A grill on a postage stamp is an embossed pattern of small indentations intended to discourage postage stamp reuse. Used in the United States in the 1860s and 1870s, they were designed to allow the ink of the cancellation to be absorbed more readily by the fibres of the stamp paper, making it harder to wash off the cancellation. In the United States The best-known (indeed only major) examples of grilling are the United States issues of the late 1860s and early 1870s, when grilling was standard for all US stamps. "Grilling" therefore remains a specialist interest only for American philatelists. While many types of grilled stamps are common, certain of the grill patterns were little-used, and define some of the great rarities of philately. In particular, the 1-cent stamp with the "Z" grill is generally cited as the rarest of all US stamps (only two copies are known), and is commonly known simply as the Z Grill. A recently discovered grill issue, the 30-cent "I" grill stamp, may be even rarer, for only one example has thus far been identified. The idea of grilling was first suggested by Charles F. Steel, a supervisor at the National Bank Note Company, with correspondence on the subject dating from 1865. The purpose of grilling stamps was to break the paper fiber of the stamp. This would cause the ink cancellation mark to soak into the paper fiber making them more difficult to wash and reuse as postage. The first type of grill to be tried, termed by philatelists the "A" grill, was applied to the entire stamp. Stamps so treated were distributed to post offices for testing in August 1867, and apparently satisfactory, as National received a contract stipulating use of grilling for all stamps. However, the actual practice of grilling on a large scale had not been completely worked out, and the process significantly weakened sheets, resulting in tearing during perforation and general production handling. National soon switched to the use of a small rectangular pattern of indentations, and subsequent grills were all of this form. National's contract did not specify the type of grill pattern, and the details changed as they experimented with equipment. Many of the details have been lost to history; in 1910s, philatelist William L. Stevenson introduced a system of distinguishing types of grills, and identifying them by letter. Later research clarified some of the details of