Grinding dried chiles in a mortar and pestle?
That sort of grinding only works with seeds and such.  Chiles are too fibrous.  I use a cuisinart mini mate chopper/grinder for all my spice milling needs.  There's a dull side to the blade for grinding stuff like peppercorns, and a sharp side for stuff like chiles.  About the only thing my mortar and pestle ever gets used for is grinding kosher salt to a more useful grain size.