Why Are Blueprints Blue?
Blueprinting is the process of transferring architectural plans from drafts onto paper as a final design that is used by contractors and construction workers to build the design. The chemical process used to make blueprints results in blue paper with white lines outlining the information inscribed by the architect. Since no ink is used, the design cannot be smudged or blurred and is not subject to fading by sunlight or other factors. The blueprinting system was begun in the 1800s and is still used by some architects today, although other methods are also prevalent.
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Process
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An architect, designer, or drafter first transfers all drafts onto tracing paper in a final design. This clear tracing paper is set against a section of blueprinting paper equal in size, and then a light is shown through the trace onto the solid paper. After several minutes, the blueprinting paper is taken and immersed in a developing solution, where it turns blue in all areas except those once covered by the lines in the tracing paper. The blueprint is then hung and dried until it is ready for use.
Blueprinting paper
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The paper used to create blueprints starts with a special coating made of ammonium iron citrate, a synthetic compound that in its un-altered state looks white, like normal paper. The most important part of this compound is the iron element, which is in an oxidation state with three oxygen atoms. These three ions are unstable and, given the correct chemical reaction, can change and move around, creating a new compound.
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Chemical Reaction
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The first chemical reaction occurs when light hits the ammonium iron citrate. The energy of the light waves passes through the iron present, and excites the oxygen atoms until one finally bursts free. The ammonium ferric citrate (with the iron that has three oxygen atoms) becomes ammonium ferrous citrate (with iron that only has two oxygen atoms) as all the iron particles of the compound lose one oxygen component...except for those not exposed to the light, the iron under the tracing lines.
When the blueprint is immersed in the potassium ferricyanide solution, all of the ammonium ferrous citrate reacts to the chemicals and becomes a completely new compound, once known as "Prussian blue" because of its blue-colored qualities. This blue chemical clings to the paper, while the mixture that still has three oxygen atoms remains unchanged. When the blueprint is washed, only the Prussian blue remains, leaving the traced outline firmly intact.
Diazotype
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Today, a more modern process is often used instead of this old method, utilizing the qualities of diazonium salt. This salt coats the printing page, but its dye qualities are destroyed by exposure to light. Only the diazonium salt under the tracing lines survives, and this is treated with a solution that allows it to bond with the paper and form in a particular color. These prints show dark lines against a white background.
Benefits
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Since full sets of blueprints often require many copies and are usually very large, it remains far less expensive to use an ammonium iron citrate or diazonium salt reaction than to print out all the pages on a large scale printer.
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