Bromine – (Br – Atomic number 35 – and one of the halogens) has one very obvious use in photographic emulsions. Of course there shouldn’t be any bromide left after development and fixing – but looking at some of the discoloration perhaps there is some here.
Bromine comes from the word meaning ‘stench’ and I remember from my lab days that it lives up to its name – not that you’d go around inhaling this stuff as it is quite toxic. Many uses of bromine, such as in flame retardants and gasoline additives in the days of leaded petrol, have stopped due to health concerns (apparently it was even used in an emulsifier in Mountain Dew!). It is still used today in many organic chemical syntheses. My topic here is its use in photography as a light sensitive compound with silver – silver bromide – AgBr. Light hitting an emulsion of silver bromide (often mixed with silver chloride, and other compounds to affect the sensitivity and grain) will turn some of the silver ions into elemental silver – the latent image. Developers then selectively produce silver in relation to the latent image – producing the negative image. The prints above were created in the same way when the light is passed though the negative to get a positive print. In the days of black and white the films would have a fairly even sensitivity across the light spectrum – but photographic papers could just be sensitive to the blue end of the spectrum, hence the use of ‘safe-lights’ – where a deep red filter would enable oe to see what they were doing in the darkroom without fogging the paper (at least with short exposure to the safe-lights. I often smile at TV films where they will show a dark room with red lights magically producing color pictures…
A photographic plate (glass with a covering of a silver halide emulsion) was responsible for helping identify a strange property of the next element in my series as I jump up to Atomic number 92 – and Uranium.
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