Four or five enquiries a week come in about black spotting, and three of those are jobs that should not be taken. The customer wants the spots gone. The spots will go, for a while, and then they will come back in exactly the same corner of exactly the same window, and the customer will conclude that the cleaner was no good rather than that the problem was never a cleaning problem.
So here is the split, bluntly. Some of this responds to cleaning permanently. Some responds temporarily. Some will not respond at all, and no product on any shelf in any wholesaler changes that.
What are the black spots?
Mould. Specifically the group of dark moulds that colonise damp surfaces indoors, and in a London flat the usual suspect is Cladosporium, with Aspergillus turning up in worse cases and Stachybotrys on anything that has been wet for months rather than damp overnight.
What matters is not the species. What matters is that these are living organisms with root structures, and they grow into porous or semi-porous material rather than sitting on top of it the way soil does. That distinction decides everything about whether cleaning works.
Spores are in the air of every building in the country at all times. They do nothing until they find a surface that stays above roughly eighty per cent relative humidity for long enough, at a temperature the organism likes, with something to eat. Indoors in winter, all three conditions turn up in the same places: the bottom corner of a window reveal, the sill, the bead of silicone where a bath meets a tiled wall, and the grout line nearest the shower head.
Why sills and silicone rather than anywhere else
Cold surfaces and dead air. A window sill is the coldest horizontal surface in the room, air moving across it slows to nothing in the reveal corners, and moisture condenses out of the air onto the glass and runs down. Whatever film of dust, skin and cooking residue sits on that sill is food.
Silicone is worse, and the reason surprises people. Cured silicone itself is not much of a meal, but sanitary sealant contains plasticisers and fungicidal additives, and the fungicide has a life of two to four years before it leaches out. Once it has gone, the sealant is a soft, slightly porous strip permanently within a few millimetres of standing water, holding soap residue, body oil and shampoo. Mould grows into it, not on it.
A bathroom in a converted house on Bushey Hill Road in Camberwell had sealant so far gone that a fingernail took a strip off the surface with black threading right through the depth of it. Nothing was going to clean that.
What does cleaning fix?
Everything that has not yet penetrated, which is more than people assume if they act early.
Fresh spotting on glass, on tiles, on gloss-painted timber, on plastic uPVC frames and on ceramic comes off completely and stays off, provided the surface is sound. Those materials are non-porous. The mould has nowhere to root, so it is sitting on a film of grime and it lifts with the film. A thorough clean, a rinse, and a dry surface afterwards will hold for months.
Grout responds well if the glaze on the tile is intact and the grout is not crumbling. Cement grout is porous, so the mould has got a few millimetres in, but the material is hard enough that a fungicidal wash with proper dwell time will kill the growth through the depth and the discolouration will lift with a stiff nylon brush. Nine times out of ten, grout that looks ruined is grout that has never had anything left on it for longer than thirty seconds.
Painted plaster is the borderline case. Modern vinyl emulsion is effectively a plastic film, so surface growth wipes off. What it also does is trap moisture in the plaster behind it, which is why the paint often blisters slightly around the spotting. Once the paint has gone, the plaster is porous and the answer changes.
The bleach question
Household bleach on silicone is theatre. It bleaches the pigment in the mould, the black disappears within minutes, the customer is delighted, and the organism is still alive inside the sealant with its colour removed. Three weeks later it darkens again and everybody blames the cleaner.
Two things work better. A thickened chlorine gel that stays in contact with the bead for four to six hours will penetrate far enough to kill rather than bleach, and that dwell time is the whole mechanism. Or a fungicidal wash with a quaternary ammonium base, left wet, wiped, and reapplied and left to dry without rinsing, so a residual film stays behind.
Never mix chlorine products with anything acidic, including limescale removers, which is a genuine hazard in a bathroom where both bottles live under the same sink.
What will cleaning not fix?
Anything where the growth has gone into the substrate, and anything where the substrate itself has failed.
Timber sills are the clearest example. A painted softwood sill in an original sash window has usually had six or seven coats over a hundred and twenty years, and the black in the corner is often growing under the paint film in wet timber rather than on top of it. Cleaning the surface removes the visible part and leaves the colony. Worse, the presence of that growth is a sign that water is getting in past the paint or past the putty, and until someone deals with the putty line, the water keeps arriving.
A ground-floor flat on Estreham Road in Streatham had spotting along the whole front of a bay sill that returned within five weeks of every clean. The putty had gone brittle on the outside face and the sill was wet from behind. Two years of cleaning and repainting did nothing at all.
Failed sealant is the same story. Once the fungicide has leached out and the surface has gone soft or the bead has parted from the tile at the top edge, water is running behind it. The black visible along the front is a fraction of what is in there. Cut it out with a hooked blade, clean the substrate, dry it for twenty-four hours and run a fresh bead of proper sanitary sealant. Forty minutes, and permanent for another three or four years. Cleaning it is a twenty-minute job that lasts a month.
Crumbling grout, blown plaster, and anything with a cold bridge behind it belong in the same category. A room in a hard-to-treat solid-wall property on Coniston Road in Muswell Hill had recurring black in one corner of a bedroom ceiling, and the cause was a steel lintel above the window transferring cold straight through the wall. Every winter, the same corner.
How to tell when silicone has to come out
Press it with a thumbnail. Sound sealant is firm and springs back. Failed sealant compresses and stays compressed, or a nail lifts a skin off it.
Then look at the top edge where it meets the tile. A hairline gap there means water has been going behind for months, and whatever is growing in that gap is not reachable by anything applied to the front. Both of those signs mean replacement, and any cleaner who says otherwise is selling a return visit.
Why does it come back in the same spot every winter?
Because the spot is the coldest square foot in the room, and cleaning does nothing to the temperature.
Condensation forms where the surface temperature drops below the dew point of the air in the room. In a London flat in January that means window reveals, the reveal corner behind a curtain, the wall behind a wardrobe on an external wall, and the ceiling corner where a bay roof meets the main wall. Air with a lot of moisture in it, which is what a flat produces from cooking, showering, drying laundry indoors and two people breathing, will drop that moisture out at the first cold surface it meets.
Cleaning removes the colony. It does not change the dew point, the surface temperature, or the moisture load. Nine times out of ten, recurrence in the same location is a building performance question wearing a cleaning problem’s clothes.
The extractor fan and the window
Most bathroom fans in London flats are the minimum-spec unit wired to the light switch with no overrun, or with a three-minute overrun that stops long before the room has dried. A shower puts something like a litre of water into the air. Three minutes of a weak axial fan removes a fraction of it, and the rest goes into the walls, the ceiling and the sealant.
A humidistat fan with a fifteen-minute overrun, or a decent centrifugal unit in a flat with a long duct run, changes the room more than any cleaning schedule will. Opening the window for twenty minutes with the door shut does nearly as much and costs nothing.
When should you stop cleaning and call someone else?
When it has come back twice in the same place after a proper clean.
At that point the money is better spent on a plumber, a glazier, a damp surveyor or an electrician than on a third visit from me. I will say so on the phone. A flat on Faraday Avenue in Sidcup had three cleans booked for a bathroom ceiling before anyone checked the loft above it and found an uninsulated section directly over the shower.
The landlord and tenant version of this argument annoys me from both directions. the cause is usually the way the property is being lived in, and the remedy is almost always the landlord’s. Drying washing on radiators in a sealed flat produces the moisture. A fan that works and a bit of insulation deals with it. Telling a tenant to open windows more in a January in a flat on Mafeking Avenue in Forest Gate is a repair being dressed up as advice.
What to write down before you complain or before you quote
Photograph the spot with something in frame for scale and note the date. Do it again after the next clean, and again six weeks later. Two dated photographs showing the same corner in the same state are worth more than any amount of description, whether the conversation is with a landlord, a managing agent or an insurer.
Note what is on the other side of the wall as well. Nine times out of ten the answer explains the pattern: an external corner, an unheated hallway, a loft with a gap in the insulation, a shower on the far side of a stud.
The sealant in Camberwell got cut out and replaced. It has stayed white since March, which proves nothing yet.