2019 was the International Year of the Periodic Table of Chemical Elements.

The Periodic Table (see more here) is a graphical display of the chemical elements, which are arranged by atomic number, electron configuration, and recurring chemical properties.

The structure of the Table shows periodic trends. The seven rows, called periods, generally have metals on the left and non-metals on the right. The columns, called groups, contain elements with similar chemical behaviour.

It is not a coincidence that the four precious metals used in photography naturally form a compact square: palladium (Pd), silver (Ag), platinum (Pt) and gold (Au).

Why green? This approximates to the colour of ammonium iron(III) oxalate, the key chemical that ultimately precipitates palladium, silver, platinum or gold in the fibres of the paper in the so-called ‘alternative’ processes.

I am grateful to Brian Minards for helping me bring this idea to the site through his graphic design expertise (and seemingly giving me the status of a new element, where mercury might be, Rp).