Month: February 2026

“It looks like someone’s been up there with dirty hands.”

That was a practice manager in Hatton Garden, standing under a run of tiles in EC1N with grey bands running along every grid line and a soft dark halo around each supply diffuser. She had it in her head that the cleaners had been leaning on the ceiling. Nobody had touched it. The marks had arrived by air, over about four years, and they had arrived in a pattern so regular that once you know what you are looking at you can read the building’s ventilation from the floor.

What she was looking at is usually called ghosting, or plate-out, and it is one of the few problems in this trade where the honest answer is that cleaning is the wrong tool.

What is putting the grey there?

Fine airborne particulate landing on a surface that is very slightly colder than the air around it.

The mechanism is thermophoresis. Sub-micron particles get pushed down a temperature gradient, from warm towards cool, because the gas molecules on the hot side of a particle strike it harder than those on the cold side. Small force, slow work – which is exactly why the result is a faint even grey rather than a splash. The particles are too light to settle out under gravity in any useful time, so they drift until they find somewhere colder to stick.

Once a particle lands on a fibrous surface it is not coming off again in the way dust comes off a desk. It has keyed into the texture. The tile is the filter.

Why does it follow the grid lines and the diffusers?

Because those are the cold spots and the turbulent spots.

The exposed grid is steel. Steel conducts heat far better than a mineral fibre tile does, so the grid and the strip of tile immediately adjacent to it run measurably cooler than the middle of the tile, and the deposition concentrates there. That gives you the grid-following bands, and it is the same physics that produces thermal tracking lines on plasterboard ceilings above unfilled joists in flats.

Around the diffuser you get a second effect on top of the first. Air leaving a supply diffuser is cooler than room air and it is moving fast, so you have both a temperature gradient and turbulent impaction driving particles onto the tile face in the throw pattern of the jet. The halo takes the shape of the diffuser’s air pattern rather than a neat circle, which is why four-way blow gives you a rough cross and a linear slot gives you a stripe.

Stand under it and look at which way the smudge points. You will know within ten seconds whether the grille is throwing evenly.

Where is all that particulate coming from?

Three sources cover almost every London office I inspect, and they are not equally weighted.

Combustion products from outside dominate. Diesel particulate is overwhelmingly sub-micron, it comes in through the fresh air intake and through every door onto the street, and in a building on Clerkenwell Road with a taxi rank outside there is a constant supply of it. This is the one nobody can do much about beyond filtration, and it is the one that produces the broad even greying across a whole floorplate.

Then there is whatever the occupants are burning. Candles are the great unacknowledged culprit in small offices and in reception areas, and a cheap paraffin candle with an oversized wick produces staggering quantities of soot. Reed diffusers and oil burners are worse than people think. In Hatton Garden specifically the picture is different again, because the workshops upstairs are running polishing motors that throw rouge and tripoli compound into the air alongside the burnout kilns, and that workshop load finds its way into the ceiling void of the office below through service penetrations that were never sealed.

Laser toner is the third, and it is the one people always name first.

Is the printer to blame?

Partly, and less than its reputation suggests.

A laser printer emits ultrafine particles during fusing, mostly volatile compounds from the toner resin that condense into particles rather than toner powder itself escaping. The emission is real and it is measurable. What it produces on a ceiling is a localised darkening in the bay above and slightly downstream of the machine, rather than the floor-wide greying most clients blame on it.

If the staining is uniform across 3,000 square feet, the printer is not your problem and moving it will not help. If there is one noticeably darker bay above the print room door, move the machine and put it somewhere with a local extract.

Why does cleaning a mineral fibre tile usually wreck it?

Because you are cleaning a soft, absorbent, air-permeable board that was never designed to be wet.

A standard mineral fibre tile is compressed mineral wool with a binder, faced with a thin painted or fissured decorative layer. In a plenum return system, air passes through the tile itself. The tile is the filter, and it has been filtering for years, so the soiling sits within the surface layer rather than on top of it.

Wet cleaning does four things, all bad. Water swells the binder and the tile sags in the grid, sometimes visibly within a day. The surface nap lifts and the fissured texture roughens, so the tile holds soil faster afterwards. You get tide marks at the wet edge that are more conspicuous than the original stain. And if you lift enough soil to make a difference, you lift some of the surface coating with it, leaving a patch of different sheen.

Then there is the performance question that nobody asks. That tile has a declared fire classification and a declared sound absorption value, both of which depend on its surface and its perforation. Coating it, painting it or soaking it changes both. A ceiling that has been rolled with emulsion to hide staining has usually lost a chunk of its acoustic absorption and may no longer meet the classification it was specified under.…