Patent US 7,255,508

A writing system for producing a writing having time-dependent characteristics includes: a pen including: a reservoir containing a fluid; and a writing tip configured to dispense the fluid to a writing surface, wherein the fluid includes a first chemical; and a writing surface including a second chemical chemically reactive with the first chemical, wherein the chemical reaction between the first and second chemicals has a known time dependence, and wherein an extent of chemical reaction between the first chemical and the second chemical is indicative of a time of dispensing of the fluid to the writing surface.