I watched a lad on a job in Peckham spray a hallway and start extracting it before he had finished spraying. He was quick, he was tidy, and the carpet came up about sixty per cent of what it should have. He had bought a good prespray. He had given it about forty seconds to do a ten-minute job.
Dwell time is the cheapest thing in the van and the first thing that gets cut. It costs nothing, it is invisible on the invoice, and there is no way to charge for it except by charging for the hour it sits inside.
That is the whole reason it disappears.
What is the pre-spray doing while it sits there?
Three separate processes, running at different speeds, and none of them instant.
The first is wetting. Soil in a carpet is bound to the fibre by a film of oil and held in place by surface tension. A surfactant molecule has a water-loving end and an oil-loving end, and before anything else can happen those molecules have to migrate to the interface between the soil and the fibre and orient themselves. That migration takes time. In a soiled traffic lane, where the soil is greasy and the fibre is coated, it can take several minutes before the solution has even reached the fibre surface at the base of the yarn.
The second is emulsification. Once the surfactants are in position they lift oily soil away from the fibre and hold it suspended in the water as tiny droplets. This is the part everyone thinks of as cleaning, and it cannot start until the first process has finished.
The third is chemical, and it depends on what is in the bottle. Alkaline builders saponify fatty soils. Solvents dissolve them. Enzymes hydrolyse protein and starch, which is a slow reaction by any standard.
The chart everybody has seen and nobody uses
Time, agitation, chemical, temperature. Every training course in the country draws the same wheel and explains that the four variables trade off against each other, and then everybody goes back to the van and manages three of them.
Reduce time and something else has to increase. In practice what increases is chemical, because chemical is the one you can buy. Stronger dilution, higher pH, a more aggressive product, and the carpet gets cleaner in the short run and re-soils faster because the residue load has gone up.
How long is long enough?
Five to ten minutes for a general-purpose prespray at normal soil levels. That is the number on most labels and it is a reasonable working figure by the clock.
Enzyme presprays want fifteen to twenty. Protease and amylase work at a molecular rate that does not care how busy you are, and an enzyme product given four minutes is an expensive general-purpose detergent with no enzymatic contribution at all. On urine work I have left product dwelling for forty-five minutes and gone and done the stairs.
Solvent-based products run the other way. A citrus or glycol ether spotter is working from the moment it lands and it is evaporating from the moment it lands too, so its effective window is short and re-application beats waiting.
Then there is the ceiling. A prespray that dries on the fibre has stopped working and started causing problems – dried detergent does not rinse cleanly, it grabs soil, and on wool an alkaline product left sitting in a warm room is doing chemical damage rather than nothing.
The rule I use instead of a timer
Wet is working, drying is damage. I look at the pile, and while there is visible moisture in it the chemistry has a medium to work in.
Twenty square metres at a time in summer, a whole lounge in February, and a mist applied over the first section if it starts to flash off before I get to it. The clock is a guide. The state of the fibre is the instruction.
Does agitation replace dwell time?
It helps and it does not replace it, and this is where I think a fair part of the trade has the balance wrong.
A carpet rake or a rotary brush does two useful things. It drives solution from the tips down to the base of the yarn, where most of the soil sits, and it breaks up soil clumps so more surface area is exposed to the chemistry. Both of those shorten the wetting phase considerably. Neither of them speeds up an enzyme reaction or a saponification, because those run at whatever rate the chemistry runs at.
What agitation does very effectively is make a carpet look agitated. The pile lies differently, the surface soil redistributes, and the operator gets a strong visual signal that work has happened. That signal is worth nothing and it is why brushing has quietly become a substitute for waiting on a lot of London vans.
My order is spray, agitate straight away to drive it in, then leave it alone for the rest of the dwell. A second pass with the rake at the end, just before extraction, undoes some of the separation the chemistry achieved.
Pre-vacuum is part of the dwell argument
Dry soil in a carpet absorbs prespray. It sits there soaking up solution that was meant for the greasy soil bound to the fibre, and it turns a correctly dosed application into an underdosed one.
Eight minutes with a twin-motor upright before anything wet happens means the prespray goes where it is aimed. Most of the complaints about products not working are complaints about products being drunk by dust.
A bad case makes the point. A four-bedroom house in Romford, two shedding dogs, no vacuum in eleven days – I filled the machine’s bin twice before spraying anything, and the second bin was as full as the first.
Why do rushed jobs re-soil in three weeks?
Because insufficient dwell and excessive product are the same mistake arriving from opposite directions, and they leave the same thing behind.
Give a prespray thirty seconds and the surfactants have not finished their work. Extraction then removes the water, some of the soil, and a portion of the detergent, and what stays on the fibre is a mix of half-emulsified soil and unspent surfactant. That film is sticky, it is hygroscopic, and it holds every footfall of dry soil that arrives after it.
Three weeks later the traffic lane looks worse than it did before the clean. The client concludes that cleaning loosened something in the carpet, which is folklore with a real observation underneath it. Nothing was loosened. Something was left.
The test for it, if you ever want to know: rub a clean white cloth hard across the affected lane on a dry carpet a month after a clean. Residue transfers as a grey smear far more readily than embedded soil does. I did that on a landing in Barnes in front of a client who had been told by two firms that her carpet was worn out, and the cloth came up grey enough to end the argument in about four seconds. That carpet had six or seven years left in it. It had been cleaned badly, twice.
I picked up a maintenance contract on a serviced office block off Bishops Bridge Road partly on this – the previous contractor was in and out of a 900 square metre floor in five hours, and the carpet was visibly dirtier at week four than at week zero, by the clock, every single month.
The rinse is not a fix for a short dwell
There is a belief that a hot enough rinse with enough vacuum will pull out whatever the prespray failed to release. It will not.
Rinse removes what is already suspended in the water. Soil still chemically bound to the fibre is not suspended in anything, and a rinse passes straight over it. Heat and pressure turned up to compensate for a short dwell add water to the carpet and add no cleaning.
What does this cost, and who pays for it?
Here is the disagreeable part, and it is aimed at my own trade more than at clients.
The average domestic carpet cleaning quote in Greater London prices a three-bedroom house at a level that assumes ninety minutes on site. You cannot pre-vacuum properly, prespray in sections, dwell for ten minutes each, agitate, extract slowly with a two-to-one dry stroke ratio, and set drying equipment, in ninety minutes. The arithmetic does not permit it. So the dwell goes, because it is the only line item nobody can see.
Most of the money this trade spends on stronger chemistry is money spent buying its way out of impatience. A mid-range prespray given its full ten minutes outperforms a premium one given two, on every carpet I have tested it on, and the premium one costs four pounds more a litre.
I price my domestic work at roughly a third above the London median and I do about half the jobs a day some firms do. Plenty of people ring off when they hear the number. The ones who do not have me back.
What a client can watch for
Ask what the prespray dwell is and whether the operator works in sections. Anyone who cares will have a number and a method. Anyone who does not will tell you their equipment is powerful.
Then watch the first ten minutes. If a wand touches the carpet inside five minutes of the sprayer being put down, you are buying a rinse rather than a clean.
Somebody standing in your hallway apparently doing nothing for eight minutes is the part of the job you are paying for. It looks like the opposite. That is the difficulty with selling it, and it is why the industry keeps quietly deleting it.
The Peckham hallway got done again a fortnight later by somebody else and came up properly. Same carpet, same soil, a prespray that cost less than the first one. Eleven minutes, by the clock.