No, the magnetite does not have to be melted. It just has to be heated above its Currie Point (where the magnetic signal gets wiped out) then cooled in the presence of a magnetic field. The trickster would take clay with a high hematite and / or magnetite content, mold it, fire it in a kiln, then cool it in a strong magnetic field. That could be a strong magnet or inside an electric coil. It'd be easily done. If they were really smart they'd change the field direction as the temperature decreases, that'd put a different signal into the different magnetic minerals. Or, as I mentionioned before, put a rare magnetic mineral into the clay, somethin' exotic enough to claim an extraterrestrial orgin.
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