Tin – Sn – Putting the reversal into reversal film

Tin (Sn – Atomic number 50) is a metal, except when it isn’t.  White Tin, or β Tin is a metal, but there is also another allotrope – α Tin – grey Tin which has no metallic properties at all.  This semi-conductor is stable under 13.2 °C, and can form from white Tin in a process known as Tin pest.  The most common uses of (white) Tin are as a component of solder and for coating steel to prevent corrosion – one example of which is to make ‘tins’ – or cans if you prefer. Solder was made from Tin and Lead – but other alloys with Tin are now used to avoid the toxic Lead.  Tin pest is more of a problem in Lead -free solder and can lead to joint integrity issues.  If you are not familiar with allotropes – carbon as diamond or graphite is a more common example.

The previous posting was about Yttrium – which had just one stable isotope – and in contrast Tin has 10 – due to it having a magic number of protons.  Tin has been used since around 3000 BC and its addition to copper to form alloys like bronze heralded in a new age.  Tin mines in Devon and Cornwall operated from around 2150 BC – with the last one closing as recently as 1998.  This made the region one of the earliest parts of Britain to trade with the rest of Europe and even the Middle East –  both before and after the Roman invasion.

The main mineral that has always been the primary source of Tin is cassiterite, and this is now classified as a conflict mineral due to the conflict waged over control of the land where large deposits are fund – such as in the eastern parts of the Democratic Republic of the Congo.  Companies in the United States that use materials that come from conflict minerals need to be able to audit their supply chains to prove the origins – following the 2010 Dodd-Frank Wall Street Reform and Consumer Protection Act.

Now for the photography connection – and Tin(II) Chloride – SnCl₂ – otherwise known as stannous chloride – is used in the ‘E-6’ photographic process for producing colour transparancies, or slides.  When I worked in a photo research lab the E-6 reversal bath was one of the formulations I worked on.  Stannous chloride is a reducing agent, and in the reversal bath it performs the same role that light does in exposing the silver halides that remains after the ‘First Developer’ has produced basically a black and white negative.  In print photography you take a negative – shine light through onto paper and then develop – for slides this all happens in the same piece of film.

The reversal bath prepares the reversed image by ‘exposing’ all the silver halide that has not been developed to a silver image in the first developer (which is a black and white developer)   and this silver halide can then be developed in the colour developer to product the positive image. The colour actually comes from dyes in the film reacting with the local oxidized developer in three layers sensitive to different parts of the spectrum.   A good article that show this effect weel is the Kodak one – Processing Solutions and their Effects.  The E-6 process applies to Kodak’s Ektachrome and Fuji’s Fujichrome and others – but probably the most famous transparency film – Kodachrome – goes through a slightly different process.  It has no dyes in the film (so the film can actually be thinner) but instead after first development it goes through a series of exposures to different coloured light and development in three different colour developers that contain the dyes – so building up the colour image without chemical reversal.  E-6 is a process you can do at home without too much difficulty – Kodachrome K-14 was much more involved – and I say ‘was’ as production of Kodachrome film ceased in 2009 and the last roles were processed in December 2010.

Next up in my photographic journey through the Periodic Table is Oxygen – and I might need to get a bit creative with a photo depicting O₂.