Hi Martin,
Cassel earth, also known as Vandyke brown and Cologne earth, is a brown earth containing iron and manganese oxides and lignites, which are organic substances from peat or coal. It was first used in European painting in the 17th century and is still in use today. This color is found in the pictures of the old masters, among them Rubens, who used it mixed with gold ochre as a warm, transparent brown, which held up particularly well in resin varnish.
So you look at Winsor & Newton and there you see they have Cassel Earth under the name Vandyke brown.
Marc