clay lover.... you are not missing anything. You get the point.
beccab......... Sorry but boiling water in the pot will do nothing to speak of to assure "safety". If the piece IS leaching, it will cause some of the material to leave the glaze and enter the water. But it will not "stabilize" the glaze if the glaze is unstable. It might decrease the initial first post-boiling water use leaching amount slightly...... but this will not make assuredly "safe". It would take a long time of repeated acid treatment to get all of the poorly bound material to come out of the glaze. And in so doing.... is deteriorating the glaze in other ways.
At a very "technicality" level, each time a glaze is exposed to a leachate (stuff that will cause poorly chemically bound materials to leave the glaze) some of the material in the glaze will leach out into that material (usually liquid). So the amount of material still in the glaze decreases very slightly. So the next time the same leachate is placed into contact with the glaze, a SLIGHLTY lower amount of the material will leach out. This pattern will continue pretty much indefinately, but will (technically) never cease.... although eventually the leaching material may go below the levels of detection accuracy.
The amount that leaches depends on the PH of the lechate and its temperaturee, and the time it is in contact. Water will do one thing, vinegar will perform differntly, tea differently again. Long time is one result.... short time another.
This is into the realm of the science side of the art. The HUGE point here is that you DO NOT KNOW the stability of that glaze unless it has been lab tested. Until you do, you are playing with a gun which you have no idea if it has a bullet in the firing chamber.
It is important to note that even if it IS leaching, it is possible that the amount is not ever going to be an issue for someone using it, but again you don't KNOW that. Now that you know about this potential..... can you NOT take precautions?
A industrial health specialist I know tells a story about how a person using a poorly formulated handcrafted mug ended up with poisoning from drinking coffee continually day after day after day from that mug. What was the poison in question........ IRON! The saturated iron glaze with sprayed itron oxide applied over it glaze was so unstable that it leached more iron than the person's body could handle.......because they had another medical condition also.
If you can post the glaze recipe here, those of us frequenting the FOrum like myself who are "tech weenies" can look at the projected post-firing chemical oxide composition and make some predictions for you........ but even those are no subsituite for KNOWING.
Note that in addition to being bitter tasting, copper also happens to be very toxic to some people with pre-existing medical conditions like Wilson's Disease.
If you want to know more clearly if the glaze is suitable for food service use, make a 3" x 4 " ID test cup, glaze the inside with the same glaze in question, fire it exactly the same way, and then send it off for actual lab testing. Since the only legal standards for the leaching of such stuff in the USA is the FDA ones for lead and cadmium, the best guide for something else is to compare to the Drinking Water Standards. This single test test will cost you about $60 plus postage. You could add in copper in addition to the lithium for about another $30. If you need an address.... PM me.
Glaze application and firing conditions can change leaching properties. The inside of the teapot and the inside of the cup likely will be sligh tly different. So if the lab tests come back at all marginal, better safe than sorry. If it comes back extremebly low....... you are good to go.
It is probably easiest to make another teapot and use a glaze on the interior that does not contain any potentially toxic ingredients. There are plenty to choose from. Then even if the glaze is not stable, it is not going to potentially harm anyone.
Give a gift certificate as the present along with the non-functional teapot... and tell them they'll get a different one that they can actually USE. (Then take back the original and smash it when you give them the new one.) When you remake the piece, keep the aspects that were already successful. From your crit feedback, address the areas that were less successful in the new one. The new one will then be a better piece than the original was. A Win-Win.