WATERMARK TRAY AND FLUID
Watermark on stamps are often very difficult to see clearly, so stamp collectors use various methods to make them easier to detect. The most common method is to place the stamp face down in a black tray or dish and pour a few drops of detector fluid onto the stamp, making the watermarked design stan out against the black background. CAUTION: Watermark detection fluids may react unfavourably with some inks and dissolve them, so extreme care should be taken in their use.