Why Mallorca is so good at making them
The U.S. Geological Survey classes water above 180 mg/L of calcium carbonate as very hard.1 Every figure we could find for Palma is well above that line, but they do not agree with one another. One table, calculated from published calcium and magnesium, gives 255 to 383 mg/L.2 A Mallorca plumbing page citing EMAYA’s analyses gives 40 to 90 French degrees, which is 400 to 900 mg/L.3 We could not get hold of an official report that settles it, and we would rather tell you that than pick the neater number. Your town’s water may differ, and water from a well or a cistern differs again. The practical point stands in every case: this is very hard water.
The usual sources on a house are sprinklers that catch the glass, the splash from a pool, an outdoor shower beside a window, a hose used to “rinse” the terrace doors on a hot afternoon, and the condensation or dripping that runs off a balcony rail. Rain is rarely the villain: water picks up its hardness moving through soil and rock,1 which rain has not yet done. It is the tap water, and the water that has run over something first.
How a mark forms
A drop lands on the pane. The water evaporates and the minerals in it cannot, so they stay in a ring where the edge of the drop was. Do that once and you have a mark that a wet cloth removes. Do it every evening for a summer and you have a layer, each ring on top of the last. Viracon, a glass maker, says routine cleaning is there to prevent the build-up of “potentially harmful mineral deposits”, and it asks for excess water to be taken off with a squeegee.4
Which stage do you have?
| What you see | What it probably is | What to do |
|---|---|---|
| White spots that fade when wet | Fresh deposit on the surface | Wash with mild soap and squeegee |
| Rings that stay when wet but feel rough to a fingernail | A built-up layer | Mild descaler, patiently, then rinse well |
| Marks that stay when wet and feel perfectly smooth | Etching: the glass itself has changed | Nothing will remove it. Keep it clean and stop the source |
The fingernail test and the wet test are our own rules of thumb, not laboratory methods, but they will tell you what you are dealing with before you buy anything. Long exposure really can damage the surface: Eckelt, a glass manufacturer, notes that long-term water effects can harm it, and that alkalis and acids acting over a long period lead to permanent etching.5
How to remove a built-up layer
- Clean the pane first with soap and water, so that you are not rubbing grit around.
- Protect what is below. Mask the frame if you can, and keep a wet cloth handy. Any acid that dissolves limescale will also attack limestone and marble sills, and metal coatings dislike it too.
- Apply a mild acid, such as white vinegar or a descaler sold for glass, on a soft cloth. Do not let it run. Work a small area for a minute or two.
- Rinse with plenty of clean water and squeegee dry. Look again. Repeat rather than scrub harder.
- Stop when it stops improving. What remains is probably etching.
Be clear about what this is. White vinegar is a household habit and our own judgement for plain glass, not a glass maker’s instruction: neither maker’s sheet we read recommends it. Eckelt says acids, particularly liquid acids, should never be used on metal-oxide coated glass, because they may irreparably damage the coating or the surface, and that acids left on for long periods etch glass permanently.5 So keep the contact short, and if you cannot tell whether a pane is coated, ask its maker or leave it. The glass trade’s bulletin says glass can be permanently damaged by improper cleaning, and asks cleaners to do one small area, then stop and look.6
What not to use
- Scouring powder or the rough side of a kitchen sponge. They scratch, and a scratch is permanent. Eckelt allows fine industrial steel wool on plain glass but rules it out on coated glass,5 and since you often cannot tell which you have, we would leave it in the shed.
- Razor blades as a routine tool. Viracon says they should not be used to clean glass, and Eckelt says they can leave fine scratches.45
- Hydrofluoric acid or other fluoride cleaners. Viracon says to avoid hydrofluoric acid cleaners on its etched and enamelled glass,4 and Eckelt says fluoride cleaners should never be used, because they may irreparably damage the coating or the glass.5
- Acid near stone. Vinegar on a marble or limestone sill leaves a mark of its own, and that one does not come off. The same goes for terrazzo.7
Stopping it coming back
Turn the sprinkler head, or change the watering time so that the glass stays dry. Move the outdoor shower’s spray away from the door. Dry the pane after you rinse it. Softened or demineralised water for the final rinse carries far less mineral to leave behind, which is the whole idea of a purified-water pole. A clean every few months removes deposits while they are still only deposits: see how often to clean.
When we clean, glass etched by years of hard water, or scratched, ends up clean but not flawless, and we point that out before we start so that nobody pays for a miracle. That is what a window clean can honestly do.