Photographic history is littered with now obsolete processing methods, like the cyanotype or the daguerreotype. One of the advantages of today's digital darkroom is being able to revisit some of these processes without needing to acquire specialist materials and chemicals and engage in strange rituals in darkened rooms. One such process that I am really enjoying is the opalotype.
Opalotypes were produced on milky white glass plates and the resulting monochrome image was then hand tinted to produce pictures that were (according to Wikipedia) "close to watercolour or even pastel in its softer coloring and tender mood." I suspect that it is probably this quality that draws me to the effect for certain subjects; like the Old Iron Bridge near Twizel in South Canterbury .