A new technique is revealing traces of lost languages that have been erased from ancient parchments.For centuries they have gathered dust on the shelves of a library marooned in a rocky patch of Egyptian desert, their secrets lost in time. But now a collection of enigmatic manuscripts, carefully stored behind the walls of a 1,500-year-old monastery on the Sinai Peninsula, are giving up their treasures. The library at Saint Catherine’s Monastery is the oldest continually operating library in the world. Among its thousands of ancient parchments are at least 160 palimpsests—manuscripts that bear faint scratches and flecks of ink beneath...