Selenium is hardly mentioned in various 'modern' darkroom books (from 60s-80s) I learned the craft from many years ago. Reading various forums online, selenium seems a must nowadays, a step that shouldn't be omited if one is serious about their processing. When I started printing, about 20 years ago, toning was considered a non essential step which can be applied to alter tones of prints. Nothing more. When did selenium toning become popular?