Article: Use ½ or 1 cup of the trisodium phosphate (TSP). Put the TSP into a gallon of warm water. This is a very strong chemical so be sure that you have all of your protection on when handing the TSP. Dip a scrub brush into the mixture. Begin to scrub the fireplace. You may have to do some tough scrubbing to remove all the buildup. Make sure to get into the hard-to-reach places and nooks and crannies. If the tough spots aren't coming off, then you can make a paste out of a small amount of water and TSP. Apply the paste directly to the spot. Scrub until the spot begins to lift. Dip a clean sponge into water. Rinse every part of the fireplace that you used the TSP on. Allow time to dry.

What is a summary?
Mix the trisodium phosphate with water. Use a scrub brush. Make a paste for tough areas. Rinse with water.