 To compensate for fluctuations in relative humidity, the case is supplemented with a bed of silica gel treated to provide 55% relative humidity.Because the "Mona Lisa"s poplar support expands and contracts with changes in humidity, the picture has experienced some warping.In response to warping and swelling experienced during its storage during World War II, and to prepare the picture for an exhibit to honour the anniversary of Leonardo's 500th birthday, the "Mona Lisa" was fitted in 1951 with a flexible oak frame with beech crosspieces.