Karen Russo sent me an e-mail with links to more invisible artworks. Thanks! Then I had to do more research. And more research always makes things more complicated. These are the preliminary results.
Art hidden behind art
Infrared reflectography can reveal drawings hidden under the layers of a painting. Many old paintings contain underdrawings - hidden ideas and sketches that are invisible in the final version. Paint layers that look opaque to human viewers are transparent in the infrared.
Classic invisible art
The most classic invisible art object is probably "The Invisibile Object" by Alberto Giacometti. It is held in the hands of a disquieting statue.
Almost imperceptible art
The most massive almost imperceptible artwork must be Walter de Maria's "Vertical Earth Kilometer". A one kilometre long solid brass rod, five centimeters in diameter. It has been sunk into the earth at the Friedrichsplatz in Kassel, Germany.
Empty space art
Zone de Sensibilité Picturale Immatérielle and Le Vide by Yves Klein.
Erased de Kooning Drawing by Robert Rauschenberg.
Exhibition of almost imperceptible art
Exhibitions of invisible art
VIDES (VOIDS) a retrospective exhibition at the Centre Pompidou.
A Brief History of Invisible Art at the CCA Wattis Institute for contemporary arts.
Going to extremes
Also on view was a work that Friedman claims took five years to create, 1000 Hours of Staring, 1992-97, a blank sheet thirty-two and a half inches square, consisting of what the catalogue calls "stare on paper."
In his enigmatic work 'Untitled (A Curse)', Tom Friedman presents the viewer with a simple plinth upon which - it appears - nothing rests. Yet, like Yves Klein's 'Void', Friedman's plinth supports both nothing and something at the same time, for the space above it has been cursed!
And then Jason Metcalf decided to erase the curse by pronouncing a blessing upon the artwork.
BRITISH taxpayers have paid £1,500 for a foreign painter to exhibit INVISIBLE pictures.
Mobile snaps reveal invisible art.
A small enigma
Invisible art detector.
Summary and conclusion
As always ... anything that you start looking into reveals an unexpected complexity. Things that first looked simple become complicated and invite further study. An evening of study brings up tons of material. This would have been impossible before the Internet. Again: invisibility + searching and finding.