This spring, the Chamber Orchestra of Philadelphia took the humble viola and put it center stage with a program that featured not one but two violists. First, Martha Mooke—a multi-genre composer and violist who has performed with classical ensembles as well as rock musicians David Bowie and Patti Smith—was soloist for the premiere of her own Invisible Hands for electric viola and orchestra. The piece was performed twice on the program, led by Music director Dirk Brossé, with Mooke at various points improvising, walking through the orchestra, and adding electronic effects. Then Born Lau, a young violist who won Astral’s 2012 National Auditions and holds degrees from the Colburn Conservatory of Music and the Curtis Institute of Music, performed Hummel’s Fantasie for Viola and Orchestra which included his own improvised cadenza. The orchestra’s 2017-18 season, dubbed “Improvisionaries,” has been exploring improvisation from the 17th through the 21st centuries, as well as links between jazz and classical music.