I get the call on the Tuesday of half term, with a vacuum running somewhere in the background. A flat in Barnes. Her nine-year-old had been making slime on his bedroom floor since the Sunday, a tub of the turquoise stuff had gone over by the radiator, and she had spent most of an hour on it with white vinegar and a kitchen cloth. The slime was off. That part had worked. What she was left with was a turquoise patch the size of a dinner plate, and the carpet had been oatmeal before the weekend.

That is the shape of nearly every slime job I get called to. The polymer is gone by the time I arrive. The colour is the problem, and the colour is there because of what was used to shift the polymer.

Why does vinegar make a slime stain permanent?

Slime holds together because borate ions from borax or contact lens solution bridge the long chains of PVA glue. Drop the pH and those bridges break. The mass goes slack, loses its grip on the yarn and comes away on a cloth. Vinegar works, which is precisely why the advice has spread so far.

The trouble is what else acid does in a carpet. Wool and nylon both carry amine sites along the fibre, and in acid conditions those sites open up and grab anionic dye. That is the method the mill itself uses: acid dye, low pH, heat, agitation. A woman kneeling on a bedroom floor with vinegar, a warm cloth and forty minutes of rubbing has reproduced the dyehouse process at domestic scale, on a patch she chose herself.

I will say something unpopular here. The white vinegar advice that circulates on parenting forums and in half the videos you’ll find on this has done more damage to more carpets than any product sold in a shop. Washing-up liquid at least only leaves residue. Vinegar changes the fibre’s willingness to take colour, and once colour is in, it is in on the same terms as the carpet’s own dye.

What the colouring in a £3 tub is made from

Most ready-made slime is tinted with food colouring, and food colouring is largely acid dye. Brilliant blue, carmoisine, tartrazine. The same family that makes blackcurrant squash and children’s paracetamol suspension such miserable calls. Craft pigment and cosmetic mica behave differently, sitting on the surface rather than bonding, which is why a glittery clear slime is a mechanical problem and a neon tub is a dyeing problem.

Look at the colour before you touch anything. Clear or pale slime, you can handle yourself. Neon pink, turquoise, that violent purple they all seem to want, be careful what you put on it. Homemade slime is usually worse than shop-bought on this count, because the tube of gel colouring from the supermarket baking aisle is stronger than anything a toy manufacturer would put near a child’s hands.

What takes the polymer out without dyeing the carpet?

Cold and patience. Add no liquid at all to begin with.

If the mass is still soft, bag up a freezer block or a handful of ice cubes in a poly bag and rest it on top for ten minutes. The slime goes brittle at the surface. Then work with the bowl of a teaspoon, or a blunt scraper if you have one, chipping inwards from the edge of the patch so you aren’t dragging material into clean pile. Work across the twist rather than along it. Vacuum between passes so you can see what’s left.

What remains after that is a thin PVA film dried into the yarn, and this is where a small amount of warm water and a neutral detergent earns its place. Dampen, leave it five minutes to swell the film, tamp with a soft brush, blot with a white towel. Isopropyl alcohol softens dried PVA where water alone is losing, and it carries no pH consequence. Keep the heat down throughout. Warm water is fine, and the steam cleaner stays in the cupboard, because heat drives dye into fibre faster than anything else you have in the house.

The scraping stage everyone rushes

Twenty minutes of chipping saves the next three hours. Nine times out of ten, when I’m told the slime “wouldn’t come out”, what happened is that somebody spent four minutes on the mechanical stage and then reached for a bottle to do the rest of the work. The bottle cannot do the rest of the work. It can only move the problem from a polymer you can remove to a dye you cannot.

Where does the glitter go, and does it ever come back out?

Down. Craft glitter is cut polyester film, somewhere between a fifth and a full millimetre across, and it carries enough static to cling to individual filaments. Trodden slime carries it past the pile surface and into the backing, and some of it will find the underlay through the primary backing’s weave.

You get it back in stages, and only some of it.

The first instinct is to attack the patch with the hose attachment, which pulls at the pile and lifts very little, because the flakes are held by static against the sides of the filaments rather than lying loose on top of them. Brush the area up with a stiff hand brush first to lift the pile and bring buried flakes toward the surface, then vacuum slowly in two directions with the beater engaged. Slowly. A fast pass moves air and leaves the flakes where they sit. Repeat over several days rather than trying to finish it in one session.

I cleaned a first-floor bedroom in Walthamstow three separate times across a spring, and glitter came up on the third visit that had not been there on the second. It comes up when the household walks it up. There is no method that gets all of it, and I would rather tell you that now than take money for a fourth attempt.

Why the vacuum brings more up a week later

Flakes sitting in the backing migrate upward under footfall, the same way sand works its way out of a hall carpet after a beach holiday. The floor is releasing what was already inside it, a few dozen pieces at a time, for months.

Does the fibre under the slime change the answer?

Completely, and it decides whether this is a twenty-minute job or an insurance conversation.

Wool takes acid dye permanently and takes it beautifully. Nylon behaves almost identically, which is why a nylon carpet and a bottle of Ribena is a job I price carefully. On either fibre, colour set by vinegar needs a reducing agent under controlled heat, and the reducer cannot tell the difference between the slime’s dye and the carpet’s own. Strip too far and the blue patch becomes a pale patch, and then you are into recolouring, which is a different trade and a different bill.

There is a second reason to slow down on wool. Reducing agents work on a timer you cannot see, and a bedroom carpet that has already had vinegar and forty minutes of rubbing has a damaged, swollen fibre surface that responds faster than a fresh one. The patch you are treating is no longer the carpet the manufacturer made.

Polypropylene is the reprieve. It has no dye sites to speak of, because the colour goes in as pigment before the fibre is extruded. Acid dye has nothing to bond to, so the colour sits on the surface and detergent lifts it. A polypropylene bedroom carpet with slime on it is a nuisance rather than a loss. Its weakness is oil, and slime has none.

Wool, nylon or polypropylene, decided in ten seconds

Pull a fibre from an edge or a wardrobe corner and hold a flame to it. Wool smells of burnt hair and crushes to ash. Nylon melts to a hard grey bead with a celery smell. Polypropylene melts to a waxy bead and smells of a candle. Failing that, look at the house. A child’s bedroom in a new-build or a recent rental is polypropylene twist nine times out of ten, and most wool in London bedrooms is either pre-2000 or in a house where somebody spent properly on the stairs and carried the same carpet upstairs.

When is paying someone to treat a slime patch a waste of money?

More often than my invoicing would like.

If the carpet is polypropylene and the slime was clear or white, do not call anyone. Ice, spoon, vacuum, a little detergent. There is nothing I bring in the van that improves on that, and a minimum charge to watch me do what you could do on a Saturday morning is money thrown away. I have talked people in Ruislip and Forest Hill out of appointments on the phone for exactly this, and I’d do it again.

If the vinegar has already been on a wool carpet, be sceptical of anyone who promises full removal. Ask what they intend to use, ask what happens to the carpet’s own colour if the reducer runs on too long, and ask what the patch looks like if it goes wrong. A pale ghost in the middle of an oatmeal bedroom reads worse than a faint blue shadow under a bed.

The other thing I turn down is the whole-room clean booked because of one patch. Cleaning the other ninety-five per cent lifts its colour and its pile, and the treated area, which now sits slightly cleaner or slightly lighter than it did, stops blending. Spot work stays spot work.

What a spot visit costs against a whole room

A spot call-out sits near the low end of most London price lists because of the minimum charge, and a bedroom clean is often only a little more. That gap is what pushes people into booking the room. Nine times out of ten they would have been better served by the smaller job and a bit of honesty about what the patch will look like afterwards.

The Barnes carpet still carries a shadow under the far corner of the bed, visible in the morning light and not much else. Her son makes his slime on a plastic tray on the kitchen floor now, which is where it should have been on the Sunday.