Sepia
Thanks to the smartphone and all its apps with filters to apply on your photos, everyone’s heard of “sepia”. Here’s a pic with the sepia effect: Sepia is a 130 year-old technique (!) of using a toner while developing photographs. It only works on black and white photos: “The print is first soaked in a potassium ferricyanide bleach to reconvert the metallic silver to silver halide. The print is washed to remove excess potassium ferricyanide and then immersed into a bath of toner, which converts the silver halides to silver sulfide.” The outcome is lots of shade of brown, but ultimately, it’s all brown and white. Therefore, technically speaking, sepia is still a monochromatic image. Why did this method evolve? For aesthetic and practical reasons. It gives the pic a slightly warmer effect. And because of those chemicals involved in the different steps, the outcome is a photo which is more resistant to exposure to the elements. Longer lasting photos, in other words....