In the bowl of a food processor, mix the flour, sugar, baking powder, and baking soda. Add the butter and combine, pulsing on and off, until the mixture resembles coarse meal. Transfer to a large bowl, add the currants, buttermilk, and orange zest, and mix with a fork until just combined. Turn out the dough onto a floured surface and knead 8 to 10 times. Shape it into a 10-inch circle about ¾ inch thick. Cut into 10 to 12 wedges and place on a greased baking sheet. Brush the tops with heavy cream or half-and-half and bake in a preheated 425 degree oven for 15 to 20 minutes.