Timothy L. Donohue and Helmut F. Frye have posted an excellent paper "Characterization and Correction of Casting Defects" (http://www.platinumguild.com/files/pdf/V5N2W_characterization.pdf).
They show cartoon images of the defects and discuss various causes. Highly recommend reading through the entire document before fixing on one defect as there are many and sometimes the differences are subtle.
