Place plastic sheets on the floor under the fireplace. Tape a strip of plastic sheeting to the walls along the fireplace.
Put on a respirator when handling and mixing dry, powdered ingredients. Mix 5 parts lime to 1 part salt by weight in a large bucket. Start out mixing small amounts. You can always mix more as you need it.
Add water and stir the mixture. Add small amounts of water until the mixture is the consistency of pancake batter. Mix in charcoal dust if you want a duller-looking whitewash. Let the whitewash sit overnight.
Wipe the fireplace with a damp sponge. The lime in whitewash penetrates more deeply if the stone or brick is slightly damp.
Brush the whitewash onto the fireplace. Vary the amount of whitewash applied to each stone or brick to produce a varied effect.
Let the lime in the whitewash begin to bond with the surface of the fireplace. Remove additional whitewash with a damp rag to increase the rustic look. The lime will continue to soak in over the next few days. Wipe off more whitewash if desired.