Go and press it. Fingernail into the bead where the bath meets the tiles, about a hand’s width from the taps, and push in hard enough to dent it.
If a bead of grey water squeezes out at the edge, or the silicone gives like a strip of foam and lifts away from the tile, stop reading and ring a plumber. If it’s firm, bonded, and the black is sitting inside a bead that won’t move, you have the other problem, and that’s the one I can tell you about.
I’ve been cleaning London bathrooms for eleven years and resealing them for about eight. The first three of those, I was making the same mistake as the plumbers I now complain about.
Is the Black Mould On the Sealant, or In It?
In it, nearly always, which is why bleach keeps letting you down.
The black staining on bathroom silicone is fungal colonisation, and the colony isn’t a film sitting on the surface waiting to be wiped off. It’s growing into the body of the sealant. Silicone contains plasticisers and processing oils, and a soft, permanently damp polymer with an organic food source in it is a fair approximation of a petri dish. Add the soap film, skin oils, shampoo and conditioner residue that coat every bathroom joint, and the top half-millimetre of that bead is the most hospitable square footage in your flat.
So when you spray a bleach gel along it and the black goes pale grey overnight, what’s happened is cosmetic. The oxidising agent has bleached the pigment in the surface hyphae. The organism underneath is intact, undisturbed, and it re-pigments over the following two to three weeks.
People then reseal, because the bleaching stopped working, and six weeks later they’re standing in the same spot with the same nail doing the same test.
The Fingernail Test That Tells You Which Job You Have
Firm bead, black inside it, no moisture: a resealing job, and it will hold for years if it’s done properly.
Spongy bead, or one that lifts, or water at the edge: the silicone is the last thing in the chain to fail and nothing you do to it will matter. Water is getting behind the joint from somewhere else and the bead is the bit you can see.
Cut a 50mm section out with a blade and look at the tile edge underneath. Wet, dark, or smelling of pond means the failure is behind the tiles.
What Does “Sanitary Grade” Silicone Buy You?
A fungicide, and a limited licence on it.
Sanitary or bathroom-grade cartridges carry a biocide dispersed through the sealant, which migrates slowly to the surface across the life of the bead. That’s the mechanism – it’s a wearing surface, like the tread on a tyre. In a bathroom with a working fan and a household that squeegees the screen, the protection realistically runs three to five years. In a windowless en-suite with two teenagers and a fan wired to the light switch, it can be eight months.
General-purpose silicone from the same shelf, £4 instead of £9, contains no fungicide whatsoever. It’s a glazing and framing product. Plenty of it ends up around baths, because the tube is transparent and looks identical on the rack, and it goes black quicker than anything else I see.
Check the cure type as well. Acetoxy silicone smells of vinegar as it cures and is fine on ceramic and enamel. It will etch marble, limestone and some natural stone trays, and it can craze certain acrylics. Neutral cure costs more and doesn’t do either of those things, which is why it’s the only kind that goes in my van now.
Why the Cheap Cartridge Isn’t the Saving
The cartridge is the smallest number in the job. Removing an old bead properly is ninety minutes of unpleasant work with a blade, a silicone eater and a lot of kitchen roll, and you are doing that ninety minutes again in a year if you saved five pounds on the tube.
Was the Joint Dry When the New Bead Went On?
This is the six-week answer, and it’s the one nobody wants, because it means the reseal was wrong rather than the sealant.
Fresh silicone laid over a joint that still holds moisture and old spores is a lid. The colony beneath carries on in the dark with a permanent water supply, and it comes through the new bead from underneath faster than it would ever have colonised it from above. That’s your six weeks. It isn’t reinfection from the air – it’s the same growth you thought you’d removed, arriving on the other side of £9 of silicone.
A plumber in Hither Green resealed a bath for a client of mine on the same afternoon he’d pulled the panel off and mopped out what was behind it. Beautiful bead. Tooled to a tidy concave and cut off square at the tap end. Black along the bottom edge within five weeks, and he blamed the cartridge.
Nothing grows on dry. The sequence that works is: cut both edges with a fresh blade, pull the whole bead, scrape the residue, wipe the joint down with silicone remover, wipe again with isopropyl alcohol, and then leave it alone for twenty-four hours with the extractor running or a fan heater pointed into the gap. Nobody baths in that period. It feels excessive for a joint you’re about to cover, and it’s the difference between four years and six weeks.
The Bath Full of Water Trick
An acrylic bath with a person and 140 litres in it drops three to five millimetres at the centre. Seal it empty, and every time somebody gets in, the bead is stretched past what silicone will take and a hairline tear opens at the tile line. Water goes in. The black starts at the split and works outwards.
Fill the bath before you seal it. Leave it filled for the full cure, twenty-four hours at minimum, then drain. The bead relaxes into compression instead of tension and it stops opening.
Is Your Extractor Fan Moving Any Air At All?
Hold a single sheet of toilet tissue flat against the grille and let go. If it doesn’t stay put, you have a fan that makes noise.
Approved Document F asks for 15 litres per second of intermittent extract from a bathroom, and a large share of the fans I see in London flats are nowhere close. The usual causes are dull. Flexible ducting crushed behind a boxed-in soffit. A duct that terminates in a loft void rather than through the wall, which quietly moves the damp problem to the roof timbers. A backdraught shutter painted shut during a redecoration in 2019. Or the fan wired straight to the light with no overrun, so it runs for the four minutes somebody is in the shower and stops the second they flick the switch, which is precisely when the room is at its wettest.
Surface humidity is what decides whether spores germinate on your new bead. Sustained damp at the joint for more than a day or two after each shower and you’ve built a growth medium regardless of which cartridge you used. Opening a window in January for ninety seconds achieves very little; a fan with a fifteen-minute overrun timer achieves most of it.
Nothing grows on dry, and the fan is the only thing in the room whose job is dry.
The Stoke Newington Flat Where the Duct Went Nowhere
Third-floor conversion, black sealant back within a month twice running, tenant convinced the landlord was buying rubbish silicone. I took the fan cover off to clean it and found 200mm of flexible duct stopping dead inside a boxed void above the cistern. Not connected to anything. Everything that fan had extracted for six years had gone into a plasterboard cupboard.
When Is Resealing the Wrong Job Entirely?
After the second time.
One reseal done properly should hold for years. If you’ve cut out and replaced a bead twice and it’s black again inside two months, the silicone is reporting a fault somewhere else: failed grout in the lower two courses of tile, a shower tray that’s moved on a springy timber floor, a bath foot that’s dropped, a waste connection weeping behind the panel. Continuing to reseal is repainting a rust patch.
Here’s the part that costs me. I took silicone mould treatment off my price list in 2021. It sat there for years as a line item at £35 and it was close to worthless, because the only honest version of that service is bleaching something pale for a fortnight. If a cleaning company quotes you to remove black mould from sealant, ask them whether they’re cutting the bead out. If they aren’t, they’re selling you two weeks.
And don’t pay anyone £160 to reseal a bath. It’s a cartridge, a tube of remover, a blade and an afternoon, and the skill is in the preparation rather than the tooling – the internet will teach you the finger-and-soapy-water part in four minutes.
What I’d Spend the Money On Instead
A fan with a run-on timer, fitted by an electrician, is somewhere around £140 all in and it fixes the cause. A squeegee hung in the shower costs £6 and takes eleven seconds a day. Nothing grows on dry.
The bead I cut and replaced in a Putney bathroom in November 2021 is still white at the tap end, and grey-ish at the plug end, which is the end where the water sits.