Everyone who owns a shower eventually meets the black line: a speckled, stubborn stripe along the bottom of the tile, right where the caulk meets the tub. You scrub it. It fades a little. Two weeks later it's back, darker. I've seen it in brand-new bathrooms and hundred-year-old ones, and it's almost always the same story.
The good news is that it's fixable, and once you understand why it happens, it's very preventable.
What the black stuff is
That black speckling is almost always mildew, the everyday name for molds growing on damp surfaces. Mold needs two things: moisture and food. A shower offers both in abundance. The water is obvious. The food is less so: soap scum, body oils, shampoo residue, and skin cells all leave a thin film on the caulk that mold is happy to eat.
If you see pink or orange slime instead, that's usually a bacterium rather than a mold, a common shower resident that thrives in the same conditions. The cleanup and prevention are the same.
Not every dark stain is mold. Mineral deposits and dirt can discolor caulk too. A quick test: dab a little mildew cleaner or diluted bleach on the spot. If it lightens within a few minutes, it's mold. If nothing changes, it's probably staining or dirt worked into the surface.
Why caulk is such a mold magnet
- It stays wet the longest. The joint at the bottom of the tile is where water runs, pools, and lingers after every shower.
- Its surface holds onto grime. Soft sealant surfaces trap soap film more readily than glazed tile, and that film is mold's lunch.
- Hairline gaps let water behind it. Where the edge of the bead has lifted even slightly, water wicks behind the caulk, and mold grows where no scrub brush can reach.
- Mold resistance wears down. "Mold-resistant" caulk contains a mildewcide that discourages growth on the caulk itself. It helps a lot, but it can't stop mold from living in the soap film on top, and its protection can diminish over the years.
Is it on the caulk, or under it?
This is the question that decides whether you clean or replace. Clean the bead thoroughly (instructions below). If the black comes up, the mold was on the surface, and you can keep that caulk. If a good cleaning, even a long bleach soak, leaves gray or black shadows inside or behind the bead, the mold is growing in the joint underneath. No cleaner will reach it. The caulk has to come out.
How to clean moldy caulk
Ventilate and protect yourself
Run the fan, open a window if you can, and wear gloves and eye protection. If you have asthma, allergies, or a weakened immune system, consider having someone else do this.
Apply a mildew cleaner or diluted bleach
A bathroom mildew spray works, or mix a diluted bleach solution; the CDC suggests no more than one cup of household bleach per gallon of water for mold cleanup. Test first on colored grout, stone, or fixtures, since bleach can discolor them. Hydrogen peroxide (the 3% drugstore kind) is a gentler alternative some people prefer.
Use a poultice for stubborn stains
Soak a paper towel or strips of cotton in the solution and press it along the bead so it stays wet against the caulk. Leave it for a few hours with the room ventilated and kids and pets out, then remove it. This long contact time often lightens stains that a quick spray won't touch.
Scrub gently and rinse well
Use a soft brush or non-scratch pad. Hard scrubbing can rough up the caulk surface and give mold more to hold onto next time. Rinse thoroughly.
Dry it
Towel off the joint and leave the fan running. A dry joint is a hostile joint, as far as mold is concerned.
Bleach mixed with ammonia, or with acids such as vinegar and some toilet-bowl cleaners, releases toxic gases. Use one product at a time, rinse between them, and keep the room ventilated.
When to replace it
Replace the caulk if you see any of these:
- Black or gray stains that survive a thorough cleaning and a poultice.
- Edges that have lifted, peeled, or cracked, even slightly.
- Soft, gummy, or crumbling sections.
- Water visibly getting behind the bead, or mold that returns within a couple of weeks of cleaning.
How to redo it so it stays white
Most black caulk comes back because the redo skipped a step. Here's the version that lasts.
Remove all of the old caulk
Every bit, plus the silicone film it leaves behind. Our removal guide walks through it.
Kill the mold in the joint
With the caulk out, treat the exposed joint with your mildew cleaner or bleach solution, let it work, then rinse.
Dry it for real
This is where most redos fail. Let the joint dry for at least a day, longer if the gap was wet deep inside, with the fan on and no showers.
Wipe with isopropyl alcohol
It removes the last traces of soap and oil and evaporates quickly.
Use the right caulk
Choose a 100% silicone labeled for kitchen and bath, with mildew resistance, such as the one in our silicone review. Siliconized acrylic is easier to work but less durable in a shower.
Fill the tub, tape, caulk, and tool
Filling the tub with water first puts the joint at its widest before you seal it. Then follow our step-by-step method.
Let it cure before the first shower
Commonly about 24 hours for silicone, though some fast formulas are shower-ready sooner. Go by the tube.
Keep it white
- Run the exhaust fan during every shower and for a good while after. Twenty minutes or so is a common rule of thumb; a timer switch makes it automatic.
- Squeegee the walls and the caulk line after showering. It takes half a minute and removes most of the water mold needs.
- Leave the curtain or door open so the enclosure can dry.
- Keep indoor humidity in check. The EPA recommends keeping indoor humidity below 60%, ideally between 30% and 50%.
- Clean the caulk line weekly with a mild bathroom cleaner, before soap film has a chance to build up.
- Fix drips and leaks at the faucet and showerhead, and replace caulk at the first sign of lifting.
When it's not just the caulk
Sometimes black caulk is a symptom. If mold keeps coming back quickly in the same spot, or you notice loose or hollow-sounding tiles, crumbling grout, a musty smell, or stains on the ceiling below, water may be getting into the wall or floor behind the tile. New caulk won't fix that. Have a pro take a look. And for mold covering a large area, the EPA suggests that anything bigger than about 10 square feet is a job to consider handing to a professional.
For everything else: clean it, and if it won't come clean, cut it out, dry it out, and start fresh. Your tub will look years younger, and so, frankly, will you.