[LargeFormat] Reciprocacy of Polaroid film (type 54)

[largeformat@f32.net]

There was a programme on TV this saturday about a 'haunted' castle in England. I decided to demonstrate to my kids how easy it is to take a photo of a 'ghost' using double exposure. I took a reading of a suitably gloomy part of the house which indicated an exposure of 15 secs at f16 for 100 asa - I was using a polaroid back with Polapan Pro 100 b&w film. I guessed a reciprocacy of around 4x, so took two shots of 30secs each.

Needless to say, all I got was nothing! What is the sort of factor I should use for this film at longer exposures? For info, the camera used was an Ebony RW45 III.