Until about 1980, philately was one of the most toxic of hobbies. Collectors used watermark fluid in great quantities. Novice and intermediate collectors used it to determine watermarks, but advanced collectors and stamp professionals used it to determine quality of stamps and whether stamps were repaired. Watermark fluid works as a wetting agent which shows comparative thickness of the paper against the black background of the watermark tray. A good philatelist can see an enormous amount in a watermark tray, similar to what a good radiologist can see on an X-ray. To make it work, the wetting agent needs to soak the stamp and allow good visibility of the paper's varying thickness. With experience, you can determine creases, thins, and even the most sophisticated of repairs. Wetting agents need to be quick drying and not damage gum and paper, so, obviously, water can't be used. The earliest wetting agents that were used were carbon tetrachloride and later benzene. Both have been known for over fifty years to be highly carcinogenic and implicated in much higher rates of Parkinson's disease. Oddly, even though old time stamp dealers such as my grandfather and many of his generation used huge quantities of these substances over their lifetimes, they lived to ripe old ages and died largely of non-cancer causes. Today we use lighter fluid and other non-cancer causing agents.