as a "Museum Opening of the Year" by Apollo. With the expansion, Glenstone opened to the public, with free tickets available online. In the year following the expansion, Glenstone admitted nearly 100,000 visitors. In 2015, Glenstone was one of several private museums questioned by the US Senate Finance Committee over its nonprofit tax status. After reporting from The New York Times had questioned the validity of nonprofit tax status for institutions like Glenstone, which, at the time, welcomed very few visitors, the committee sought to investigate whether high-value individuals and families were using private museums as a form of tax shelter. Committee Chairman Senator Orrin Hatch said in a letter to Glenstone and other institutions (including the Rubell Museum in Miami and The Broad in Los Angeles) that, "Some private foundations are operating museums that offer minimal benefit to the public while enabling donors to reap substantial tax advantages." After 2018, Glenstone continued to acquire adjacent properties, including lots containing residential homes. Speaking on Public Radio Tulsa's Museum Confidential program in 2019, curator Emily Wei Rales said that plans do not include substantial expansion, and will likely be limited to "one or two smaller buildings to house artworks, maybe in the woods, maybe on an adjoining property". She also mentioned potential plans to build a conservation lab on the campus. To encourage the usage of public transportation, Glenstone successfully lobbied Montgomery County to add a bus stop near its campus. The museum admits visitors without tickets who arrive on public buses. In August 2018, Glenstone Foundation Inc., the foundation that manages the museum, was sued by HITT Contracting, the company that managed the construction of the 2018 expansion, for $24 million to cover cost overruns. In October 2018, Glenstone countersued HITT Contracting for $35.9 million, claiming that the project was over budget and that construction issues had delayed the expansion's opening. In July 2021, Glenstone announced a purpose-built expansion to house Richard Serra's large-scale sculpture Four Rounds: Equal Weight, Unequal Measure (2017) that the museum had acquired from the artist. The building, designed by Thomas Phifer, the architect of the Pavilions, was created in consultation with Serra to fit the acquired piece. The expansion opened in 2022. Rales donated $1.9 billion to Glenstone in February 2023. Collection and exhibitions The museum's collection of about 1,300 post-World War II works from around the world consists of paintings, single-artist installations, video installations,