Why Are Blueprints Blue? (And Why Today’s Plans Print on White Paper)
Short answer: old blueprints were blue because they were cyanotypes, a contact-printing process introduced by Sir John Herschel in 1842. The chemistry turns exposed paper Prussian blue, and the drawing’s lines stay white. The process is obsolete. Today a “blueprint” is any printed construction drawing, usually black lines on white paper.
People still say “blueprints,” but almost nobody gets a blue print anymore. The name comes from a photographic copying process that made white lines on a dark blue background. Here’s where the blue came from, what replaced it, and what you’re actually ordering when you print plans today.
Order prints: upload your PDF, pick size and color, and check out. Or start from the homepage.
How a real blueprint was made
The original drawing was traced in ink onto translucent paper. That tracing was laid on top of paper coated with a light-sensitive iron compound, clamped under glass and set out in daylight:
- Where light passed through the tracing, the coating turned into an insoluble blue dye (Prussian blue).
- Where the ink lines blocked the light, the coating stayed soluble and washed away, leaving white lines.
- The sheet was washed and dried. The result was a negative of the drawing: white lines on blue.
Because it was a contact print (tracing pressed directly on the paper), the copy came out at the same scale as the original. It couldn’t reproduce color or shades of gray.
Why everyone used them
Before blueprints, copies of drawings were traced by hand or made by photolithography, and both were expensive. By the late 1890s, a blueprint cost about a tenth as much as a hand-traced copy in American architectural offices. It was a far less expensive way to make many accurate copies of one original, so builders and engineers used it for over a century to share construction and industrial drawings.
What replaced blueprints
| Process | What it looked like | Status |
|---|---|---|
| Blueprint (cyanotype) | White lines on blue | Obsolete for construction drawings |
| Whiteprint (diazo, “blueline”) | Blue or black lines on white | Largely abandoned by architects and engineers by 1999; prints faded in light |
| Large-format xerographic copiers (around 1975 on) | Black toner on plain paper | Replaced the chemical processes |
| Plotters and large-format printers | Black or color on plain paper, printed straight from CAD or PDF | How plans are printed today |
Diazo “bluelines” were the in-between step. They gave a white background, but they used ammonia to develop and they faded with exposure to light, so offices had to reprint working sets. Large copiers and then printers driven straight from CAD files took over. Large-format printers are still sometimes called “blueprint printers.”
What “blueprint” means now
Architects, engineers and drafters often call them drawings, prints or plans. Most other people say blueprints. Either way, today’s set is printed from a PDF:
- Black and white for most construction linework.
- Color when the drawings use it (site plans, landscape, MEP color coding, renderings). See black and white or color plan prints.
- Full size, usually 24×36. See blueprint sizes and the 24×36 sheet explained.
New to reading a set? Start with how to read blueprints and the construction drawing glossary.
What printing your “blueprints” costs today
We print on 20 lb bond paper from PDF uploads, at the same price at all sizes (18×24 to 36×48), with no minimum order. Printing only, before shipping:
| Example | Sheets | Printing | With code WELCOME20 (20% off your first order) |
|---|---|---|---|
| 1 black and white sheet | 1 | $1.95 | $1.56 |
| 10-sheet black and white house plan set | 10 | $19.50 | $15.60 |
| 10 sheets, 3 copies | 30 | $58.50 | $46.80 |
| 8 black and white + 2 color site sheets (separate items) | 10 | $23.10 | $18.48 |
WELCOME20 takes 20% off printing on a first order. Students get 25% off printing with STUDENT25. One code per order.
FAQ
Why are blueprints blue?
They were cyanotypes. Light turned the coated paper Prussian blue everywhere except under the drawing’s ink lines, which stayed white.
Who invented the blueprint?
Sir John Herschel introduced the cyanotype process in 1842.
Are blueprints still blue today?
Rarely. The process is obsolete for construction. “Blueprint” now means any printed plan, usually black lines on white paper.
What is a blueline or whiteprint?
A diazo copy with blue or black lines on a white background. It replaced the blueprint and was itself replaced by large copiers and printers.
Do you print blueprints?
We print construction plans from your PDF in black and white ($1.95 per sheet) or color ($3.75 per sheet), on 20 lb bond paper, at all sizes from 18×24 to 36×48. We don’t make cyanotypes.
What file do I need?
Upload a PDF. We print it to scale, exactly as uploaded. Need to print from another file type? Email us at support@printmyplans.com.
Printing it
Got the PDF? Upload it on the order page, pick the size and black and white or color, and check out. Order by 3 PM ET, ships same day, arrives as soon as the next day. If we make a mistake, we reprint and ship it free of charge.
