Cologne Earth (Cologne Umber)
The application of the term “umber” to this earth can only have been based on a certain similarity in colour to true umber. In chemical composition, however, the two are quite different, Cologne earth really consisting of a mixture of humic substances. It is well known that the rotted wood found in the interior of decaying trees is often a handsome brown colour; and all woody matter, after lying a very long time, finally acquires this colour, owing to the transformation of the wood into dark-coloured compounds richer in carbon. This effect can be seen on the large scale, in Nature, in the case of coal, brown coal and peat.
Now Cologne earth consists of a brown-coal mould, dark brown in colour, of earthy character and of such low cohesive power that it crumbles with ease. Owing to this character, Cologne earth can be easily ignited by the flame of a candle, and then burns with a strong, smoky flame, leaving very little ash and disseminating the peculiar bituminous smell given off when brown coal is burned.
The geological characteristics of Cologne earth enable one to conclude that, where similar conditions prevail, materials of analogous nature may be discovered. This earth is found embedded in a deposit of brown coal, in which it forms pockets, and occasionally large bodies. Now, brown-coal deposits of enormous extent occur in very many localities, as for instance in Upper Austria and in Bohemia; and many of these mines are sure to contain pockets of brown-coal mould, which have perhaps been overlooked, but might very well be utilised in the preparation of colours of very similar character to Cologne earth.
The preparation of this material is very simple. The earth coming from the deposits is put through a simple levigation treatment which leaves, as residue, lumps of semi-decomposed wood, mineral admixtures, sand, etc. The levigated earth is sold in the form of cubes.
Cologne earth comes into the market under various other names, such as: umber, Cassel brown, Spanish brown, etc.
The fiery brown which was so greatly preferred by the famous painter Van Dyck, and named Vandyke brown after him, was of very similar composition to Cologne earth, and is said to have been obtained from a deep brown peat earth. The Vandyke brown of the present day, however, is almost invariably a ferric oxide pigment, toned to the proper shade by suitable calcination.