What Happens to Gallery Humidity When You Clean the Carpet in a Bond Street Showroom

Fifty-two per cent at half past six, and sixty-nine per cent by twenty past seven. That was the trace off a logger sitting on a plinth in a first-floor room off New Bond Street while a contractor extracted about sixty square metres of wool carpet in the same room. Seventeen points in under an hour, in a space holding four canvases on loan.

Nobody was cleaning the paintings. Nobody touched them. The damage risk in that room, if there was one, arrived through the air.

Most carpet cleaning quotes for Mayfair showrooms are written as though the floor were the only thing in the room. It rarely is, and in this postcode the contents are frequently worth more than the building.

What does a hygrometer do while you extract a gallery carpet?

It climbs, and faster than most people expect, because a showroom is a small sealed volume and water evaporating from a carpet has nowhere else to go.

Take a typical Mayfair first-floor room: twelve metres by nine, four and a half metres to the cornice, so somewhere around 480 cubic metres of air. At 20°C, saturated air holds roughly 17.3 grams of water per cubic metre, which means the room at 100 per cent would be carrying about 8.3 kilos of water in total.

Now leave one litre behind in the carpet. One litre evaporating into that volume adds around two grams per cubic metre, which is about twelve percentage points of relative humidity if nothing removes it.

One litre. A hot water extraction on sixty square metres of cut pile applies far more than that and recovers most of it, and the fraction that stays in the pile and the underlay is measured in litres rather than millilitres. The arithmetic is why the trace on the hygrometer moves the way it does.

The other half of the effect is temperature

Evaporation cools. A wet floor pulls heat out of the room, the air temperature drops a degree or two, and because cooler air holds less moisture at saturation, relative humidity climbs again for a second reason on top of the water you added. Two mechanisms, same direction, both of them arriving inside the hour. Turning the heating up to compensate is the reflex, and it works on the number while doing nothing about the water, which is still in the room and still coming out of the underlay at midnight.

Why does that number matter to a registrar?

Because it’s written into the loan agreement.

The environmental guidance most institutions now work to, coming out of the Bizot Group’s Green Protocol and the parallel IIC and ICOM-CC declaration, sets a range of 40 to 60 per cent relative humidity and 16 to 25°C for many classes of hygroscopic material, with fluctuations of no more than about ten percentage points of RH in any 24 hours. The older prescription was tighter, 50 per cent plus or minus five, at around 21°C, and plenty of lenders still work to something close to it for individual objects.

Loan paperwork turns that guidance into a contractual term. A standard institutional exhibition agreement requires the borrowing venue to keep the work within those bands and to notify the registrar immediately of any excursion outside them. A gallery running a data logger in the room, as most serious ones do, has a record of the excursion whether they report it or not.

Seventeen points in an hour breaches that condition comfortably, and the conversation afterwards is with a lender’s registrar rather than with a facilities manager.

Rate of change matters more than the number

The Bizot position is careful about this and it’s worth understanding rather than quoting. The group’s own refreshed guidance says the 40 to 60 band has no fundamental scientific meaning as a threshold and should be a starting point for a risk-based approach rather than a new rule to replace the old one. Sensitive objects need tighter control, determined by a conservator, and other objects tolerate far more than the tradition assumed.

What conservators consistently worry about is speed. A slow seasonal drift across twenty points does less to a panel painting than a fast swing across ten, because the swing puts differential stress between the surface and the core of a hygroscopic material before it can equilibrate. A carpet clean is a fast event by definition.

Which things in a Bond Street room care?

Fewer than the anxiety suggests, and the ones that do are the expensive ones.

Panel paintings, marquetry and veneered furniture, musical instruments, anything with animal glue in its construction, parchment and vellum, ivory, and stretched canvas where the fabric responds to moisture faster than the paint film on it. Wood moves across the grain and the joint is where it shows. A veneered console table against the wall of a showroom has a thin sheet of one timber glued to a substrate of another, and the two respond to a moisture swing at different rates. Lifting veneer along a front edge is the classic result and it appears weeks later, which is why nobody attributes it to the night the carpet was done.

In a watch salon the concern is different. Steel movements, unlacquered brass, and case interiors that spend their display life in a controlled cabinet with silica gel doing quiet work. Raise the ambient in the room for eight hours and the cabinets buffer it, mostly. Do it repeatedly and you are asking a lot of a desiccant that gets changed when somebody remembers.

Jewellery is largely indifferent. Silver tarnishes on sulphur rather than water. Textiles and historic costume are not indifferent at all, and a room hung with historic dress needs the same caution as one hung with panel paintings.

The one I refuse

An unglazed canvas hanging low on a wall in a room I’m being asked to clean. I won’t do wet work in that room while the work is on the wall, and I say so at the quoting stage, because the moisture gradient near the floor is higher than the number on the hygrometer at plinth height suggests and I’ve no way of proving otherwise afterwards if a lender asks.

That position has cost me two jobs on Cork Street. Both galleries booked somebody else and I’d make the same call again.

Can’t you just run a dehumidifier?

You can, and the correction is its own risk.

A low grain refrigerant dehumidifier will pull the room back down, and it will also add heat, which shifts the temperature side of the equation while it fixes the moisture side. Run it hard and you overshoot. A room taken from 69 per cent down to 38 in three hours has now had two rapid excursions instead of one, in opposite directions, and the second one is the direction that opens joints and cracks gesso.

The workable approach is small and slow. Modest dehumidification, gentle air movement, a target of drifting back toward the starting figure over several hours rather than resetting the room in one. Watch the trace on the hygrometer and let it come down at a few points an hour.

The dust nobody costs

Air movement in a gallery lifts particulate off the floor and puts it in suspension, and what goes up settles on horizontal surfaces including the top edges of frames and the shoulders of anything on an open plinth. Aggressive pile agitation does the same thing.

Vacuum first, with a machine on a HEPA filter, and keep air movers low, angled along the floor rather than up into the room. A gallery technician who finds a grey line on a frame top the morning after your visit will connect the two events correctly.

So when should a showroom carpet be cleaned?

Between shows, in the deinstallation window, with the walls empty.

Commercial galleries turn exhibitions every four to six weeks and the room stands empty for two or three days between the crate collection and the next fit-out. That gap is the only comfortable time to put any moisture into the room, and it’s usually enough. Book it as a standing arrangement with the gallery manager rather than as a reactive job, because by the time a carpet looks bad enough to prompt a phone call the next show is already hanging.

October is a blackout month in this postcode. Nobody is closing a Mayfair gallery for a floor during Frieze week, and the fortnight either side of it is spent installing and then dismantling. Ask in September or ask in November. The same applies either side of a major evening opening, when the room takes three hundred people, their coats and a drinks table, and the humidity trace from that night is worth looking at before anybody blames a cleaning contractor for anything.

The maintenance answer between those windows is dry: compound, encapsulation with minimal moisture, or in a room with something sensitive in it, a thorough vacuum and nothing else. There are floors I’ve maintained on Albemarle Street for four years without wetting them once, and they look better than the ones that get extracted annually.

Hang a logger a week before

Ask the gallery to put a data logger in the room seven days ahead and leave it running through the job and for 48 hours afterwards. Two reasons. It tells you what the room does on its own, which on a first floor above a shop with a glazed frontage and a door onto Dover Street can be a twenty-point daily swing before anybody touches the carpet. And it gives both parties a record.

If the room already swings that much unaided, the conversation with the gallery changes shape entirely, and it stops being about the carpet.

That room off New Bond Street came back under 60 per cent by half nine that evening with two dehumidifiers running. The registrar was notified. The show opened on the Thursday.