The dirt that comes out dried hard onto crockery is just tiny bits of food debris that have not properly drained off.
Weetabix and mashed potato are good culprits - best to rinse off as much food as possible before loading dishes into the machine especially for heavy loads - best to load to give good water flow around crocks - very important to use good quality rinse aid.
I have discovered that rinse aid is not to make crockery shiny but to break down surface tension in the water to allow final draining to take away any debris before it has the chance to become cement like during the drying cycle. Droplets of water with food debris in them dry in the steam heat leaving the debris baked on.
I am following this regime (for the past week) and our dishwasher is much improved. No more crud inside our glassware.
Peccavi
March 2009