There's a particular kind of disappointment in spotting a black speckled line running along the silicone where your bath meets the tiles. You wipe it, it fades, and three days later it's back. That's normal, because mould doesn't simply sit on sealant — it grows into it. Silicone is slightly porous, and the film of soap scum, shampoo and body oils that builds up along the seal feeds it beautifully. Warm, steamy air does the rest. The good news is that most light to moderate growth can be dealt with using things you already have in the cupboard, and you'll know within a couple of days whether the sealant is worth saving or better off replaced.

Why mould settles into sealant in the first place

Surface mould sits on top of the silicone and comes away with a scrub. What you're usually looking at, though, is staining that has worked its way into the body of the sealant, which is why a wet cloth and a bit of bathroom spray does nothing. Silicone develops microscopic pits and scratches over time, and once mould gets a foothold in those, it keeps regrowing from below.

Age matters too. Sealant that has started to lift at an edge, feel slightly slimy, or pull away from the bath creates a gap where water sits. That's a much bigger mould problem than the sealant itself, and no amount of cleaning will fix it. For most people, mould on a sealant line is a cosmetic nuisance rather than a health issue, but if anyone in the house has asthma, allergies or a weakened immune system, it's worth taking seriously — and worth getting a professional opinion if it keeps returning.

What you'll need before you start

• Bicarbonate of soda and a small dish
• White vinegar or lemon juice in a spray bottle
• An old toothbrush and a few cotton buds
• Cotton wool and cling film (for the bleach method)
• Kitchen roll or a clean microfibre cloth
• Rubber gloves and eye protection
• A silicone sealant remover, if replacement looks likely

Open a window and put the extractor fan on before you touch anything. And one absolute rule: never mix bleach with vinegar, lemon juice, descaler or anything ammonia-based. The resulting fumes are genuinely dangerous.

The gentlest option: a bicarbonate of soda paste

This is where I'd start, because it lifts surface mould without attacking the sealant. Put two or three tablespoons of bicarbonate of soda in a dish and add water a few drops at a time until you have a paste about the thickness of toothpaste. Too runny and it slides off the vertical surfaces; too dry and it won't cling.

Smear it along the sealant line with your finger or a cotton bud, then work it in with a damp toothbrush using small circles. Leave it for ten to fifteen minutes, scrub again, rinse with warm water and dry the seal properly with kitchen roll. Repeat every other day for a week if the staining is stubborn.

You'll see plenty of advice about adding vinegar to the bicarb for the fizz. The reaction is fun but it mostly cancels both ingredients out. Use one or the other.

Vinegar and lemon juice for light staining

Neat white vinegar sprayed along the seal and left for twenty to thirty minutes will deal with fresh, surface-level growth and leave the bathroom smelling sharp rather than chemically. A soaked strip of kitchen roll pressed against the sealant line works better on vertical joints, because it keeps the liquid in contact instead of letting it run down the tiles.

Two cautions. Vinegar is acidic, so keep it away from natural stone such as marble or travertine, and test it on coloured grout first. And don't leave it sitting on the sealant for hours on end, day after day — repeated long exposure can dry out and harden the silicone, which is the opposite of what you want.

Using bleach carefully when nothing else works

For staining that has sunk in properly, a bleach-based bathroom cleaner is the realistic answer, but it needs handling with respect. Gloves on, eye protection on, window open, fan running. Never combine it with any acidic cleaner.

1. Twist cotton wool into a rope roughly the length of the joint.
2. Soak it in diluted bleach and press it along the sealant line.
3. Cover with cling film so it doesn't dry out.
4. Leave for a few hours — not overnight, and certainly not for days.
5. Remove the cotton wool, ventilate the room well and rinse thoroughly.

Keep bleach off chrome and other metals, where prolonged contact can pit the finish. And don't make this a weekly habit: repeated bleach soaking breaks down silicone, leaving it brittle and prone to leaking, which creates a far worse problem than the mould you started with.

When to stop cleaning and replace the sealant

If the black staining returns within a few days, if the sealant has cracked, shrunk or lifted away from the bath, or if you can see mould behind it, you're wasting your time. Cut it out and start again.

Score along both edges with a sealant remover or a sharp knife held flat against the surface, pull the old bead away, then clean the joint thoroughly with a mould-killing product and let it dry completely — a full day is ideal, because trapping moisture under fresh silicone guarantees a repeat performance. Apply a good-quality anti-mould sanitary silicone, cut the nozzle at an angle, run a steady bead in one pass and smooth it with a silicone tool. Leave it to cure for the time stated on the tube before anyone showers.

Keeping it clear afterwards

• Run the extractor fan for ten to fifteen minutes after every shower.
• Squeegee the walls and the sealant line, then dry the silicone with a towel.
• Deal with dripping taps and leaking shower screens promptly.
• Wipe the seal weekly with a mild cleaner rather than waiting for a deep clean.
• Skip abrasive scouring pads — they scratch the silicone and give mould somewhere to hide.
• Don't leave wet towels or bath mats resting against the seal.

A final word on safety and when to call someone in

Ventilate, wear gloves, never mix cleaning chemicals, and test anything acidic on a hidden patch first. If mould covers more than the sealant line — ceilings, walls, or the corner behind the shower — the cause is almost certainly damp or poor ventilation rather than dirty silicone, and it's worth having a plumber or damp specialist take a look before you reseal anything.

Photo: Brett Jordan / Pexels