How do colours appear ?
As one might expect colouring is not only used to uplift the appearance of a map. It could also be used to hide shorting comings of a map.

Washing and/or bleaching a map is done in order to remove dirt and stains. After this treatment the paper acquires an unnatural whiteness, similar to that of photocopying paper. Colours applied on such paper very often appear very bright in comparison to old colouring.


Recto
The colourist has applied the colours skilfully.  At a first glance they seem authentic.


Verso
The tin paper shows no traces of oxidation. The colours are therefore modern.

Defects in the paper also reveal whether colours are original or not. If you have a close look at a tear whereby the surface is evenly coloured, the colouring has been applied prior to the tear. If colouring has been applied after the [restored] tear the colours around the tear will be more intense. A tear or damaged spot causes the paper to fibre, which consequently absorbs the colours more.

This also applies for the folds of a map. When the colouring around the fold is more vibrant than the rest of the colouring, you may presume the colouring is modern.


This map has not been preserved that well. The oxidation traces of the green colouring on the right side, have left their traces on the left part of the map.

Conclusion
Determining the originality of colours depends on a number of factors of which the most important have been outlined in this article. My advice would be to try to obtain an experienced eye by inspecting as many maps as possible. Sources for gaining experience are your local antiquarian book store, specialised map fairs and of course specialised dealers. Consulting map collections at the major national libraries and/or specialised museums is, regretfully, getting increasingly difficult due to security measures.

© Dieter Duncker

Reference :
  • Collecting Old Maps by F.J. Manasek
  • Wikipedia.org