http://www.saa.org/Portals/0/SAA/publications/SAAbulletin/16-3/SAA16.html

I think try to get the highest dpi that you can.  Also, I don't know if there are any with a recessed lid but if not try and find a good box or something that can fit over the scanner so you can have it closed when you put your artifacts on it, as the lid will be propped up and let light in.

http://archaeologyfieldwork.com/AFW/Message/Topic/13285/Discussion/photographic-tool

I don't know if this will help at all but I had a similar question last year, not a lot of info here but it may give you some suggestions.

In my experience a camera will do a better job, cheaper, faster. Lighting is the critical factor.

I would look into the EPSON Perfection V500 Photo Scanner. The software is user friendly and the zoom capability allows you to capture incredible detail. It has a lid, which I leave open, but I will place a piece of felt (color varies depending on the artifact) over the artifact. Then, using Adobe Photoshop, I clean the image by removing the background.

At the moment, you can get it from EPSON for $99.00 with free shipping!