Best acid to clean up fireplace brick

[Hugo Stiglitz]

Namely soothing, tar, everything really.

[OutInTheShed]

Acid is good for removing chalky stuff, cement etc.
For tarry stuff or soot, you probably want a detergent or solvent.

Or even a strong alkali?

[Astacus]

Soot comes off quite well with a simple wire brush on a drill, without damaging the brick

[Hugo Stiglitz]

Astacus said:

Soot comes off quite well with a simple wire brush on a drill, without damaging the brick

Tried it... Nada.

[dhutch]

Very little that will eat away carbon black.

So you are on to mechanically removing it, with or without detergent/surfactant to assist.