A Wilton in a first-floor drawing room on The Ridgeway, Mill Hill, went the colour of weak tea about six days after somebody cleaned it. Somebody else, I should say. The firm had arrived with a rotary and a drum of something the invoice called commercial strength alkaline traffic lane cleaner, and by the time the owner rang me the whole centre of the room had gone dull and warm-toned, with the swirl pattern of the machine still legible in the discolouration.
That carpet was 80/20 – eighty per cent wool, twenty per cent nylon, the standard British blend sitting in about half the good houses I work in across Greater London. A forgiving construction in almost every respect. Push the pH up and it punishes you for it slowly, then all at once.
What actually happens to wool above pH 8?
Wool is keratin and keratin is protein. The fibre holds its shape through disulphide crosslinks between cystine residues, the same chemistry that decides whether hair holds a curl. Those bonds are stable in mildly acid conditions and unstable in alkali. Wool sits at its isoelectric point somewhere around pH 4.5 to 5, which is where the fibre is most chemically settled and, as it happens, roughly where the sheep left it.
Take it past 8 and alkaline hydrolysis starts cleaving those crosslinks. The reaction products include lanthionine and a family of degradation compounds that absorb light at the blue end of the spectrum. Absorb blue, reflect yellow. That is the whole mechanism, and it is a change in the fibre rather than something sitting on it – no amount of extraction lifts a chromophore that has become part of the wool.
Heat multiplies everything. At 20°C and pH 9 you might have a couple of hours before anything shows. Run a truckmount at 65 or 70°C through the same solution and you have got minutes, give or take.
The scales lift as well. An alkali-swollen cuticle catches on its neighbours, and agitation on top of that gives you felting, matting and a handle gone harsh underfoot. Clients complain about the colour. By then the structural damage has already happened underneath it.
Why the 20% nylon buys you nothing
Nylon 6,6 shrugs off alkali far better than wool. That fact has convinced a lot of people that a wool-rich blend is somehow tougher than a hundred per cent wool broadloom, and it is the wrong conclusion. A blend behaves according to its weakest component when the damage is chemical. What you get from the nylon is uneven survival: the wool component yellows while the nylon holds its tone, and the result reads as a flat, muddy cast rather than a clean discolouration.
Both fibres take acid dyes, and both dye systems loosen their grip as pH climbs. On a mid-blue Axminster in Chislehurst, BR7, an over-alkaline prespray pulled enough dye out of the wool to leave the nylon reading brighter than the wool beside it. Under LED downlights that reads as a stain. It is dye loss, and it is permanent.
So where is all this alkalinity coming from?
Almost nobody sets out to put pH 11 on a wool carpet. It arrives by accident, through products that were never labelled as the problem.
Sodium carbonate and sodium metasilicate are standard builders in general-purpose presprays, and plenty of those sit at 10 to 11 at working dilution. Enzyme presprays are commonly formulated around 9 to 10 because that is where protease is happiest. Oxygen boosters based on sodium percarbonate hydrolyse into hydrogen peroxide and sodium carbonate, and the carbonate half takes the solution to about 10.5. Encapsulation polymers, sold hard as the gentle option for maintenance, frequently land between 8.5 and 9.5.
Then there is residue. Alkaline salts left behind after a clean are hygroscopic – they pull moisture out of the air and hold it, which means the reaction carries on quietly for weeks after the van has driven off. A carpet that looked fine on Friday can be visibly off by the following Thursday. That is the signature of an unrinsed job, and it is why I distrust any process that has no rinse step at all.
What is going into your rinse tank
London mains water runs hard through most of the north and east – somewhere around 280 to 320 mg/l as calcium carbonate, and out of the tap at roughly 7.5 to 8, give or take the day and the postcode. That is already at the top of the range wool tolerates, before anything is added and before it goes through a heat exchanger.
A rinse of plain hot tap water on a wool-rich carpet is therefore not a neutral act. It leaves the fibre sitting somewhere it should not be. Every wool job I do gets an acid rinse buffered to 4.5 to 5.5, measured. A pack of pH strips costs less than a tank of diesel.
Is that yellow patch browning, or is it alkaline damage?
This is where the money is, and where most of the trade gets it wrong.
Cellulosic browning comes from lignin and tannins in a jute secondary backing, or in jute weft threads on an older Wilton. Overwet the carpet and the moisture carries those compounds up the pile as it dries, depositing them at the tips. It has a look you can learn: tea-brown rather than yellow, with defined edges where the wet area ended. Tide-marks. It follows the drying pattern, strongest around the perimeter of a spill or along the last strip somebody cleaned before packing up.
Alkaline yellowing looks different in every one of those respects. It is field-wide rather than edged, because the detergent went everywhere the detergent went. It follows the tool – wand strokes, rotary swirls, the arc of somebody’s spray pattern. The colour is duller and greyer-yellow rather than warm brown. It sits down the shaft of the fibre as well as at the tip, so parting the pile with a thumbnail shows you discolouration at depth rather than a clean line near the surface.
Here is the opinion that gets me disagreement at trade events: the industry has spent thirty years teaching cellulosic browning and it has made everyone worse at diagnosis. Browning is the first thing anyone reaches for because it is the thing they were taught, and roughly two-thirds of the browning jobs I get called to on wool-rich carpets are not browning at all. They are alkaline damage. The browning treatment – acid plus a reducing agent – lightens the chromophore just enough that the cleaner believes the diagnosis, takes the money, and gets a call four weeks later when it has settled back in.
The difference matters because the prognosis is different. Browning corrects. Alkaline yellowing is fibre damage, and damaged fibre does not un-damage.
The five-minute test I run before quoting a correction
Wet a small area with distilled water, press a pH strip flat into the pile for thirty seconds, read it in daylight. Above 8 on a carpet nobody has touched in a month means residue, and residue means the last process, not the spill. Then take a white cotton towel, dampen a corner with a 2 per cent acetic acid solution, lay it on the discoloured area and leave it ten minutes under a weight. Browning transfers – you will see it on the towel. Alkaline yellowing does not shift, and the towel comes up clean while the carpet stays yellow.
Two tests, both cheap, and they decide whether I quote for a correction or a conversation.
Can an acid rinse actually reverse yellowing?
An acid rinse arrests it. That is worth doing on its own – bringing the fibre back to 5 stops the hydrolysis continuing, which stops the discolouration deepening. It will not rebuild a cleaved disulphide bond.
Where there is genuine improvement to be had, it comes from a reducing agent. Sodium metabisulphite or sodium dithionite in an acidified solution will break down some of the chromophores and lift the tone, sometimes dramatically on a first pass. I mix at label strength, dwell for ten minutes, and rinse it out thoroughly with the acid rinse, because leaving a reducing agent in wool creates its own problems later.
Hydrogen peroxide gets suggested a lot for yellowed wool. It needs an alkaline environment to do anything, which reintroduces the exact condition that caused the damage, and on a carpet already down to bare keratin the second round is worse than the first.
Knowing when to stop
Two passes with a reducing agent, and if the third pass would be needed, the third pass is not going to fix it either. Diminishing returns arrive fast, and every cycle costs you a bit more fibre strength.
If a cleaner has quoted you for three correction visits on a yellowed 80/20, do not pay for the third. Have the honest conversation about a rug over the worst of it, or a section reweave from the offcut in the loft.
What should you ask before anyone puts a machine on an 80/20?
Ask for the pH of the prespray at use dilution, and ask what the rinse is buffered to. Anyone who cleans wool regularly answers both without hesitation and will probably show you the strips. Ask whether air movers arrive with the wand or afterwards as an upsell.
WoolSafe approval is worth something, and less than people think. The certificate belongs to the product, not the operator, and I have watched an approved detergent used at four times the label dilution on a stair run in Kew, off Sandycombe Road, by a man who was very proud of his certification sticker.
Between cleans
Vacuum often, with the beater bar set high enough that it agitates rather than scrapes. White vinegar diluted one to five handles most fresh spills. Keep the oxygen-based supermarket stain removers away from wool – percarbonate, alkaline, the same problem in a smaller bottle.
The Mill Hill drawing room came back about seventy per cent after two reduction passes and an acid rinse. The owner moved a Heriz over the darkest section near the fireplace and said she had stopped looking at it. Three months and one insurance claim, after a job that should have taken an afternoon.