Crystallisation
Scale does not settle onto a tap, it grows there. A crystal nucleated on a surface is bonded into it, which is why wiping was never going to work.
In detail
Dissolved minerals leaving solution do not simply fall out as dust. They nucleate — begin forming a crystal at a favourable point, usually a scratch, an existing deposit or a heated surface — and then grow, incorporating the substrate's own texture into the crystal's base. That is a fundamentally different attachment from a particle resting on a surface, and it explains most of what is confusing about limescale. It appears fastest where water is hottest, because heating drives calcium bicarbonate out of solution, so kettles, elements and shower heads scale while cold pipes do not. It grows preferentially where scale already exists, which is why the problem accelerates once started. It is not soil, so it is chemically indifferent to detergent, and mechanical removal fights the substrate as much as the deposit. What actually removes it is a chemical conversion — an acid turning the insoluble carbonate into a soluble salt — and the honest corollary is that anything sharing that chemistry with the deposit shares its fate, which is the whole marble-and-descaler problem. Prevention works because nucleation needs the water to be there: dry the surface and no crystal starts.
Why
- Limescale is explained by Crystallisation
- Descaling is explained by Crystallisation
- Mineral salt soil is explained by Crystallisation
Sources for this page
Sources
How do I descale my Philips Steam Iron?
Philips · Manufacturer documentation
Relied on for: That the harder the water in an area the faster scale impurities develop and solidify inside a heating appliance. (states this directly)
Care & Cleaning of Natural Stone
Natural Stone Institute · Trade or professional body
Relied on for: That acids dissolve calcium carbonate, which is what both limescale and calcareous stone are made of. (states this directly)