Munch employed a rather special method of coming to grips with the reality of nature – he allowed nature to patinate his pictures, leaving them exposed to the elements during the working process. This drastic method tested their resistance to the real world. ‘Paintings should be able to take up the fight with the sun and the moon’, he remarks.

… he was furious with the Nasjonalgalleriet in Oslo for varnishing his pictures. In his eyes, this treatment prevented the paintings from breathing, causing them to choke upon their own illusion of self-sufficiency. He experiments with various techniques of painting – amongst other things, with casein as a binder – ensuring that the greatest possible degree of pigment adheres to the canvas. Porosity becomes something to strive for – a living, breathing technique[….] Munch experiments, and masters various forms of porousness in his painting – he thins the paint with so much turpentine that the binder is sucked out of the priming and the colour is left almost without adherence.

The double-sided fatal and careless relationship to his work may derive from a resigned awareness of their place in the world. They exist, which is why he could casually throw them on the floor and trample all over them, or lay them like lids on a boiling pot of soup – which happened every now and again at Ekely.

from Poul Erik Toejner’s essay in Munch in His Own Words