My assumption that the basement document safe had an effective door seal turned out to be wrong, so I replaced the bagged desiccant with a tray of granules, sealed the door with masking tape, and tried again:
The jagged black curve shows the Basement Laboratory temperature trending toward the usual mid-50s winter level. The dead-flat horizontal blue line at 15% RH shows the tray of desiccant can keep up with whatever air leakage might occur around the tape and through the floor bolts.
I cannot find the table (that I once had and know exists somewhere) which lists various desiccants and their terminal humidity levels in a sealed container. I’m pretty sure the low humidity means it’s one of the clay-based desiccants, not silica gel.