Showers, limescale and soap scum

Where the obvious answer is wrong

Cases where the treatment that removes the problem is the treatment that ruins the surface it is on.

• How do I remove limescale from a marble shower without ruining the marble?

  Marble is also calcium carbonate. The acid cannot distinguish between carbonate deposited by hard water and carbonate that is the stone itself, so it dissolves both. The result on the stone is etching: a dull, slightly rough patch where the polished surface has been chemically removed. It is not a stain and there is nothing left on the surface to clean off.

• Can I use a limescale remover on the grout in my shower?

  Cementitious grout is alkaline based and is dissolved by acids — the tile trade states this plainly and draws the conclusion itself, that manufacturers do not advocate acids because they attack the cement in the grout. Grout is also porous, so an acidic product does not merely touch the surface. The damage is gradual and looks like wear rather than like chemical attack, which is why it is rarely attributed to the cleaner that caused it.

• How do I get hard water spots off a bathroom mirror without wrecking it?

  A mirror is not a pane of glass. It is glass with a reflective coating and a protective backing, and both are exposed at every edge. The glass industry's mirror division states that acid or alkali cleaners can attack the front surface, the edges and the backing, and names cleaners containing ammonia or vinegar specifically as ones not to use. The failure is called black edge, it starts where liquid runs into the perimeter, and it is degradation of the mirror rather than dirt on it.

• Can I use a limescale remover on my bathroom taps?

  The visible part of a modern tap is a plated, physical-vapour-deposited or powder-coated finish a few microns thick, not solid metal. The manufacturer's care sheet states that PVD finishes must not meet cleaners containing ammonia, bleach or acid, which excludes most limescale removers and a good share of bathroom sprays. Because the finish is microns thick there is no margin: material lost is coating lost, the dulling is progressive, and it cannot be polished back because there is nothing underneath to polish.

• How do I get limescale off a shower screen without damaging the glass?

  The glass association's cleaning paper excludes acids from glass cleaning solutions as a class, naming hydrochloric and hydrofluoric as its examples, and warns that glass can be permanently damaged if it is infrequently or improperly cleaned. It also rules out the two things people escalate to when the acid does not work: abrasive cleaning materials, and razor blades or metal scrapers, which are excluded from routine cleaning altogether. The mechanism it gives for scratching is not the cleaner but trapped grit, so scrubbing a gritty screen is worse than leaving it.

Assessed and found wanting

Widely repeated techniques in this area that the evidence does not support, or supports only in part.

• Natural cleaning products are safer

  Does not work

  Natural describes where something came from and predicts nothing about what it does. The UK regulator lists this belief among the myths it exists to correct.

• Mixing vinegar and bicarbonate of soda

  Does not work

  The most popular cleaning combination on the internet, and the two ingredients largely cancel each other out. Both are useful — separately.

• Vinegar as an all-purpose cleaner

  Disputed

  Genuinely excellent at one job and promoted into a universal one. The surfaces it is most often recommended for online include several it permanently damages.

• Granite is acid-proof, so vinegar is fine on it

  Disputed

  The stone genuinely resists kitchen acids. The worktop is not only the stone, and the trade still does not recommend acidic cleaners on it.

• Using a limescale remover on shower grout

  Does not work

  It removes the scale and dissolves the grout. Cement is alkaline, and the tile trade says acids attack it — which is what most bathroom products are.

• Mould needs bleach

  Does not work

  The environmental regulator does not recommend biocides as routine, directs detergent and water, and says the part everyone misses: killing mould is not enough, because it must also be removed.

• Vinegar for mirrors and glass

  Does not work

  The glass industry's mirror division names vinegar explicitly among the things not to use. So does it name ammonia — which is what the shop-bought alternative contains.

• Never use an abrasive in the bathroom

  Disputed

  Right about the bath, the taps and the mirror. Wrong about the toilet and basin, where the manufacturer permits a soft abrasive outright.

• Putting a cleaning block in the cistern

  Does not work

  The manufacturer's instruction is not that it works poorly. It is that it can damage the flush valve and other working parts.

• Toothpaste as a metal polish

  Does not work

  A mild abrasive in a tube, and the tap manufacturer names it specifically among the things that must not be left on the finish.

• Scraping glass clean with a razor blade

  Does not work

  The glass industry excludes blades and metal scrapers from routine cleaning outright, and the scratches may only become visible months later at the right sun angle.

• Cloudy dishwasher glasses can be cleaned

  Disputed

  Sometimes they can, and sometimes the cloudiness is etching — which the manufacturer states is permanent. The two look identical and have opposite answers.

• Descaling a steam iron with vinegar

  Does not work

  Widely recommended, and prohibited by the appliance manufacturer by name — with the damage described as the same symptoms scale itself produces.

• Soaking a shower head in descaler overnight

  Disputed

  The chemistry is right and the fitting is the problem. Most shower heads are plated, and the trade excludes acidic cleaners from plated finishes.

• One spray for the whole bathroom

  Does not work

  The most consequential belief in the house, because a bathroom puts five materials with contradictory rules within arm's reach of each other.

The records

• Shower

  Thing to clean

  Four incompatible surfaces within arm's length of each other, permanently wet, and the two commonest deposits need opposite chemistry from the grout that joins them.

• Shower tray

  Component

  Where everything that ran down the walls collects, in a material that is usually acrylic or stone resin — and abrasives are excluded from both.

• Shower head

  Thing to clean

  Blocked by the same carbonate deposit as a kettle, and finished in the one coating that most descaling products are excluded from.

• Shower hose

  Component

  A flexible metal or plastic conduit with a rubber liner inside it. The liner is what a long acid soak destroys, invisibly.

• Limescale

  Stain or problem

  A hard, chalky calcium carbonate deposit left by hard water. Dissolved by acids, essentially untouched by detergents.

• Soap scum

  Stain or problem

  Not scale and not dirt: a metal soap formed when hard water meets true soap. It is the reason a shower screen films over in one house and not another.

• Insoluble metal soaps

  Science

  A metal ion meets a fatty acid and makes something that will not dissolve in water. It builds up invisibly, then oxidises — which is why some stains appear long after the event that caused them.

• Water hardness

  Science

  Dissolved calcium and magnesium picked up from the ground. It decides how much detergent you need, what your kettle looks like inside, and whether surfaces dry clean.

• Evaporative deposition

  Science

  The mark is made when the water leaves, not while it sits. Everything dissolved in it stays behind on the surface.

• Descaler

  Cleaning agent

  A product category rather than a chemical: an acid chosen to dissolve carbonate scale. Which acid decides which surfaces the bottle will ruin.

• Squeegee

  Equipment

  A rubber blade that removes water before it can evaporate. It is the difference between a clean window and a window with the map of its own drying on it.

• Grout

  Material or surface

  Cement-based grout is porous and alkaline, and acids dissolve it. That single fact rules out most of what bathroom cleaning advice recommends for it.

• One spray for the whole bathroom

  Claim or technique

  The most consequential belief in the house, because a bathroom puts five materials with contradictory rules within arm's reach of each other.