Safety gear like goggles and gloves is a must.

1. Make sure the bricks are wet with water.
2. Combine 4 cups of water with 1/2 cup of ammonia and 1/4 cup of dish soap in a cleaning bucket or large spray bottle.
3. Apply the cleaner to the bricks.
4. Scrub with a natural bristle brush. ...
5. Rinse well with warm water.

On the exterior of my 170 year old home, a restoration company used a very soft bristle brush and mild dish soap, per recommendations of the historic district.