Evil Mad Scientists have demonstrated a great application for their Watercolorbot -- a plotter that paints with water colors. By drawing on a Buddha Board (a board whose coating goes transparent when wet, then reverts as it dries), they can produce ">a Watercolorbot Clock that very slowly counts away the hours.
We built a evaporating-hand water clock using a WaterColorBot fitted with a Buddha Board. The Buddha Board is a black board with a gray ceramic coating that becomes transparent when wet, so you can paint on it with plain water to make black marks that disappear as the water evaporates. (And, it fits nicely in a WaterColorBot with the appropriate jig.)
A WaterColorBot Water Clock [Windell/Evil Mad Scientists Labs]