The period for the fermentation of rum is longer than that for the manufacture of whisky, and this is recognized in the regulations, which allow a longer period for fermentation in a distillery surveyed for the manufacture of rum than when surveyed for the manufacture of whisky or alcohol.

While, as is said above, molasses may be considered the base supply for rum, other materials derived from sugar cane are used in many countries in its preparation, wholly or in part. The skimmings which are taken from the boilers of the old open-kettle processes of manufacture are often mixed with the molasses, and there is also found employed in the manufacture of rum the counterpart of the sour mash process of making whisky. In other words, a portion of the residue from the previous distillation, known in some countries as "dunder," is added to the mash. The rum which is produced wholly from refuse molasses is of a very inferior character, and the same term is applied to it in many countries as was applied to the low-grade whisky made in this country, namely, "nigger rum."