Cobweb paintings were invented by 16th- century Austrians. Monks and peasants collected cobwebs from attics, sheets of grass, and tree branches then stretched them onto postage-stamp-size canvas frames and carefully painted them with opaque watercolors. The artform died out with the onset of WWI, and fewer than 100 known paintings exist today. Source Source 2
On the left, you can see the fibers at the edges of the cobweb painting. On the right, the same cobweb painting is held up to the light.