Recently I had a house fire and the smoke that vented from the soffit and siding ended up on the brick.  The fireplace brick was stained from the efforts of the fire department to put out the fire that brought down dirty sooty water down on to the hearth and surrounding brick.

What is one to do and not do when attempting to clean brick?

The DON'Ts of cleaning brick:

1) Don't use a wire brush.  You will remove the finish of the brick and you will be left with the core of the brick exposed.  This typically doesn't match.
2) Don't use sand paper for the same reason.
3) Don't breathe the fumes

The DO's of cleaning brick:
1) Work your way through the cleaning solutions.  I tried several different things. 
    A) Water: the universal solvent.  It should remove some of the dirt and grime but it probably won't get it clean.
    B) Soap.  This should remove more and any oily residues as well.
    C) Goof off or some kind of heavy duty cleanser.
    D) Muriatic Acid.  If this doesn't get it off, it's not coming off.
2) Use a hard bristle brush.  I used a deck brush head (minus the pole) and it worked well.
3) Wear safety goggles
4) Wear gloves
5) Have a bucket of water nearby
6) Have a couple of rags nearby
7) Have some baking soda nearby
8) Ventilate the area

I jumped directly from water to muriatic acid because it was cheap and I knew that it would work.  I do caution you, however, that this stuff is very caustic.  Just opening the bottle, you could see whisps of the acid coming out.  This is 30% Hydrochloric acid from Home Depot.  You have to buy it in two gallon increments (comes in 2 one gallon bottles) and it costs about $10.

First you need to make a concentration that won't eat straight through your hand or the brick.  If you remember back to junior high science class, you never pour water into acid, only acid into water.  If you do, the acid will attempt to boil and if it does, it will spray out and most likely get all over you.  I bought a 1/2 quart spray bottle and used it to apply the acid to the brick.  The acid I purchased suggested using 1 pint of acid to 1 gallon of water for concrete/mortar cleaning.

Realize that the reaction between mortar and muriatic acid generates gases.  The lime in concrete and mortar will react with the acid giving off hydrogen and in some cases chlorine gas.  Chlorine gas was used as a poison gas in World War I to suffocate the enemy.  Take precautions to avoid breathing it. 
Once you've got all your stuff together and your safety measures are in place, you can begin to spray some of the brick.  Start in an inconspicuous spot, spray it and test to make sure that the brick itself is not reacting with the acid.  Once you've determined it's safe to use on your brick, spray a small area and allow the acid to sit for a couple minutes then take your hard bristle brush and scrub using very small motions with the brush directly over the area to be cleaned.  I moved the brush about 1/4 of an inch to reduce the splashing of the acid caused by the bristles.  Rinse and repeat as necessary.

As a word of caution, although I did the work indoors, it is not recommended to use muriatic acid indoors.  If you do, take even more care to have safety measures in place.

Good luck and be safe.

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