How to Buy a Watermark Tray
The purpose of a watermark tray is to help identify the watermark on a stamp. Holding the stamp up to the light or placing it face down on a dark surface can detect some watermarks. Others, printed on colored paper, cannot be identified that easily and a watermark tray is necessary.
The normal way is to place the stamp face down in the tray and add a small drop of watermark fluid into the tray. As the fluid penetrates the paper, the watermark will come into view. This happens because the thinner paper where the watermark is located has a faster penetration than does the normal paper.
If that moment is passed over, or if you want an additional look at the watermark, then the stamp should be allowed to dry and the process repeated.
Black glass trays work best. These can be special trays sold by stamp stores for this purpose. Or a black saucer works fine. Orange and yellow stamps may require a blue background tray for easier viewing. Some collectors have been known to use jar tops as watermark trays.
Sometimes, the watermark fluid will dissolve various additives in the stamps and deposit this on the bottom of the tray. This can also come from remnants of hinges. Clean the trays frequently and wait until they are absolutely dry and clean before using them again.