Cleaning brick looks like the least risky thing you can do to a wall, which is exactly why so much brick gets damaged doing it. The brick face is hard. The mortar between it is deliberately softer, and on an older wall it is much softer. Every cleaning decision is really a decision about what that mortar can survive.
The mortar is the soft part, and that is the whole problem
A masonry wall is designed so that the joints are the sacrificial element. They take up movement, and when something has to give, they are meant to go first because repointing a joint is far cheaper than replacing a spalled brick. That design intent works against you during cleaning: pressure that a modern brick face shrugs off will erode a lime joint noticeably in a single pass, and once the joint is scoured back the wall starts taking water where it never did before. Damage from cleaning shows up as a leak the following winter, not on the day.
Start with the mildest thing that works
The sequence that avoids nearly all of the trouble is dull: water, time, and a soft brush first. Soaking the wall and letting the dirt release before touching it does most of the work on ordinary soiling. Move up only if that fails, to a mild detergent, then to a proprietary masonry cleaner chosen for the specific stain. Test everything on a small area out of sight and give it a week before judging, because some cleaners look fine wet and leave a bloom once the wall dries.
Identify the stain before you buy a chemical
Different stains have opposite fixes, and using the wrong product often sets the stain permanently. White powdery deposits are usually efflorescence, salt carried out by water, and they respond to dry brushing plus fixing whatever is letting the water in. Green or black film is biological growth and wants a biocide with dwell time, not scrubbing. Orange-brown streaks are usually iron, which acid will make worse rather than better. Paint and coatings need a poultice or a stripper made for masonry. Guessing here is what turns a dirty wall into a stained one.
Where a pressure washer goes wrong
The tool is not the problem, the settings are. A narrow tip held close concentrates enough energy to erode mortar, drive water deep into the wall, and take the fired skin off the brick face, and that skin is the part that resists weather. Once it is gone the brick starts absorbing water and spalling. If a washer is used at all, it should be a wide fan at low pressure, kept moving, held well back, and never aimed straight into a joint. On a wall with open or eroded joints, forcing water in also means forcing it out somewhere inside. A useful discipline is to set the machine at the lowest pressure that achieves anything and then never increase it, because the temptation to turn it up arrives exactly when the wall is at its most vulnerable. If low pressure is not working, the answer is a different cleaner and more dwell time, not more force.
Older brick needs a different rulebook
Hand-made brick from the nineteenth century and earlier is softer, more porous, and far less tolerant than anything produced today, and the lime mortar around it dissolves in acid. On those walls, acidic cleaners, abrasive blasting, and pressure washing are all capable of causing damage that cannot be undone, and blasting in particular removes the fire skin permanently. Where a property sits in a historic district, cleaning method may be regulated as well. The conservative approach on old brick is water, patience, and accepting that some staining is age rather than dirt.
Frequently asked questions
Is acid safe on brick? Only in specific circumstances, diluted, on pre-wet masonry, with a thorough rinse, and never on lime mortar or old brick. Acid attacks the cement matrix and can leave a permanent bloom or discoloration. On anything historic, treat it as unsuitable.
Does the wall need wetting first? Yes, for any chemical cleaning. Saturating the masonry keeps the cleaner in the surface where the dirt is rather than letting it soak deep into dry brick, and it makes rinsing far more effective.
Should cleaning happen before or after repointing? After. Cleaning a wall with open joints drives water into the core, and new pointing will be marked by whatever cleaning follows it. Repoint first, let the mortar cure, then clean gently.