Once your first step is completed you can use a rounded acrylic burr or a #6 round burr to hollow out the internal of the shell. Upon completion, the provisional should be very thin once the internal is thinned out – which is exactly why this technique is called an “eggshell” provisional. The palatal and buccal walls of the provisionals usually measure around .5mm thick. - See more at: http://www.speareducation.com/spear-review/2013/04/eggshell-provisionals-full-arch-cases-part-ii/#.U3cFjfl_ttw