Spectrum Scientifics Blog

Sunprint Kits are perhaps the most popular items in the store. Its not surprising. The concept is simple and fun for both adults and children.

How it works is this: You take an object, like a leaf or a key. You place it on top of the photographic paper and under a sheet of acetate. Then you place all of that in the sun for 1-5 minutes to expose it (depending on the strength of the sun). The sun seems to fade the paper where it lands but it will continue to fade unless you ‘develop’ the sunprint by putting it in water. When you do that the colors reverse and you end up with a white image of the object’s shadow on blue paper.

The Sunprint is actually a process known as a Cyanotype. Cyanotypes were first invented by English scientist and astronomer Sir John Herschel back in 1842. However, the technique was actually popularized by botanist Anna Atkins by publishing a book of Cyanotypes.

The Sunprint paper is coated with a combination of Ammonium Iron (III) and Pottassium ferricyanide (note, the paper is not poisonous despite the word ‘cyanide’ being in one of the chemicals). When ultraviolet light from the sun (or a strong UV bulb) the light causes a chemical reaction, creating a blue dye known as ferric ferrocyanide, also known as Prussian blue.

When you flush the paper with water, the water soluble Iron salts are washed away, while the insoluble Prussian blue remains in the paper. This process was actually used to create ‘blueprints’ in the 20th century before advanced computer CAD systems and printed methods replaced the technique. Nonetheless, the term ‘blueprints’ is still used to refer to drawings used to build buildings.

Shadows of leaves and keys are not the only thing you can make images of. Many folks have used sunprint kits to make unique images from film negatives. An especially fun thing to do is find very old, large negatives and make images on Sunprint paper with them. Its a technique that is tricky, and what you end up with is a monochrome image of the negative. But it is fun with old film you never knew you could have!

If you enjoyed this post, make sure you subscribe to my RSS feed!

Filed under: Chemistry, City Science, Solar