Why aer blueprints drawn/written in blue?

Answer 1

The term "blueprint" was originally derived from the use of the printing process called cyanotype. Cyanotype was primarily used as a method of copying. The original would be placed between a strong light and a sheet of paper soaked in a photosensitive chemical (reacts to light). The light would be turned on and the chemical would turn blue where the light hit it. Because the lines on the original were black, no light would get through and the chemical would remain unchanged. The copy was then washed and the blue remained, but the unchanged chemical would wash away leaving a white line where there was a black one on the original. This would make a copy of the original of white lines on a blue background (i.e. "blueprint"). The chemicals were rather nasty, and it is not used very often any more. The electronic printer and copier are used much more now.

Another method is called "whiteprinting" which leaves blue lines on a white background. Explanation of that process is for a different question...

Answer 2

In an era of Computer Assisted Design (CAD) it's hard to believe that any 19th century technique to do anything is still used, yet blueprinting is one. The word, in fact, has come to signify more than just reproducing copies of a drawing, it also describes any sort of master plan. How's that for perseverance?
How Blueprints are Made

The process of blueprinting goes something like this: A designer or draftsman first makes an original drawing on tracing paper or tracing cloth.



Original Tracing

This tracing is placed over chemically modified blueprinting paper. We'll get to the chemistry of the paper in a moment.



Layover
A bright light is shone on the paper for several minutes.



Hit it with Light
The bluprinting paper is then placed in a developing solution where, viola, the paper turns blue, except where the original tracing blocked the light. The bluprint is then washed and dried yielding a perfect negative of the original, give or take a few millimeters after washing and drying.



Finished Blueprint
The Chemistry of Blueprinting

Blueprinting paper isn't just ordinary paper. It's coated with ammonium iron citrate. The formation of this compound is shown below. Amazingly enough, the structure of this inorganic compound isn't known, but we do know that iron is in the +3 oxidation state.



Formation of ammonium iron citrate
Next, the blueprinting paper, with the original drawing on top, is exposed to light. All the iron +3 ions, except those directly under the tracing lines, are reduced to iron +2.



Reduction of Iron +3 to Iron +2
The blueprinting paper, which is still white, is placed in an aqueous solution of potassium ferricyanide. This compound reacts with ammonium ferrous citrate and forms a compound called prussian blue. This compound, in it's hydrated form, is blue. The structure of prussian blue, also known as Turnbull's blue, is still under debate.



Formation of Prussian Blue
Now deserving of it's name, the blueprint is washed. Prussian blue adheres to the paper while the unreduced iron +2 compound washes away.

Try this experiment!

Blue-line Prints

Blue-line prints get their names because they have a bluelines on a white background. It sounds almost too simple, doesn't it? These prints are made in the same way as a blueprint, but since they have inverted colors, an original with inverted colors is used. This inverted original is called a brownprint.

Newer Methods

The methods described above, though still used, are old and have given way to more sophisticated techniques.

Whereas blueprint reproductions are negatives of an original drawing, the diazo (whiteprinting) process provides positive images like those of blue-line prints, except the lines can be of almost any color on a white background.

The paper is coated with a diazo compound that is sensitive to ultraviolet light. Just as in blueprinting, the lines of the original protect areas beneath it. Whiteprints are developed in an ammonia atmosphere, but in this case, the unexposed areas, the lines in this case, change color.

Where're the polymers?

So you're scratching your head and wondering what all this has to do with polymers, huh? Well, to start, paper is made of polymers, and. . . Aw heck, you caught us. We weren't trying to pull a fast one, honest. The topic just seemed so darn interesting. And if you're reading this far, perhaps you thought so too.

Answer 3

Blueprints are/were actually white lines on a blue background.
Black on white to white on blue was the first available mechanical copying technique for large sheets

Answer 4

Because of the process of how they used to be made. Before we had photocopying machines, we had to use paper that could transfer to other sheets just by writing on them, and in the case of blueprints the paper was blue.

Answer 5

Cause they are called BLUE prints. But I never seen one written/drawn in blue, I have seen some on ble paper and with white writing/drawings.

Answer 6

One CANNOT make xerox copies of that blue print. Only the original, can be xeroxed. Originals are not drawn in blue.
Even from CAD programs, the prints are blue.

Answer 7

I believe that stems back to the days when they used duplication paper to duplicate drawings. That was before large copiers or printers.

Answer 8

Because of the reproducing chemicals. The original of course was black. You can make them black and white but it costs much more.
Vaya con DIOS

Answer 9

It was the old chemical process of reproducing images that turned the color of the paper blue.

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