Until the mid-twentieth century, most world postage stamps were printed using the intaglio method (also commonly called gravure or line engraving). The intaglio process requires that a master engraver carve the stamp design into a block of metal (typically steel) called a die. At various stages in the creation of this die, proof prints are pulled to check the quality of the engraving.
These die proofs are highly sought after by stamp collectors. Because they are pulled one at a time from the original engraving, they often exhibit crisp, deep lines and brilliant colors far superior to those of finished postage stamps. They are usually printed on paper stock that is either thicker and/or whiter than stamp paper, and sometimes they even exist in different colors than the issued stamp. These are trial color proofs which are test printings to determine what color best suits the stamp’s design and level of detail.
Die proofs are on view throughout the philatelic galleries, but the largest grouping of them is in the Ernest R. Ackerman Collection, which can be found in pullout frames 2-15 through 2-37 in the "U.S. and International Stamps Gallery."
Written by Daniel A. Piazza