There is a cream wool twist in a first-floor front room on Fortis Green Road in Muswell Hill that has a stripe across it. The stripe runs at an angle, about eighty centimetres wide, and it stops dead at the shadow line thrown by the bay. The carpet inside the stripe is a warm buttery yellow. Outside it, the same carpet is the colour it was in 2019.
The owner assumed sun bleaching and rang me to ask whether cleaning would even it out. It would not, and the sun was only half the culprit. The other half arrived in a bottle three years ago, applied by somebody who did a perfectly competent job and left the room with the carpet looking better than it had in a decade.
What is an optical brightener doing to a carpet?
Nothing chemical to the soil. That is the first thing to understand about the entire category.
An optical brightening agent – a fluorescent whitening agent, if you read technical sheets – is a compound that absorbs ultraviolet light in the region of 340 to 370 nanometres, which the human eye cannot see, and re-emits it as visible blue light at around 420 to 440 nanometres, which the eye sees very well indeed. Most of the ones in cleaning products are stilbene derivatives: diaminostilbene disulphonates, distyrylbiphenyls, the Tinopal family and their relations.
Fabrics and fibres yellow as they age. Yellow is the visual opposite of blue. Add blue light to a yellowing surface and the two cancel out, so the surface reads as neutral, and because the brightener is emitting light rather than reflecting it, the total light coming off the carpet can exceed what fell on it. That is the reason a brightened carpet looks not merely clean but faintly luminous. Blue over yellow.
The deposit is the whole of the effect
None of this removes anything. The soil that made the carpet look tired is still in the pile, minus whatever the detergent alongside the brightener lifted, and the brightener is a deposit sitting on the fibre to correct the appearance of what remains.
I have no objection to the effect in laundry, where the garment goes back in the machine next week and the deposit is refreshed and redistributed constantly. A carpet is washed every eighteen months if it is lucky, gets uneven light, and stays where it is put. Whatever you leave on it in March is still exactly there in October, on exactly the same tufts.
Where do brighteners come from if you never bought any?
The largest single source in London homes is washing powder poured into a hired carpet machine. People do it constantly – the machine wants a detergent, there is a box of biological powder under the sink, and consumer laundry detergent is heavily loaded with brighteners because that is a selling feature on clothes.
They also turn up in supermarket carpet shampoos, in a good number of aerosol spot cleaners, in some upholstery foams, and in the cheaper end of the trade prespray market. Ask a supplier directly and most will tell you straight. The ones who go quiet are answering the question.
The hire-machine problem, in more detail
A hire unit leaves a high proportion of what you put down in the carpet, which I have written about elsewhere in the context of moisture. The same applies to everything dissolved in that moisture. Brightener that would have rinsed away in a washing machine has no rinse cycle here – it dries into the pile and bonds to the fibre surface, and stilbene brighteners have real substantivity for cellulosics and reasonable affinity for nylon and wool.
Worse: application is by hand, in overlapping passes, at inconsistent dwell. So the deposit is uneven from the first day. You will not see that unevenness for a year or more, because a fresh deposit is doing its job everywhere it landed. The pattern of your passes is written into the carpet in a chemical that is currently invisible, waiting for daylight to develop it.
Why does the carpet go yellow a year later?
Stilbenes are not stable under prolonged ultraviolet exposure. They are conjugated organic molecules with a double bond that ultraviolet light is very good at attacking – the trans form isomerises to the cis form, which does not fluoresce usefully, and then the molecule oxidises further into breakdown products that are themselves yellow.
So the compound put down to cancel yellow becomes yellow. Blue over yellow, until the blue stops and the sum is yellow twice over: what was there originally, plus the residue of what was masking it.
This takes between about eight months and three years in a British interior, depending almost entirely on how much direct sun the area receives. A north-facing back bedroom in Stratford may never get there. A south-facing bay in a Victorian front room, with modern glazing that passes more UVA than the old glass did, will get there inside a year.
And the degradation is only in the sunlit part. That is what produces the stripe on Fortis Green Road, and it is why these faults look so much like sun damage that nobody investigates further. Sun damage to the carpet dye would fade the colour towards pale. This does the reverse – it drives the colour warm and dirty, most visibly on cream, ivory, oatmeal and pale grey, which is most of what gets specified in London sitting rooms.
There is a second version of the fault that has nothing to do with windows. Uneven deposit across the whole floor degrades at different rates according to loading, so a carpet cleaned with a brightener-heavy product develops a soft mottled appearance over two or three years – blotchy in a way that reads as poor cleaning rather than as chemistry. Owners then have it cleaned again, usually with the same class of product, and the new deposit hides the old fault for six months before both come through together.
The bay-window line
Any hard shadow boundary that stays in the same place is where the fault will show first. Bay window returns, the edge of a rug, the line under a sofa that has not moved since the last clean, the dark strip behind a full-length curtain. I look for those boundaries before I look at anything else on a yellowing job, because a fault that respects a shadow line is not a soiling fault. Soil does not care where the light falls, and a stripe that stops at a shadow line was put there by something in a bottle.
A Chiswick job last year had a perfect rectangle of pale carpet where an ottoman had stood, surrounded by yellowed carpet, and the owner had convinced herself the ottoman had somehow bleached the floor. The ottoman had protected it.
Can you tell whether a carpet has been treated?
Yes, in about four seconds, and this is the one piece of practical kit I would put in every householder’s drawer.
Buy a 365-nanometre ultraviolet torch. Not the cheap 395nm ones sold for spotting pet stains, which flood everything with visible purple – a proper 365 with a filter costs perhaps £25. Close the curtains, switch it on, and sweep the floor.
What the torch shows you
Brightener deposit fluoresces a bright, cold, slightly electric blue-white that is nothing like the response of the fibre itself. Where somebody has worked in overlapping passes you will see the passes. Where somebody has spot-treated with a supermarket product you will see a bright patch with a hard edge, often three times the size of the original spill. On a cotton rug fringe treated with a whitening product, the fringe will light up like a filament while the rug body stays dull.
The same torch shows other things. Urine fluoresces a dull yellowish-green, quite different from the blue-white of brightener, and once you have seen the two side by side you will never confuse them. That is worth the £25 on its own.
Can brighteners be taken back out?
Partly, sometimes, at a cost – and here is where I talk myself out of work.
Fresh deposit rinses out reasonably well with repeated hot water extraction using a plain acid rinse and no detergent at all. Two or three passes, checking under the torch between each. If you have just had a carpet cleaned and the torch shows a blue glow, ring the contractor back that week and it can mostly be undone.
Old deposit that has already photodegraded is a different proposition. The yellow breakdown products are on the fibre, and taking them off means an oxidising treatment – hydrogen peroxide-based, worked carefully, with heat – which on wool carries genuine risk of dye damage and fibre damage, and on a mid-value carpet costs more than the carpet is worth. I have done perhaps six of these and I would not describe more than three as successes.
When to leave it alone
If the carpet is over eight years old, or if the yellowing is even across the whole floor rather than patchy, leave it. Cleaning will not touch it, stripping might ruin it, and you will spend £250 to be told what you already knew.
Which brings me to the thing I would say to my own trade if anyone were listening: a product that makes a carpet look better without removing anything from it should not be in a professional’s van. Brighteners exist to win the doorstep verdict – the customer looks at the floor twenty minutes after the machine goes off, and blue over yellow beats an honest rinse every time in that particular test. It loses badly in the test that runs for three years.
Ask whoever cleans your carpets whether their prespray contains optical brighteners. If they do not know, they have not read the safety data sheet for the product they are spraying in your house.