Why won't the green come off my stone?

Read the pattern, not just the colour. Algae is a living film that spreads across everything in the damp, shaded spots — the stone, the mortar, the joints — and it feels slightly slimy when wet. Vanadium is a mineral deposit that sits in patches on the face of the brick or stone only, not the mortar, and it turns up in dry, sunny areas too, where algae would never survive. The clincher is the wash test: a moss and algae cleaner or a bleach solution knocks algae back, at least for a while. If you've scrubbed, bleached and pressure-washed and the green hasn't budged, you're almost certainly looking at vanadium, and the next move matters a lot.

Vanadium is a trace metal that occurs naturally in the raw clay and minerals of many light-coloured bricks, some porcelain pavers and some natural stones such as sandstone and lighter granites. It sits in the material as soluble vanadium salts. When water moves through the stone — rain, groundwater, reticulation, or the water used during construction — it dissolves those salts and carries them to the surface. As the surface dries, the water evaporates and leaves the salts behind on the face. In an acidic environment they crystallise green. It's the same salts-riding-water mechanism as efflorescence, only the deposit is coloured instead of white.

Because acid is exactly what turns vanadium green and sets it. The salts are close to invisible until the conditions tip acidic, and then they crystallise into that green deposit. Hydrochloric (brick) acid is a very common step in cleaning brick and paving, so people unknowingly trigger the very stain they're trying to remove, or take a faint stain and lock it in hard. Even naturally acidic rain can bring it on. That's why the instinct that works on general grime — hit it with acid — is the worst thing you can do here, and why so much vanadium staining is self-inflicted during a clean-up.

Often, yes, but with the opposite chemistry to what most people reach for. Because the green forms in acid, it's neutralised with an alkali — a sodium or potassium hydroxide based cleaner, or a purpose-made vanadium remover, used carefully and rinsed properly. A stain that hasn't been acid-fixed will also fade on its own over months as clean rain slowly leaches the soluble salts out of the surface. What doesn't work is bleach, a stiffer brush or more acid. On valuable stone, get it identified and treated by someone who works with masonry stains rather than experimenting with the shed chemicals.

A seal isn't a cure — the vanadium is inside the material and a coating goes on the outside, so it can't lift a stain that's already there. What a breathable coating does, once the surface is clean, stable and dry, is cut down the water soaking in and cycling through the stone, and it's that water movement that dissolves the salts and carries them up. Less water in, less salt to the surface, the same way a breathable seal slows efflorescence. The key word is breathable: you never want to trap a film over a salt-laden surface. Deal with the vanadium first, get it dry, then seal to keep future water from feeding it.