Calima and Saharan dust: how to clean glass safely

AEMET, the Spanish weather agency, defines calima as reduced visibility caused by dry particles in suspension, such as dust or smoke.¹ AEMET’s study traces the dust in Spain’s mud rain to the Sahara, carried by air masses moving ahead of depressions to the west and south,³ and you see it as a yellow-brown sky and a colourless sun. That part is harmless to a house. The trouble starts when it settles.

If a little rain falls through the dust, the drops bring it down with them and leave spots of mud as they dry: the “lluvia de barro” that turns every car on the island brown. AEMET’s study of mud rain in Spain, using 54 stations from 2003 to 2017, found the highest count of any station at Menorca, our neighbour, with 14 days a year.³ Across the peninsula, the Balearics and Melilla the main season runs from March to June, with May the most frequent month, and a second one from June to September.³ The study gives no figure for Mallorca itself, and we will not guess one. What it does tell you is when to keep an eye on the forecast.

Every glass maker’s cleaning sheet says some version of the same thing. Viracon asks for extreme care that no abrasive particles are trapped between the glass and whatever you are cleaning it with.² Eckelt warns against abrasive agents and says sharp tools can leave fine scratches.⁴ Dust from the desert is exactly the kind of grit they are worried about, and a dry cloth or a dry feather duster drags it across the surface.

You will not usually see the harm the first time. You see it years later, when low sun shows a fine haze of scratches that no clean will remove.⁵ A scratched pane stays scratched, which is why we mention it before we start on any job where we find it.

1. Wait. Check the forecast and let the episode finish. Cleaning between two waves of dust is a pleasant way of doing the job twice.
2. Rinse first, and generously. Soak the pane with clean water from the top down and let it carry the grit away. A hose on a gentle setting is fine at ground level. Do not use a pressure washer on glass with seals around it.
3. Rinse the frame and the sill too. Otherwise you will drag what is sitting there back across clean glass at the next step.
4. Wash with mild soap. A soft, clean, grit-free cloth and a mild, non-abrasive solution are what Viracon asks for. Avoid strongly alkaline cleaners, and rinse your cloth often in fresh water.²
5. Rinse again and squeegee straight away. Excess water should come off with a squeegee rather than being left to dry.² Mallorca’s water is hard, so what evaporates on the pane leaves marks of its own. Wipe the rubber blade after each pull.
6. Dry the edges with a separate clean cloth, then look along the pane in low light for anything you missed.

• No dry dusters, kitchen paper or newspaper on a dusty pane.
• No scouring pads, steel wool, razor blades or scrapers. Viracon says plainly that razor blades, scrapers and putty knives can scratch glass and should not be used to clean it.²
• No abrasive cleaners, polishes or steel wool on aluminium frames either: they can remove the coating, and Technal lists them as things to avoid.⁶
• Not in the full heat of the day if you can avoid it. Water dries faster than you can squeegee, which is how you turn a dust problem into a limescale problem. The glass trade’s bulletin also warns against cleaning tinted and reflective glass in direct sun, because the surface can be too hot.⁵

The dust that misses the glass lands in the tracks of sliding doors, where it mixes with salt and turns to a fine paste. Technal says to remove what collects between the rails with the small tip of a vacuum cleaner and its brush attachment, because debris there affects both how the door runs and how rainwater gets out. It also says not to lubricate the wheels of sliding doors, which are self-lubricating.⁶ So do not reach for the oil if a door grinds after a dust storm: vacuum first.

One ground-floor flat is an hour and a bucket. A villa with a wall of sliding glass, upper floors and a balustrade is a day, and the top of a ladder is a poor place to be careful about grit. If you would rather not, window cleaning is priced by count and size, with €7 for a small window and €24 for an extra-large one or a sliding door, and you see the total before you book. A shower the next day rarely spoils the result, but a fresh dust episode will, so it pays to wait for the forecast to clear.