Traces of iodine and bromine in the box will erase part of the latent image during the mercury developing procedure.
Nineteenth Century Daguerreotypists sometimes used this procedure to correct for overexposure of a plate.
You really don’t need a box for the iodine and bromine fuming if your boxes are well sealed with a tight fitting ground glass lid. However if you feel you must use the box for fuming, remove the fuming boxes before you put your mercurizer in the box.