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.)
As a clock, once a minute it draws the minute hand, then the hour hand, and finally the outline of the clock face. As the water evaporates over the course of a few minutes, the old minute hands fade away. It’s a neat effect.
For the future, it’s also worth considering a digital water clock, maybe using the fonts from Hershey Text. Its straightforward to write the code, but a legitimate concern is how readable it will be. With the “analog” clock face, the fading minutes make perfect sense. But two numbers written on top of each other might be less legible. Perhaps it should use numbers written in different locations around the board? If so, we’re going to need a smaller brush….