How Were Copies Made?
Some possible methods for copying portolan charts are identified: one prescribes using oiled tracing paper and "smoked carbon paper."
Another method involves using a sheet of paper over the portolan chart to be copied and making small pinpricks on the coasts. Then the paper would be placed over the new vellum skin and a fine flour spread over the paper and pushed into the pinpricks in it, making a pattern on the new vellum for the cartographer to follow. One advantage of this method would be that the paper model could be used several times.
Still another method would be to place the model portolan chart underneath the new vellum on which the copy was to be made. Then, with a light source underneath, the outlines for the second copy could be made by copying directly onto the new vellum (Campbell 1987, 391).
The problem with any of these methods of copying is that so far no two portolan charts with precisely the same coastlines have been located, to the present, which would have resulted from any of the processes! Since few portolan charts have survived, and especially in one location, it is difficult to know how to make a good analysis of the way they were made. It should be remembered that, since the production of portolan charts was a business, any process that could make it possible for artisans rather than artists to copy maps would have been welcome.