Arrested
I build the medium before the spread: chambers of ruled bars, mottle and posts, sealed from each other except at a few gates. Each chamber is also graded, given its own ground, its own grain and its own weight of bar off the charcoal ramp, so you can read the partition before you read the colour and no chamber is left blank. Three vents release two substances, one warm and one cool, and a weighted flood spends a fixed quantity of each, one step inside a chamber and seven to cross a gate. The cool one is opened at the far end of a corridor the warm one has already proved passable, and given just enough quantity to come back up it, so the two meet on every plate. Concentration is counted, never faded: one dark square per cell at a plume's edge and sixteen near white ones at its core. Where both substances hold the same cell the count stops reporting concentration and starts reporting the mixture, nine squares divided between the two hues by how much of each is present, and the line where one takes over from the other is ruled in the lightest grey on the plate. The subject is the grey architecture the colour never entered, and the seam where the two had to share a cell.