This "practice calligraphy" paper has a special coating that amplifies the contrast between light/dark when a wet brush is applied.
The physics behind this effect has been explained:
- Why do wet objects become darker?
- Why does paper become translucent when smeared with oil but not (so much) with water?
From my understanding the refractive index of the liquid and roughened surface should be as close as possible (Eg: Glycerol 1.4729, and PMMA 1.4896) in order to maximize this effect. But this Pilot paper works better than any of my trials.
Can anyone suggest how it's made, or what materials/chemistry would work best?