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In 1979 I visited Egypt, and among other things, I saw about 2/3 of the old Cairo Museum (not the Grand Egyptian Museum that is about to open). One of the exhibits I saw in there was the treasures from the tombs found by Montet at Tanis. Unlike the treasures of Tutankhamun, the Tanis collection, including the silver coffins of Psusennes and Shoshenk, was small enough to fit in one museum gallery of normal size. When I saw the Tanite stuff, my first impression was: “That’s not so great.” I had already seen what Tutankhamun had (what did not go on tour, anyway), and the artwork on the artifacts of Tanis was cruder.
As for why the mummy of Psusennes was not preserved, keep in mind that the royal mummies we have came from the western Egyptian desert, the Valley of the Kings and neighboring areas. As long as flood waters did not get to them, they were kept absolutely dry. By contrast, the Tanis tombs were buried under a temple in the Nile delta, where the humidity is higher.