Three years ago, having outgrown its stacks space, the Ryerson & Burnham Libraries at the Art Institute of Chicago made arrangements with Northwestern University Library to house a portion of its collection at NU’s book storage facility in Waukegan, Illinois, 40 miles north of Chicago. The terms of the arrangement are fairly straightforward: NU houses our material in a state-of-the-art facility and they load our MARC records in their catalog so that their students and faculty may use our materials. When we want one of our books, they send it by overnight courier and our patron has it in hand the next day. At present, approximately 40,000 books have been sent off-site; in order to obtain adequate elbow room in the stacks, we would like that number to be closer to 100,000.
As one would expect, the most difficult aspect of this process has not been working out the logistics or fielding patron concerns, but deciding which materials to send off-site. Our goal has been to inconvenience our researchers as little as possible and to minimize the staff time necessary to request and return off-site materials; in other words, we have tried to determine which items are in low demand, and likely to stay that way. Our selection criteria have hinged on imprint date and circulation data.
Three years and 40,000 books later, we have picked all the low-hanging fruit and sent it to Waukegan. We are now at a stage where we must review our selection criteria. In addition to simply broadening the categories of eligible materials to send off-site, we might also consider developing a deeper reciprocity agreement with NU that would allow us to withdraw certain items from our collection. Another possibility is to initiate a digitization project to provide virtual access instead of physical access to parts of the collection. The challenge is to think creatively about how best to preserve the strengths of the collection, serve our patrons, minimize the impact on library staff, and save money. What selections criteria have other institutions used to make these painful decisions?
ARLIS/NA Collection Development SIG Blog 
This is very helpful for other art libraries going through this process. It is good to have validation that this is an accepted practice that makes priority collections more visible.