a brick and stone cleaner would be a good place to start.
What exactly are you trying to clean off & are these "glazed" or??? Besides what @KMS Woodworks mentioned I have seen some good results with steam & also dry ice blasting though I would check first with the person doing it if it is a good idea in your case
One of the best things I have seen for soot is scrubbing bubbles and a stiff brush. Put down plenty of old rags to soak up the mess! No bad fumes with these and the bubbles do most of the work.
Go to Wal-Mart, buy the Great Value oven cleaner. Get a few cans its very cheap. Just spray it on and let it set. You may need to use a brush to get into the tiny holes of the brick. It will remove the black that gets on the brick around a fireplace. I have also used oven cleaner on my backdoor steps where the kids get mud on the steps and it builds up. Works great and cheap.
You did not say if the bricks were painted or raw. You need to be careful when using chemicals and some cleaners to remove any soot stains. To much can cause the soot to bleed into the bricks making it even harder to remove. Also chemicals can be dangerous to use. Fume build up can get you quite sick. I would suggest going the route that Kevin stated a brick and stone cleaner.
Do not expect to get the bricks really clean. Being porous as they are the soot can easily get into the brick and not come out.
I would suggest to take photos of the front of the fireplace before you clean. Then show them to a professional chimney sweep if you clean the fireplace before they come by. A properly operating fireplace should not soot the outside of the brick if its properly constructed and sized. Fixes if there are issues can be as simple as a glass door or panel front to decrease the overall opening in the front, to extending the fireplace chimney higher to generate additional draw to pull the soot spillage into the fireplace instead of allowing it to spill out.
easy off oven cleaner works like magic for me :)
Thanks to ALL that replied to my question. There are some very good suggestions for cleaning my fireplace....:)
I would use dawn power disolver. I would spray it on and then use brush to scrub. Then rinse with vingar and lemon.
Our chimney sweep told us to use ammonia in a spray bottle. Absolute magic!
I think I'll try the Vinegar and see how that works, Thanks
Lemon ammonia and water. Better than anything else. Vinegar is not potent enough to cut the black smut. The ammonia does all the work!