How to write a message using light in 1 minute

How does cyanotype work

We’re used to draw­ing with pens and pen­cils – but how about us­ing chem­istry and light? In this ex­per­i­ment, we’ll show you how to write us­ing these two rather un­ex­pect­ed el­e­ments.

Safe­ty pre­cau­tions

Wear pro­tec­tive gloves, glass­es, and a mask. Work in a well-ven­ti­lat­ed area.

Reagents and equip­ment

  • 10 mL 1.5 mol/L cit­ric acid so­lu­tion;
  • 10 mL 1 mol/L am­mo­ni­um iron(III) sul­fate;
  • 10 mL 1 mol/L am­mo­ni­um car­bon­ate so­lu­tion;
  • 10 mL 1 mol/L red prus­si­ate of potash (potas­si­um hex­a­cyano­fer­rate(III)) so­lu­tion;
  • 2 glass bowls;
  • 1 flash­light;
  • cot­ton ab­sorbents;
  • wa­ter;
  • 1 sheet of wa­ter­col­or pa­per.

Step-by-step in­struc­tions

Pre­pare your light-sen­si­tive so­lu­tion in the first bowl: mix the so­lu­tions of cit­ric acid, am­mo­ni­um iron(III) sul­fate, am­mo­ni­um car­bon­ate, and red prus­si­ate of potash (potas­si­um hex­a­cyano­fer­rate(III)). Mix thor­ough­ly. Make sure to work in dim light­ing to keep the so­lu­tion from re­act­ing pre­ma­ture­ly. Next, take a sheet of wa­ter­col­or pa­per and ap­ply an even coat­ing of the so­lu­tion us­ing a cot­ton ab­sorbent. Use your flash­light to write a mes­sage. Watch the so­lu­tion trans­form! Fill your sec­ond glass bowl with wa­ter and rinse the treat­ed pa­per thor­ough­ly to re­move any ex­tra so­lu­tion.

Process de­scrip­tion

Am­mo­ni­um iron(III) sul­fate and cit­ric acid re­act to form light-sen­si­tive am­mo­ni­um iron(III) cit­rate. When ex­posed to light, cit­rate ions re­duce iron(III) ions to iron(II) ions, which re­act with potas­si­um hex­a­cyano­fer­rate(III) to form a strik­ing blue pig­ment known as Prus­sian blue. This re­ac­tion is the foun­da­tion for a print­ing method known as cyan­otype, which was once used to print plans and var­i­ous doc­u­ments. Such prints were there­fore known as blue­prints.

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