You probably need to speak with someone that "throws clay". I work with a kiln and poured clay so I am more familiar with that type of product.

What you have was probably not fired in a kiln, but I have not seen it and they did have kilns 500 years ago.

My first suggestion is to speak with someone at a museum. If no luck there, look for someone that teaches throwing clay on a wheel.

If you still have no luck, then I can tell you what I would do myself. Just get some clay slip and repair the piece yourself. If you just use slip, the repair would be temporary - it also would not ruin the value of the piece since the repair is temporary. Once you use the slip to glue the piece together again, just don't move it. Someday you will find a professional to repair it. Good luck.

Whatever you do DON"T glue it, you will loose any value the piece has as an antiquity. I work with clay 7 days a week and I know repair is very difficult. There are many ways of repairing and few look good. Find someone that repairs this type of work. Your best bet is a museum or an internet search to find someone. Make sure you work with a qualified person. Something as old as your piece deserves to be treated well. Take your time to find the right person.

Monica I would be leary of using slip since it is so fragile and may let go causing more damage.

500 years ago the firing methods ment that the clay was fired at a low temperature and thus more fragile and porus. That said it did last 500 years.

I have a few very old pieces myself and they are so very wonderful. To think that someone made it with their hands so many years ago.
Good luck and treat it well.