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A weekly show where we endeavor to answer one of your big questions. This week, David Lev asks, "Why are blueprints blue?"


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Hi I'm Craig I'm an ex-Blue Man, not part of the group, I was just a man who was blue and this is Mental Floss on YouTube. Today I'm going to answer David Lev's question "Why are blueprints blue?" 

Nowadays, blueprints refers to any sort of plan whether they are blue or not, but originally blue prints had to be that color because they were copies that could only be made in blue. Let's get started. Or did we just answer the question? Let's get started.


The process of making copies may seem simple now but architects used to have to do it by hand tracing copies whenever they needed one. Experts estimate that the invention of blue prints reduced the cost of copies to one tenth of what it originally was, and you did not have to frigging you know just draw all the time.

Blue prints were invented in the mid-nineteenth century. People needed a better way to reproduce plans for architecture and other projects, and some scientists found that a combination of ammonium, iron citrate, and potassium ferrocyanide created a solution that could help with this. How did they find that out? 

They just throwin' together, well this ammonium citrate and potassium ferrocyanide and [magic noises] blueprints. Scientists are amazing.

So to make this a blue print you coat a sheet of paper with that solution and allow it to dry, then you make the original plan on cloth, or tracing paper. You place a sheet on top of the paper that will be blue printed. Then sine a bright light onto both, and that is when the solution of ammonium, iron citrate, and potassium ferrocyanide do their thing.

And by do their thing I mean they create a new compound, blue ferric ferrocyanide. the reaction turn the paper blue, and the lines on the original plan have blocked the bright light, so the blue print stays white where those lines are. 

Basically a blue print is a negative, and the reason why blueprints are blue is because the chemical reaction does not allow another color or even shades.

Of course nowadays it's pretty easy to use a photocopier, technology invented in the 30s that became mainstream in the 60s. But it took architects and engineers awhile to switch away from blue printing. In-fact a majority of those two fields turned to photocopies during the early 2000s. 

And if architects or engineers want a blueprint like copy nowadays, they tend to create a white print instead. It's basically the opposite of a blue print, white paper with blue lines.

White prints requires diazonium salt and Azo dye, which are less toxic then the chemicals used in blue printing. I wouldn't drink them though.

Thanks for watching Mental Floss on YouTube made with the help of all of these nice diazonium salts. If you have a big question of your own that you would like answered, leave it below in the comments. See you next week.