Roller
From PyroGuide
Wooden rollers are used to prepare the materials before mixing (particularly to crush lumps of oxidizers but also to pulverize chemicals). This is carried out on a hard, non sparking surface e.g. a wooden table with a duralmine plate. Wood should be avoided because the pores become contaminated with chemicals over the time. A polished stone plate may also be used as a surface to grind individual chemicals.
Smaller amounts of material are quickly ground by hand against what milling (with consequent cleaning) would be a waste of time. However some chemicals seem to be unsuitable for hand grinding e.g. potassium nitrate, red gum etc. and are best milled. The rollers should be turned out of a single piece of wood. Versions assembled from multiple pieces should be avoided, because the possible pressure is not sufficient to grind hard materials.
As with other cases a division of tools is compulsory and it is very unwise to use the same roller for both grinding sulphur or sulphides and chlorates.