Don't know if it's a combination of heat (Josepi's link?) and the
chemical agent, but using K2Cr2O7 in direct sunlight will darken your
wood a heck of a lot faster and with a darker shade, than applying it
in a non-sunlit shop/environment. My uncle would apply it and, while
still wet, flash it in direct sunlight for about 30 to 60 seconds and
you could watch the darkness evolve in that time frame... darker and
faster darkening, than when not exposed to sunlight.

Sonny