d2dd4b77___Stamp_Collecting_Accessories__Description
[Accessory] Watermark Detector (Tray and Fluid) [Price Range] $10 [Description] 
Identifying watermarks is important for collectors as again, stamps of the same design can vastly vary in price depending on the type of paper and watermark used – specifically, inverted watermarks are highly collectible. In general, watermarks on stamps are largely invisible to the naked eye. For some it helps to place the stamp upside down on a dark background. But for most others, the best bet is to use a watermark fluid. They dry fast and do not leave marks and so are deemed safe. There are more expensive alternatives like optical watermark detectors but most collectors employ watermark fluid because of their reliability.
One Family's Blog: Stamp Collecting Accessories
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