edkrueger wrote:You should just test the bottom of the pot.
I used to work in the Quality Assurance testing lab in Kmart International Headquarters in Troy MI. (No longer exists. Kmart is a fragment of the company it once was.)
I did lead testing as part of the testing process for Childrens clothing. We tested buttons, snaps, zippers and anything else that was painted. Our sister lab tested anything that wasn't associated with clothing.
Testing the bottom of a pot with one of these contact tests will only show lead if there are large enough amounts of lead that the tiny amount of acid tin the kit can leach some out in the brief time you are rubbing the test area. Thais is a HUGE amount of lead that is readily releasable. Lower quantities of lead and lead firmly bound up in less soluble substrates is likely to go undetected
The legal test for determining lead content in paint requires removal of a sample of the paint using mechanical friction or chemical solvents. This permanently damages the painted surface.
The test for China dinnerware and other food grade items involves putting an acid solution into the object, allowing it to soak for a predetermined time, and then testing the acid solution for lead. In the lab I worked in we used a infrared spectrophotometer to determine the precise quantity of lead released into the acid solution or contained in the paint sample.
Such preciseness isn't required for home testing of antiques, but unless you are leach testing, the results you are getting from home testing kits aren't likely to mean a thing- the amount of time the chemical has to react with the substrate is probably insufficient to leach enough lead to get a strong color reaction unless truly huge amounts of lead are present.
Personally, I'm pretty comfortable using Yixing clay pots. I'm not comfortable using antique glazed and painted pots and cups. The CPSC website has many warnings on it regarding fancy painted Chinese porcelain, but I've seen none regarding any yixing-esque earthenware pots.
If anyone wants the ASTM test numbers for the methods we used I can look them up, but I'm sorry, I don't know them off the top of my head.