Invisible web

Making a secret Halloween picture

Safe­ty pre­cau­tions

Only un­der adult su­per­vi­sion.

Equip­ment and reagents

  • 5 mL 1% tan­nin so­lu­tion;
  • 5 mL 0.1 mol/L iron(II) sul­fate so­lu­tion;
  • emp­ty mark­er;
  • cot­ton cylin­ders;
  • wa­ter­col­or pa­per;
  • pipette;
  • glass­es.

Step-by-step in­struc­tions

Use a pipette to fill an emp­ty mark­er with 5 mL 0.1 mol/L iron(II) sul­fate so­lu­tion. Draw a col­or­less web on a sheet of pa­per and de­vel­op it us­ing a cot­ton cylin­der soaked in 1% tan­nin so­lu­tion.

Process de­scrip­tion

Tan­nins are a class of plant-based or­gan­ic com­pounds found in leaves, nuts, oak wood, and the bark of oth­er trees. Iron(II) sul­fate so­lu­tion mixed with tan­nin so­lu­tion forms the well-known iron gall ink. Upon ex­po­sure to air, this grey so­lu­tion forms an in­tense­ly pur­plish-black iron(III) tan­nin com­pound that fix­es firm­ly to the pa­per. Such ink dates back at least 1000 years, and was used all the way up to the 20th cen­tu­ry. And did you know? A sim­i­lar ex­per­i­ment is in­clud­ed in the “Chem­istry for spies” set from the MEL Chem­istry sub­scrip­tion!