The painting has been in the museum’s collection for more than 250 years and the hidden Cupid had been known about since an x-ray in 1979 and infrared reflectography in 2009. It had been assumed that the artist himself had altered the composition by covering over the painting of Cupid.
But when a major restoration project began in May 2017, conservators discovered that the paint on the wall in the background of the painting, covering the naked Cupid, had in fact been added by another person. When layers of varnish from the 19th century began to be removed from the painting, the conservators discovered that the “solubility properties” of the paint in the central section of the wall were different to those elsewhere in the painting.
Following further investigations, including tests in an archaeometry laboratory, it was discovered that layers of binding agent and a layer of dirt existed between the image of Cupid and the overpainting. The conservators concluded that several decades would have passed between the completion of one layer and the addition of the next and therefore concluded that Vermeer could not have painted over the Cupid himself.
https://kottke.org/21/08/a-restored-vermeer-painting-now-with-bonus-cupid
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