Black mould along a line of silicone is stubborn in a way that ordinary bathroom grime is not. You scrub it, it fades, and a fortnight later it is back. That happens because much of the mould is not sitting on top of the sealant so much as living inside it, feeding on soap residue and body oils in a room that never quite dries out. The good news is that plenty of mouldy silicone can be rescued, and the cases that cannot be rescued are usually easy to spot once you know what to look for.

Why silicone goes black in the first place

Silicone is flexible and slightly porous, which is exactly what makes it good at sealing a bath or shower tray and bad at staying clean. Every shower leaves behind a film of soap, shampoo and skin oils, plus minerals from hard water. Mould spores are in the air constantly, and they only need a damp surface and a bit of organic residue to settle in and start growing.

The sealant joint is the perfect home for it. Water collects in the tiny channel where silicone meets tile, and again where it meets the bath. Once mould gets a hold behind the edge of the bead, no amount of surface spraying will reach it. That is why a cheap cleaner can make the line look better for a week and then appear to fail completely.

You will also notice the black comes back fastest in bathrooms with no window, a weak extractor fan, or a shower head that sprays directly onto the sealant. Fix the moisture and the cleaning becomes much less of a chore.

Clean it or replace it? How to tell

Before you buy anything, press the sealant with your thumb and look closely along the whole joint.

• Clean it if the black marks sit on the surface, the silicone is still soft and springy, and the bead is firmly stuck to both the tile and the bath. Surface staining on healthy sealant usually responds well to treatment.
• Replace it if the bead has lifted or peeled away at either edge, feels hard or crumbly, or moves when you press it. Any gap at all means water is getting behind it.
• Replace it if the mould returns within a few weeks of a thorough clean. That is the clearest sign it has grown into the material rather than onto it.
• Look wider if there is mould on the ceiling, on the grout, or in the corner of the room, or if the wall behind feels soft or smells musty. That is a ventilation or damp problem, not a cleaning problem. For anything beyond a small patch, it is worth getting advice from a qualified damp or ventilation specialist before you simply reseal over it.

What you will need

• Rubber gloves, eye protection and an open window or a running extractor fan
• An old toothbrush and a small bowl
• Bicarbonate of soda
• Thin household bleach or a proprietary mould remover that contains it
• Cotton wool pads or cotton buds
• A clean cloth for drying, plus kitchen roll

The basic clean, step by step

1. Open the window and put the extractor fan on. Keep the door shut so children and pets stay out of the room.
2. Wash the area with warm soapy water and a cloth to lift off soap scum. Rinse and dry it properly — cleaners work far better on a dry surface.
3. Mix bicarbonate of soda with a little water to make a paste. Work it along the silicone with the toothbrush, using small circular movements. Do not attack the edges with a knife or scraper.
4. Leave the paste for about ten minutes, then rinse and dry the joint thoroughly.
5. For any remaining surface mould, dab on bleach or mould remover with a cotton bud, following the instructions on the bottle. Leave it for the stated time, then rinse and dry.
6. Dry the whole area with a cloth rather than leaving it to air dry, and run the fan for a while afterwards.

If the black marks have faded but not gone, and you can see the mould is sitting in the surface layer, the next method is worth a try before you reach for the sealant remover.

The soak method for stubborn black staining

This is the trick that rescues most borderline cases. Soak cotton wool pads in thin bleach, squeeze them out slightly so they are damp rather than dripping, and press them along the whole length of the silicone line. Cover them with cling film to stop the bleach evaporating, then leave for four to twelve hours — overnight is ideal.

Keep the room ventilated throughout, shut the door, and wear gloves when you take the pads off. Rinse the joint well with clean water, then dry it. If the line still looks grey or black after that, the pigment has worked its way too deep into the sealant and no product on the shelf will shift it.

A safety note worth repeating: never mix bleach with vinegar, lemon juice or any other acid, and never mix it with ammonia-based cleaners. The fumes produced are genuinely dangerous. Bleach can also discolour coloured silicone and mark natural stone such as marble, so test on a hidden corner first.

Stopping it coming back

Once the sealant is clean, the aim is to keep it dry. Run the extractor fan for ten to fifteen minutes after every shower or bath, and check occasionally that it is actually pulling air — hold a sheet of tissue near the grille and see if it holds. Leave the door ajar or the window open when the room is empty.

Squeegee the shower screen and wipe the bath edge with a cloth after use. Do not leave wet bath mats, sponges or bottles pressed against the silicone, since they trap moisture against it. A weekly spray with diluted white vinegar, left for a few minutes and then rinsed, keeps the film of soap and minerals down without harsh chemicals. If you have a tap or shower head that sprays onto the sealant, a small adjustment can make a real difference.

When to cut it out and reseal

If you have decided to replace the bead, do it properly. Remove the old silicone with a sealant remover or a craft knife held flat against the surface, clean the joint with methylated spirits, and let it dry completely — ideally a full day with the fan running. Apply a sanitary-grade silicone, smooth it with a sealant tool or a wetted finger, and leave it for at least twenty-four hours before anyone uses the bath or shower.

Skimping on the drying time is the single most common reason a new bead fails within a month. New silicone will not bond to a damp surface, and it will not bond to old sealant residue either.

A sensible rule of thumb

Try one thorough clean. If the mould has gone and you can keep the room dry, you have saved yourself an afternoon's work. If the black marks return within a month, or the bead is lifting, hardened or crumbling, replace it — you will spend less time overall than fighting it every few weeks. And if the mould extends well beyond the silicone, or anyone in the household has asthma or a weakened immune system, treat it as a ventilation and damp issue and take professional advice rather than just cleaning harder.

Photo: Max Vakhtbovych / Pexels