Ce n'est pas un pinceau.
I'm not sure, but the depicted tool may be une brosse; it's a 3/4 inch flat brush, which makes it fairly large for an art brush. From the descriptions I found online, un pinceau is a delicate item, a small brush used for detail.
But I digress. It was painted with a knife. But there are some bits of art supplies catalogue in there that describe brushes.
Acrylic on box canvas, 40 x 40 cm