The United States 1c Z grill is the rarest of all US stamps, with only two known to exist. On October 8, 1998, the stamp set a world record for a single U.S. stamp, realizing $935,000. During the 1860s, the postal authorities became concerned about postage stamp reuse. Although there is little evidence that this actually occurred much, many post offices had never received any cancelling devices, and improvised by scribbling on the stamp with an ink pen ("pen cancellation"), or whittling designs in pieces of cork, sometimes very creatively ("fancy cancels"), and poor-quality ink could be washed from the stamp. A number of inventors patented various ideas to solve the imagined problem. The Post Office eventually adopted the grill, a device consisting of a pattern of tiny pyramidal bumps that would emboss the stamp, breaking up the fibers so that the ink would soak in more deeply, and thus be harder to clean off. While the patent survives (No. 70,147), much of the actual process of grilling was not well-documented, and there has been considerable research trying to recreate what happened and when. Study of the stamps shows that there were ten types in use, distinguished by size and shape (philatelists have labelled them with letters A-I and Z), and that the practice started some time in 1867 and was abandoned around 1871. A number of grilled stamps are among the great rarities of US philately. (http://encyclopedia.thefreedictionary.com/US%20postage%20stamps) (http://tinyurl.com/ywp29)

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