditional?) Chinese pottery cookware? I would like to know if it contains lead and if it is safe to cook acidic food in it. (For those who are not familiar with the cookware I'm talking about, it's a light brown or beige earthenware with a rough, porous texture. It is also used for serving certain dishes in Chinese restaurants.) Please reply by posting.
A: Lead is used as a flux in low temperature glazes. Lead glazes are soft, brilliant, slightly yellow in colour. this yellow tine affects other colours which ma be added to lead glazes. Lead is used in the temperature range 900 deg C to 1200 degC on earthenware. If this pottery is definately earthenware and not stoneware it is more likely to to contain lead in the glazes. Other low temp fluxes for glazes are avaiable and lead should not be used in cookware. If you are still worried get it tested. Your local health dept should be able to help with this. ps lead volatizes and leaves a glaze at 1250 deg. From your description this glaze could be a salt glaze but this is usually done on stoneware from 1190 to 1285 degC. Salt glazes have an " orange peel" texture. I don't know much about ceramics so I might have misused the terminology. My dictionary says stoneware is well vitrified and nonporous, and earthenware is "made of slightly porous opaque clay". The pottery I was talking about was porous. Sometimes you can actually see a small amount of liquid bleed through it. The color of the unglazed surfaces is beige, close to that of a manila folder, and much more yellow than any unglazed parts of coffee mugs. I don't know if that's enough clue for you to tell whether it's earthenware. I just took a closer look at the dark brown (almost black) glazing. It's overall smooth but has fine, irregular, dimples-like imperfections. It's somewhat like "matte" finish on printed pictures--except that the texture is finer and more irregular. Is that what you meant by "orange peel" texture? This sounds like earthenware, as it is usually buff or red coloured clay and porous when unglazed, whereas stoneware is hard and vitrious and able to hold water when unglazed. The glaze is probably not salt. A brown glaze may contain iron, manganese, copper and cobalt and possibly lead so unless you know where or who made it, its probably best to get it tested. A matt surface is often produced by the additiion of barium, zinc or alumina.
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