for all the things you do...
I've been collecting stamps since I was knee high, but I still cringe when it's time to break out the watermark gear. Confirming the presence of a watermark is often a challenge even for the most experienced stamp collector. Even more challenging can be those times when we want to confirm there is no watermark to be found. However, since a watermark is sometimes the only feature to distinguish a common stamp from a rare one, watermark detection is an important skill for all but the most casual of philatelists.
Watermarks are made during the paper manufacture process by intentionally impressing an image into the still-wet paper. Where the image is impressed, the paper is thinner and the image can be seen by holding the paper up to a light (or setting it against a dark background). Over time, postal agencies throughout the world have used watermarked papers as a security measure against counterfeiting. For some countries the period of watermark usage is relatively brief. The United States, for example, only used watermarked paper from 1895 until 1916. For other countries, such as Great Britain, watermarked paper has been a factor during their entire postal history.
Frequently over the lifetime of a stamp issue, there are changes in the paper supply. It is common for stamps that look similar to exist in watermarked and un-watermarked varieties. There are also many times when stamps that look similar can be found with one of several watermark patterns. It is nearly universal among collectors to treat these varieties as "different" stamps. All of the major stamp catalogs throughout the world will assign these varieties unique catalog numbers.
Not all watermarks are difficult to see. In fact some, such as the various wavy line and web pattern watermarks on many German issues, are visible with no special equipment at all. The remaining watermarks fall somewhere in a continuum from tricky to downright aggravating. The "single-line" watermarks used for United States stamps between 1910 and 1916 can be extremely difficult to find. This watermark pattern consists of the letters USPS repeated across the sheet and many stamps have only a small portion of one or two of the letters. A partial U or P might appear as just a thin line towards the edge of a stamp.
Watermarks are best viewed from the back of the stamp. The basic method of watermark detection is to place the stamp face down in a specially manufactured black plastic tray, and place a few drops of "watermark fluid" on the stamp. The fluid evaporates very quickly and because it doesn't contain any water, it does not harm either used or mint stamps. As the fluid evaporates there is a moment just before the stamp dries when the watermark is most visible. This magic moment is key to watermark hunting and veteran collectors know to watch for it.
Modern watermark fluid has been specially formulated to be safe for both your health and the environment. Unfortunately it is not the best formulation for viewing watermarks. The modern formulation evaporates too quickly and the magic moment described above is often frustratingly short. A little secret among stamp collectors is to use Ronsonol brand lighter fluid. It evaporates more slowly and will reveal watermarks that the commercial fluids miss.
There are a few mechanical devices on the market that can help the most serious in their watermarking pursuits. Morley Bright makes two very similar products: the INST-A-TECTOR and the ROLL-A-TECTOR. The systems use a small, clear sachet (a plastic pouch) filled with blue ink. When the stamp is placed under the sachet and pressure is applied, the ink settles into the watermarked area which is very slightly thinner than the rest of the stamp. The result is the watermark impressed on the ink sachet much like an old gravestone rubbing or similar etch. I'll confess that I was a little surprised to find that these toys actually work! A major weakness in the Morley Bright system is the relative short life of the ink sachet which will become dirty and brittle and must replaced periodically.
The other machine commonly sold is the Safe Signoscope which comes in two sizes, a larger size that is plugged into household electricity and a smaller, portable unit that runs on batteries. The system consists of a clear plastic block hinged to a metal plate on which the stamp is placed. Pressure and lighting are then applied and the watermark is revealed. Signoscopes are not cheap and cost is certainly a deterrent to many in using this system. Other complaints are the need for electricity in one form or another and the occasional bulb replacement.
It should be noted, that neither the Morley Bright nor the Signoscope are as effective a method for finding watermarks as is good fluid in skilled hands. However, there are compelling reasons for their use. First, even the modern "safe" fluid has an unpleasant odor, is awkward to work with, and may not be all that safe anyway. If you can find 90% of the watermarks you seek using a Morley Bright, then you can spare yourself the hassle of fluids 90% of the time. Second, unlike fluid which has a magic moment and then the watermark image is gone, these devices allow you to freeze the image temporarily for study. This capability is especially handy when trying to distinguish among watermarks which are very similar to one another watermarks of British Commonwealth and certain Latin American countries come immediately to mind.
One final method that is worth a mention is the use of a computer scanner and graphics software to capture watermark images. The basic approach here is to scan the reverse of the stamp against a black background and then use software to "tease" out the watermark by manipulating the contrast, and color saturation. I have not had any success with this method and find that I'm really only able to scan watermarks that are visible without enhancement. A number of philatelists are working on techniques in this area, and although some people claim success, the scans that I have seen are not all that remarkable. One problem with improving the method is the fact that with so many different scanner and software combinations available, it is difficult for people to share results and learn from one another. Perhaps in the future we'll see special software optimized for watermark study.
Hopefully I've provided some information here which will help you in identifying and studying your stamps. Over time you'll find that the right tools combined with practice and experimentation will build both your skills and your confidence; your inventory of properly identified stamps will grow and you pile of "unknowns" will shrink and one day disappear altogether.