In a bowl whisk together flour, baking soda, salt, and ground spices. In another bowl with an electric mixer beat together butter and sugar until light and fluffy. Add molasses, egg, flour mixture, crystallized ginger, and zest and beat until a dough forms.
On a sheet of wax paper pat dough into an 8-by-5 inch rectangle, about 1 inch thick. Chill dough, wrapped in wax paper, 4 hours, or until firm.
(After the dough has been formed into a rectangle it can be kept in the refrigerator for one week or the freezer for one month. Thaw the frozen dough in the refrigerator until sliceable, about four hours. The baked cookies keep in airtight containers at room temperature for four days.)
Preheat oven to 350° F. and lightly butter 2 baking sheets.
Cut dough crosswise into slices about 1/6 inch thick and arrange about 2 inches apart on baking sheets. Bake bars in batches in middle of oven until deep golden brown, 8 to 10 minutes, and transfer to racks to cool. The baked cookies keep in airtight containers at room temperature for four days.
