Sometimes knowledge is not meant to be shared.

As Lloyd Brown, in his The Story of Maps, writes: "At the outset, sea charts, by their very nature, were destined to be removed from the academic realm and from general circulation. They were much more than an aid to navigation; they were, in effect, the key to empire, the way to wealth. As such, their development in the early stages was shrouded in mystery, for the way to wealth is seldom shared" (Brown 1949, 121). Might this be the reason that so few portolan charts have survived? In the beginning portolan charts were created for commerce. Though their decoration is often spectacular as art, their business was business.

No doubt thousands of portolan charts were made. But portolan charts were used at sea. They went down with ships; they got wet and were damaged; they became outdated and were discarded. One problem is the material from which they are made: vellum, which is animal skin prepared for writing. Vellum was expensive and was not wasted. Sir Thomas Phillipps, the nineteenth-century collector of all the manuscripts he could get his hands on, found that his competition included "goldbeaters, glue makers and tailors, all of whom derived some advantage from the destruction of vellum manuscripts" (Campbell 1987, 173). Bookbinders used vellum in binding new books. The combination of reuse, damage, the discarding of outdated portolan charts, and the utilization of vellum in other ways help to explain their rarity.

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