We might think we know the colour of dust. Wisps of grey at the edges of rooms, a dull powder that coats unloved surfaces, it is a trace of our bodies (our skin cells, hair, bacteria) as they rub against the world (pollen, mites, soil). Household dust has no use and no beauty: it must be prevented against, swept away. Dust also appears memorably in Genesis as a reminder of the transience of mortal flesh: you are dust, and to dust you shall return. No wonder we are so phobic about its presence in our homes…