Philatelic expertisation is the process whereby an authority is asked to give an opinion whether a philatelic item is genuine and whether it has been repaired or altered in any way. Forging and faking, regumming and reperforating of stamps is common in the philatelic marketplace, and increasingly buyers demand an expert certificate before buying a valuable item. Some items are so often faked or altered that they may be almost unsaleable without a certificate. The process of expertisation Experts may be individuals or committees but they will all follow a similar process to determine whether an item is genuine. The opinions of experts differ and have evolved over time. Conflicting expert opinions, such as on colour shade or whether a stamp has been reperforated, can have a huge effect on a stamp's value. Stamps Experts will attempt to determine whether a questioned stamp is a genuine example of a particular stamp issue; that is, whether it was printed at the same time from the same plate or stone as that stamp issue. Among the factors they may consider are: Is the stamp's design identical to those of genuine ones? Forged stamps almost invariably differ to a greater or lesser degree from genuine ones. Is the size of the stamp correct? Was the stamp printed using the same printing method, e.g., lithography, engraving, etc.? Is the paper identical to the paper used for genuine stamps? Is the thickness, color, and type of paper (laid or wove) correct? Does the stamp contain the correct watermark as do genuine ones? Are the perforations correct, e.g., is the spacing of the perforations correct; are they of the same size, type, and shape as those on the genuine stamp? Is the color of the stamp correct? Is it printed with the right inks? Some inks may react to ultraviolet light, for example. Does the stamp have the correct tagging? Does the stamp have the correct gum? Even if the stamp was printed from the original plates or stone, is it a reprint made later, either officially or unofficially? There may be subtle differences in color, paper or design. Experts also will ask whether the stamp been altered in any fashion: Has the color of the stamp been changed? This sometimes can be done chemically. Have perforations been added or removed to make it appear imperforate or coil? Has the design been changed in any fashion?