Just as it is with laundry, drying often follows washing and is used to rid the solvent of any water that it absorbed. All solvents absorb some water although you can't see it. It is a good idea to rid the solvent/product of any water, especially before distillation,, because it's one less thing to worry about and because there is an off-chance that water may form a slight amount of byproduct when heated with product oil under distillation conditions.
All one has to do to dry something is to take a sheet of filter paper or paper towel, fold it into a cone and place it into a funnel. About a shot glass full of sodium sulfate (Na2S04) or magnesium sulfate is dumped into the filter and the solvent poured through it. It is also a good idea to follow up by rinsing the Na2S04 with a little extra, clean solvent to insure that everything gets washed out of the drying agent. The Na2S04 is a white crystalline salt that tastes like table salt. It binds water molecules without readily dissolving and leaves the solvent water free. This way of drying is also an option for making some solvents and pure liquids anhydrous (wai ter free, bubba!) for some of the recipes in this book that call for such things.
Drying isn't absolutely necessary, but it is a good bet that if a recipe stresses its use then an evil underground chemist will do so.
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