|Willard L. Metcalf's Note to Framers and Dealers|
I'm doing some research on the varnishing of oil paints right now. I've known for some time that the French Impressionists were the first to consider varnishing optional. Part of the reason was aesthetic, in that they liked the look of dried, matte paint. It gave the painting a more atmospheric feeling. Also, they felt they had more control over the final effect, since any application of varnish would darken and increase the saturation of some colors. Finally, it was a revolt against the traditions of the Academy and its Salon.
But what I didn't know is that some artists, such as Degas, preferred the look so much that they went at great lengths to "leach" the oil out of their paints with turpentine prior to painting or spread the paint on newsprint so it would sop up some of the oil, making the paint "short" (having less oil). Some enhanced the effect by using an absorbent ground to pull the oil away from the visible layer. All of this dramatically increased the matte look. In fact, it looked a lot like pastel.*
Interestingly, to protect the surface and to also provide a harmonizing filter through which to view the work, some of the artists framed their oils under glass. (This practice ended around 1890.)
Unfortunately, not all art dealers understood the artists' intent in not varnishing. Thinking that brighter colors and more contrast would help sell the paintings, the dealers varnished the paintings anyway.
Toward the end of the Impressionist age, the anti-varnishing movement came west to the US, where some American Impressionists took up the practice. Willard L. Metcalf wrote on the back of one paintings: "UNDER NO CIRCUMSTANCES should VARNISH or other mediums be applied to this canvas as they will change certain values and therefore ruin it."
*Readers have asked about the soundness of the approach of leaching oil out of paint. It's not a good practice, since leaching or thinning the oil too much with solvent will ultimately weaken the paint film and cause flaking or cracking.