Pound good birch coals into small pieces in a mortar and sieve the pieces. For 5 lbs butter, use 2 teacups of the best pieces without ashes. Sprinkle them into butter that has been melted, but not boiled, on top of the stove. Set aside for a full 24 hours in a place warm enough so that the butter and coals will remain as thick as sour cream without congealing. The next day melt the mixture on top of the stove, but do not let it boil. Strain while still hot. By this method, the butter will lose its bitterness.