Science Poetry

Light Verse

“The substance of print photochemical...
Cyanotype’s image, accessible;
Its process is fated
When illuminated
By sunlight steadfast: print indelible.”

The 12 April 2020 post returned to the main form from the 2019 project, attempting to illustrate a chemical principle in the five lines of a limerick.  

“The substance of print photochemical… /
Cyanotype’s image, accessible”
The first line introduces the theme of this poem: photochemistry, a term which refers to reactions caused by light.  In an artistic context, this type of reaction can be used to create a photochemical print.  Many different types of these prints exist; one is called the cyanotype for the cyan color it achieves.  These first two lines were intended to emphasize sunlight’s crucial role in creating a photochemical image on a photosensitive surface.  

“Its process is fated /
When illuminated /
By sunlight steadfast: print indelible.”
A photochemical print process generally achieves a permanent image through three steps: first, photosensitizing a surface (creating a surface that can react with sunlight); second, exposing that surface to sunlight to achieve that reaction; third, fixing the image on the surface to ensure its permanence.     

For cyanotypes, the photosensitive surface is prepared by mixing potassium ferricyanide [K3[Fe(CN)6] and ferric ammonium citrate ((NH4)5[Fe(C6H4O7)2]) together, in a process devised by  Sir John Herschel in 1842.  If paper is painted with this mixture and then dries, its resulting surface is a yellowish color.  When exposed to sunlight, the print turns a bluish color wherever light hits it, and a stencil or object can be used on the print to create a negative image.  After the reaction is done and the remaining photosensitive mixture (any excess reactant) is rinsed away, then the photochemical print deepens to a cyan color by oxidizing: reacting with the oxygen in the air.  

The last three lines of the poem focus on the second step in detail; the overall process ultimately leads to a “print indelible.”