Do you write with the assumption that the artist has no idea what an uncommon item is? Writing steampunk variant I will have several such things. Or should I mention the item and have a glossary that the artist can refer to?

Extra description will boost your page count, that's for sure.
Unless you feel comfortable letting the artist take the reins, put in as much description as possible.
At least, if it's a contrived sort of device. If it gets too long to be in the script, you could always make a separate appendix for it all...