What if white sugar gets lumpy?

Granulated white sugar will harden if it’s exposed to moisture or high humidity, causing the surface to dry. To prevent lumps, always keep the sugar in an airtight container. To bring it back to its original texture, break the hardened pieces with a meat tenderizing hammer or heavy mixing spoon.  If necessary, use a food processor or blender to blend the pieces until smooth.  Sift it to remove lumps. If confectioner’s or icing sugar becomes lumpy, roll it with a rolling pin, and then sift it before it is measured.
 
Freeze for prolonged storage; it will thaw rapidly.  
 
Sources: 
Pope School Cookbook by Antoinette and Francois Pope, 1958.
The Sugar Association