The building's surface was made out of concrete to reduce the cost, inferior to the stone finish that Wright had wanted. The small rotunda (or "Monitor building", as Wright called it) next to the large rotunda was intended to house apartments for Rebay and Guggenheim but instead became offices and storage space. In 1965, the second floor of the Monitor building was renovated to display the museum's growing permanent collection, and with the restoration of the museum in 1990–92, it was turned over entirely to exhibition space and christened the Thannhauser Building, in honor of one of the most important bequests to the museum. Wright's original plan for an adjoining tower, artists' studios and apartments went unrealized, largely for financial reasons. However, as part of the restoration, architects Gwathmey Siegel and Associates analyzed Wright's original sketches to design the rectangular 10-story limestone tower, that stands behind, and taller than, the original spiral building (replacing a much smaller structure), which has four additional exhibition galleries with flat walls that are "more appropriate for the display of art." Also in the original construction, the main gallery skylight had been covered, which compromised Wight's carefully articulated lighting effects. This changed in 1992 when the skylight was restored to its original design. Funding for the alterations was raised partly through the controversial sale of masterworks by the foundation in 1991.