Watermarks are thin spots in the paper that some stamps are printed on. These thin places are pressed into one side of the paper during its manufacture. Since the supply of watermarked paper is usually tightly controlled, the watermark paper becomes a security device that helps impede the production of counterfeit stamps.

Identifying watermarks is a way to identify different printings of stamps. Just as different perforations define different stamps, so do different watermarks define different stamps. Not all collectors worry about watermark and perforation differences. Many topical collectors just collect face different stamps. However, for the majority of stamp collectors, watermarks and their differences are important.

The easiest way to identify watermarks is to hold a stamp up to a bright light source with the printed side of the stamp toward the light source. With a little practice, many of the common and clear watermarks are apparent enough to allow their identification.

For the vast majority of watermarks you will need more than a bright light. It is here that you will need a couple of tools of the trade. For the beginner, a watermark tray available from most stamp supply sources and a can of lighter fluid available from your local drug store will get you through most of the watermark work that you will ever have to do. The lighter fluid comes in a neat can with a drip tip that precludes spills. For safety reasons, don't pry the cap off. You simply place your stamp face down in the tray and add a couple of drops of lighter fluid to the stamp. The fluid "wets" the stamp and makes the watermark easier to see. It will also show up many forms of damage such as creases and thin spots. Practice first with stamps where you can see a watermark with a bright light and then try harder ones. Make sure that the stamp might have a watermark by looking it up in a catalog. This way you don't waste fluid. Always do watermarking with adequate ventilation. Lighter fluid contains naptha which is very flammable. In addition, naptha is a solvent and all solvents are to some extent toxic if you breathe enough of them. The key to safe use is ventilation and using the minimal amount of watermark fluid possible. One or two stamps in your stamp room is probably OK; but when I have to do a lot of stamps, I wait for a warm day and do them outside.

On some stamps it's harder to find the watermarks than on others. The newer issues of the British Commonwealth are sometimes impossible to find in a tray with solvent. Stamps printed in yellow, such as the 10-cent Washingtons from the Washington-Franklin series, are very hard to detect. In some of these cases, the expensive electronic watermark detectors are of help. Personally, in these few cases, I either set the stamp aside or make a guess. As you practice doing watermarks, try different kinds of lighting. I have found that raising the tray until it is almost level with the light source sometimes helps. The oblique lighting in these cases helps make the surface differences of the paper more visible.

A little chemistry lesson. If you stay with the hobby, you will hear all kinds of discussions about watermark fluids. Here is a little information to help you make your choices. There is a commercial watermark fluid out in the world that many dealers carry. Its big advantage over anything else is that it is not flammable. It contains CFC-113 which is also known as Freon 113. The chemical name for this solvent is trichlorotrifloroethane. Its drawbacks are that it is a Freon, and by its nature it will be expensive. It is also related to the Freons that have recently been banned. If you believe the stuff about the ozone hole and worry about Freons, then you should also worry about this one. I personally don't believe in the ozone hole so I would use this product except that I am cheap. You can get almost four times as much lighter fluid for the same money, so in my mind there is no choice. If you smoke while you do stamps, then you might consider the Freon product. One other point: I have read comments about using watermark fluids under the kitchen hood so that you don't breathe anymore of it than is necessary. This does not really afford you much additional protection because both these solvents are heavier than air and will flow along the surface of the stove and out into the room. The hood air velocities are just too low to contain the fumes. In addition, if you use the lighter fluid and spill it, the hood fan motors are not explosion proof. Not a likely scenario but a possible one.

You will eventually hear some old timer talk about the old days and Carbona. Carbona was a dry cleaning product that contained carbontetrachloride. Carbon tet in large quantities or in low exposure over long periods of time is liver toxic. You cannot get it anymore unless you are a chemist. It was a nice solvent because it was not flammable. Carbona was replaced as a watermark fluid for a number of years by benzene. You can't get benzene anymore because it can cause leukemia after long periods of exposure. (You get exposed to benzene in small quantities every time you pump gasoline unless it has been removed from that recently.) In discussions about watermark fluids, the term benzine will come up. It is always confused with the benzene mentioned above but it is not the same thing. Benzine is a European term for what is called petroleum ether in this country. This is a great watermark fluid although it is somewhat more flammable than gasoline. It is also not good for your health if you breathe a lot of it over a long time. What this is leading to is that all solvents are toxic if you breathe enough of them. Use ventilation if you do more than a couple of stamps. It is your health that you will wreck if you are stupid.