Why does bathroom silicone turn black?

Mould spores are everywhere indoors. They grow where moisture doesn’t dry out and there is something to feed on. The silicone isn’t the food; the soap scum on top of it is. The US EPA’s advice on mould comes down to moisture: dry wet areas quickly and fix leaks.

Corners where water pools, beads tooled into a hollow that holds water, and bathrooms without an exhaust fan go black first.

Look closely in good light. Marks on the surface of a bead that is still firmly stuck are surface mould, and you can clean them. Black inside clear silicone, behind it, or following an edge that has lifted means water is getting under the bead. Cleaning won’t reach it. The bead has to come out.

Run a fingernail along the edge. If it lifts, peels, or water squeezes out, replace it.

The EPA recommends scrubbing mould off hard surfaces with detergent and water, then drying them completely. Use a soft brush rather than a scourer, which roughens the surface so dirt sticks faster. Never mix cleaning products: bleach mixed with acid or ammonia cleaners gives off toxic gas.

Black that has soaked into the silicone usually won’t come out. If it bothers you, replace the bead.

Cut out all the old silicone and scrape off every trace, because new silicone won’t grip an old silicone film. If the joint was wet behind, leave it to dry for at least a day with the fan running. Our step-by-step replacement guide covers the details.

Reseal with Pereseal AF. It is a neutral-cure silicone with added fungicide, made for baths, showers, sinks and kitchen worktops. It is tack-free in about 15 minutes, but cures from the outside in, about 10 mm deep in 7 days. Keep the shower off for as long as you can, and overnight at the very least.

AF slows mould down. It won’t stop it on a joint that never dries. Run the exhaust fan, squeegee the tiles after a shower and wipe the joint down regularly.

If a new bead goes black or lifts within weeks, look for a leak: a dripping mixer, cracked grout, failed waterproofing under the tiles, or water tracking behind the shower screen. Stains spreading on the wall next door or on the ceiling below, or loose tiles, point the same way. That’s a job for a plumber or waterproofing contractor, not more sealant.