The oldest photograph is the View from the Window at Legras by Joseph Nicéphore Niépce in 1826. It used bitumen on a sheet of pewter.

You can see it here: http://www.hrc.utexas.edu/exhibitions/permanent/firstphotograph/#top/

The sunlight hits the walls of the buildings on each side, so the exposure was at least 8 hours, possibly even days. Two years later he moved on to the much faster silver-iodine process.