Dust
A window pane under a coat of dust, to draw in with your finger. A fingertip pushes the dust aside into ridges and drops a little heap where it lifts; sticky dust smears instead of lifting clean. Choose the dust (Saharan, household, sand, soot, pollen, ash), let rain fall on it, tilt the pane so gravity takes over, and run the clock forward until your drawing is buried again. Behind the glass are slogans from George Orwell’s 1984 and the film They Live, or a picture of your own.
Things to try
- Write your name. Then look closely along the edges: the dust your finger pushed aside stands in ridges, and a heap sits where you lifted off. Switch the light to "from behind" and the same strokes turn bright.
- Sticky versus dry. Draw in soot or pollen and the finger smears a film; draw in sand and it lifts clean, but the loose ridges on a steep pane slide down and bank against the frame. Raise the humidity and everything gets stickier.
- Rain on dust. Turn on rain over a dusty pane: each drop gathers the dust under it and, as it dries, leaves it in a ring at its edge (the coffee-ring effect). Big drops run, scour a clean channel and stop in a muddy head. Tilt the pane flat and nothing runs.
- Dusty rain. Wash the pane, choose Saharan dust and switch on dusty rain: this is how Saharan dust usually reaches Europe, and why cars wake up covered in orange spots.
- Breathe on it. Fog the glass, then draw while it is fogged; the fog dries, and dust that was wet clumps into a crust.
- Abandon it. Turn up the dust in the air and the clock: your drawing fills in and fades, like the old writing on the windows of a ghost town.
How it works
The pane is 600 by 400 mm in 0.8 mm cells. Each cell holds loose dust and dried crust as a mass per area (grams per square metre) and the mix of dust kinds in it, so colour, stickiness and grain blend when kinds meet. What you see follows from that mass: the light let through falls off exponentially with it, at a rate set by the dust (soot blocks far more light per gram than sand), and sunlit dust scatters its own colour back. A coat of a few grams per square metre, like a Saharan dust day, is enough to hide most of the view.
The finger lifts most of the dust under its pad, less if the dust is sticky, and pushes it sideways into ridges; it carries a small heap ahead of it, smears a little behind and drops the rest where it lifts. A cloth soaks dust up; a brush flicks it away; blowing moves only dry, loose dust. Raindrops are particles: a drop takes up the loose dust it lands on, and when it dries its edge stays pinned, so most of the dust ends in a ring. A drop starts to run once it is too big for its edge to hold it on the tilted glass, and it stops when it has left enough of itself behind. Dry dust slides when it is piled thicker than the slope lets it hold.
Settling and drying use the clock, which you can run faster than real time; drops running, dust sliding and breath fog drying happen at their real pace. The numbers are simplified but chosen to stay in realistic ranges.