Black mould on grout and silicone

Black specks along the edge of a bath or in the grout lines of a shower are the most common complaint about any bathroom, and the most commonly mistreated. Cleaning them is treating a symptom. They return because the conditions that grew them have not changed.

Grout and silicone are different problems

Grout is porous. Mould grows into it, below the surface, so a cleaner that bleaches the colour away has not removed the growth. It looks fixed for a few weeks and then reappears in exactly the same lines.

Silicone is not porous, so mould on silicone is usually growing on the surface, in a film of soap and water. That genuinely does clean off. If the black is beneath the surface, or shows as dark staining within the rubber itself, the silicone has failed and cleaning will not bring it back.

What actually cleans each one

For surface growth on silicone and tile faces, a mould-specific cleaner left to dwell for the time stated on the bottle does the job. Dwell time matters more than scrubbing: these products work chemically, not mechanically.

For grout, cleaning is at best partial. A stiff narrow brush and a dedicated grout cleaner will lift what is near the surface. Where staining is deep, the realistic options are raking out the top few millimetres and regrouting, or accepting the appearance.

Never mix cleaning products, particularly anything containing bleach with anything containing acid. The combination produces a genuinely dangerous gas in exactly the sort of small unventilated room where people do this.

Replace silicone rather than cleaning it twice

If a seal has gone black beneath the surface, it is finished. Cutting it out and laying a new bead takes an hour or two and gives a result that cleaning never will. Use a sanitary grade silicone with a mould inhibitor, remove every trace of the old bead first, and let it cure fully before the room is used.

Then deal with why it grew

Mould needs moisture sitting on a surface. Remove that and it stops coming back. The usual causes, in order of how often they turn out to be the answer:

Ventilation that is absent, too weak, or switched off with the light rather than running on afterwards.
A shower screen or curtain that holds water against a seal instead of letting it drain.
Towels or bath mats left damp against a cold wall.
A cold external wall where warm wet air condenses, which is a condensation problem rather than a bathroom one.

Wiping a shower down after use sounds fussy and does more to prevent mould than any product sold for removing it.

When it is not a surface problem