(Received 18 January 1985; accepted 16 June 1986)
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The relation between the depth of the stamp mark and the depth to which the stamp mark can be restored has been determined using both etching (Fry's reagent) and magnetic techniques. The depths of restoration increased with increasing depth of stamp mark. Increasing the stamp size also resulted in an increase in the depth of restoration. For new stamp marks, the magnetic technique was more sensitive, giving, in some cases, depths of restoration about 10% greater than the maximum depths determined by etching. For old stamp marks, the sensitivity of the magnetic technique was considerably reduced, whereas the etching technique was unaffected and therefore was the more sensitive. The etching technique could also often restore stamp marks which had been obliterated by overpunching, whereas the magnetic technique could not. A recommended procedure for the restoration fo stamp marks is given.
Senior research scientist, Materials Research Laboratories, Defense Science and Technology Organization, Ascot Vale, Victoria
Stock #: JFS12368J