Unless I knew with certainty that the glaze was stable and safe, I wouldn't put edibles of any kind in glazed pots. Maybe you could seal the inside with something safe (whatever that might be), but, given the difficulty in truly assessing the risk, I just don't see the point in taking any chances with an unknown glaze (either for your health or for the health of the trees).

How about lining the pots with a heavy plastic bag? That should minimize the loss of volume while separating the pot from the soil in a stable and reversible manner. Put the bag in the pot. Fill w/soil, trim the top, pierce the bottom via the drainage holes, push a few wicks in.

Or you could use the terra cotta as cache pots and get some really cheap plastic pots to go inside them. This would also make them easier to move as terra cotta full of soil is very heavy. I do this so I can change the displays around seasonally.

How could outside glazing be harmful. We use all kinds of coffee cups, mugs, dinner plates .. with glazing. !!!

Second : sealing the inside is , IMO, totally unnecessary plus it could actually cause contamination.

JMO

Pottery intended for food typically uses glazes without lead or other contaminants. I have read that inexpensive pots (these were!) are fired at low temperatures which require special low temperature glaze which may contain lead or other heavy metals. There was a scare a few years ago about cookware from outside the USA using lead glaze. I don't know what's in the Malaysian glaze but since the pots are for plants there's unlikely any control. The pot interiors are permeable (I tested with a few drops of water which soaked in) so I am concerned if water can reach the exterior glaze from inside that there may be long-term transfer of lead or cadmium (which I don't know is _not_ there) or whatever to the inside.

Plastic liner pots is a good idea but it's hard to find ones to fit the containers without dropping in size so much I am losing 1/4-1/3 the volume of the container. I'll hunt around since you are right, it does make them much lighter to move if it can be done as two pieces, but this isn't exactly the best time of year to go hunting for pots in Minnesota!

I bought water-based acrylic finish and painted on 2 layers. I did this to all the pots so they are generally sealed against water soaking into the pottery (I could end up storing empty pots outside in freezing temperatures). For one of the fruit trees I also lined the pot with a piece of old shower curtain so there's now 3 layers between the plant and the pot. At least the plant is happy now and if I can find a plastic liner pot that fits I can transfer it later.

I'm with seysonn on this one - to a point.

In fact, some cookware and dishes do have lead in them (I read up on a lot of this stuff a few weeks ago when I was trying to figure out a suitable replacement for a pet's water bowl that has worn-off glaze.) and I don't think you're necessarily in the clear even if something is made in the US. Besides, even if you could guarantee your dishware was safe, could you guarantee that items from restaurants, friends/relatives, etc. were?

http://www.dhs.wisconsin.gov/lead/doc/Porcelain%20and%20Ceramic2.pdf

And about that shower curtain according to this site (which may or may not be reliable) there are plenty of issues there too: http://watoxics.org/files/VolatileVinyl.pdf

Anyway, my point is if it were me I'd use whatever pot I felt like and pat myself on the back for increasing and varying my fruit intake. In my opinion unless you're already doing everything else you can to be healthy as far as diet, physical activity, and prevention/early detection of health issues then worrying about any negligible risk of lead contamination from pots is a waste of time.

Just as an update--I did some reading about lead and cadmium contamination in soils and in both cases fruit is the way to go. Lead exposure from contaminated soil is usually via inhalation of dust or ingestion of soil particles. It isn't readily taken up by plants and when it does it tends to be in the roots and leaves but not in the fruit. Cadmium is taken up more easily than lead; however it is also mainly in the roots and leaves but not in the fruit. Of course there could be other stuff in the glaze, but at least for these the lemon is probably safe.

Here are a couple links:

Cadmium (old paper)
Lead (handout from UCD)

Thank you for all the postings everybody, and the extra research sf_rhino. As I mentioned earlier, I have already applied two coatings of acrylic to the inside, mostly to prevent water from soaking into the ceramic, but it will act as a barrier. I liked the idea of using liner pots which I already did with one plant a few years ago. It does make it easier to move around and more flexible in changing planter-plant combinations from indoor to outdoor seasons. However, the planters and the pots are very different shapes, in particular the base diameter. These plants will need decent size containers and I would lose too much volume with liner pots, or I'd have to buy much larger planters for modest size pots. As a compromise I re-lined one of the planters with a plastic bag (+ the acrylic seal), then trimmed the bag down to soil level so it is almost invisible (and will be when I get something decorative to cover the soil). This will add more barrier but also make it easier to transplant at some stage.

Hi
I use this to seal my papercrete pots:

Here is a link that might be useful: liquid glass

Unless you're going to subsist on only the kumquats and lemons you grow in these containers, I wouldn't be concerned.