I agree, they are quite beautiful. I particularly like the way they seem to shift between looking like photographs and paintings.
The process he used most often (mediobrome) isn't all that well documented, but it seems to be bromoil with the original silver image partly redeveloped. The rest is his skill with adding and removing pigment.
There's a photographic museum in his home town of Charlerois that I believe has a collection of his work, but I don't know how much of it is on permanent display.
(Unrelated: Charlerois is also famous for its metro, especially the architecture of some of its stations.)