Manet

From: The Judgment of Paris by Ross King

p 52

the viewing public was accustomed to standing close to paintings and studying them minutely and marveling over the delicacy of the handiwork. ... But manet's apparently clumsy brushstrokes and lack of clarity did not lend themselves to this sort of appreciation. The work looked lackadaisical and incomplete because inplaces the undercoat of white primer and the weave of the canvas could clearly be seen. Elsewhere the marks of the paintbrush were visible. Most other painters used thin glazes and fine brushes made of sable to cover their traces, in effect brushing themselves out of their work.

The vast majority of pigments sold in France were no longer mixed with linseed oil which yellowed with age, but rather with poppy oil whose use resulted in more buttery, textured pigments than the smooth ones produced with linseed oil.
Painters using pigments bound in poppy oil therefore needed to work harder to eliminate the bristle marks from their canvases, though a number of them, notably Delacroix and Couture had been leaving behind the visible sign of the brush as a kind of signature of their individuality and workmanship. They were following in the tradition of painters of the Italian Renaissance, such as Leonardo and Tiotian, some of whose works showed how they even smeared paint with their fingertips. But by the nineteenth cetury this seemingly spontaneous approach had disappeared.