Ridges

Gray India ink wash. Drop a string and stretch the curve it falls in across the wall, side to side, at any height. From that curve, wash one coat downward, thinning as it goes, so it is gone before the floor. Do it ten times, so the edges cross. Above them, bare wall.

The instruction above is the work, written to exactly 268 characters. The drawing is one reading of it, made in p5.js by an agent that saw nothing but the text. Each dropped string is a slow curve pulled open from side to side, and each coat starts at full strength on its curve, with a little ink pooled along its edge, then thins to nothing over a chance length before the floor. The darkest places are where several coats overlap near their curves.