# Why are Blue Prints blue? **URL:** \ **Category:** Factual Questions\ **Created:** [October 5, 2000, 5:02pm UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802 "2000-10-05T17:02:04Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Zebra](https://avatars.discourse-cdn.com/v4/letter/z/c0e974/32.png) [@Zebra](https://boards.straightdope.com/u/Zebra)\ **Post date:** [October 5, 2000, 5:02pm UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/1 "2000-10-05T17:02:04Z")
Well what gives? Over fifty posts! Yeah! Woopie! Wahyoo! Top that man! ---
**Author:** ![ticker](https://avatars.discourse-cdn.com/v4/letter/t/d07c76/32.png) [@ticker](https://boards.straightdope.com/u/ticker)\ **Post date:** [October 5, 2000, 5:06pm UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/2 "2000-10-05T17:06:06Z")
In the days before CAD, before Xerox even, plans were copied by an early photographic process known as Cyanotype, invented by Sir John Herschel in 1841. Cyanotypes are blue. ---
**Author:** ![AWB](https://avatars.discourse-cdn.com/v4/letter/a/bb73d2/32.png) [@AWB](https://boards.straightdope.com/u/AWB)\ **Post date:** [October 5, 2000, 5:45pm UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/3 "2000-10-05T17:45:35Z")
Because they absorb all other wavelengths of light besides blue. _Over fifty posts! Yeah! Woopie! Wahyoo! Top that man!_ Zebe, take a look to the left. šŸ˜› ---
**Author:** ![Ike\_Witt](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ike_witt/32/283_2.png) [@Ike\_Witt](https://boards.straightdope.com/u/Ike_Witt)\ **Post date:** [October 5, 2000, 5:59pm UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/4 "2000-10-05T17:59:40Z")
If they were any colour other than blue, they wouldnā€™t be blueprints would they? ---
**Author:** ![plnnr](https://avatars.discourse-cdn.com/v4/letter/p/7ea924/32.png) [@plnnr](https://boards.straightdope.com/u/plnnr)\ **Post date:** [October 5, 2000, 6:47pm UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/5 "2000-10-05T18:47:42Z")
You donā€™t see real blueprints (white lines on a blue background)much anymore. You do see blueline drawings (white background with blue (or another dark color) lines)quite a bit. A blueline machine most commonly uses a mylar film or velum film image and a sheet of paper that has been coated with a substance that is reactive to ammonia. You lay the mylar over the sheet, feed it through a set of rollers that expose the sheet to the ammonia vapors (fixing the outline onto the sheet) and then another set of rollers that expose the sheet to a ā€œblackā€ light (which exposes all of the other areas of the sheet except the outline). Paper goes in colored (ususally yellow) and comes out white except for the image, which is in blue lines. They are called Diazo machines (or at least the ones I use are) and the ammonia is incredibly strong (much, much more so than household ammonia). I made the mistake of knocking a bottle of it over in the office and the whole building had to be evacuated, the hazmat team called, etc. But that may have been more than you ever wanted to know about blueline machines. ---
**Author:** ![Zebra](https://avatars.discourse-cdn.com/v4/letter/z/c0e974/32.png) [@Zebra](https://boards.straightdope.com/u/Zebra)\ **Post date:** [October 5, 2000, 6:53pm UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/6 "2000-10-05T18:53:30Z")
> [@](#): > > \*Originally posted by ticker \* > \*\*In the days before CAD, before Xerox even, plans were copied by an early photographic process known as Cyanotype, invented by Sir John Herschel in 1841. Cyanotypes are blue. \*\* Would that be like a ā€˜ditto sheetā€™? Boy think of that! One huge ditto sheet, I would just wrap it around my head for hours. ---
**Author:** ![plnnr](https://avatars.discourse-cdn.com/v4/letter/p/7ea924/32.png) [@plnnr](https://boards.straightdope.com/u/plnnr)\ **Post date:** [October 5, 2000, 6:54pm UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/7 "2000-10-05T18:54:12Z")
You donā€™t see real blueprints (white lines on a blue background)much anymore. You do see blueline drawings (white background with blue (or another dark color) lines)quite a bit. A blueline machine most commonly uses a mylar film or velum film image and a sheet of paper that has been coated with a substance that is reactive to ammonia. You lay the mylar over the sheet, feed it through a set of rollers that expose the sheet to the ammonia vapors (fixing the outline onto the sheet) and then another set of rollers that expose the sheet to a ā€œblackā€ light (which exposes all of the other areas of the sheet except the outline). Paper goes in colored (ususally yellow) and comes out white except for the image, which is in blue lines. They are called Diazo machines (or at least the ones I use are) and the ammonia is incredibly strong (much, much more so than household ammonia). I made the mistake of knocking a bottle of it over in the office and the whole building had to be evacuated, the hazmat team called, etc. But that may have been more than you ever wanted to know about blueline machines. ---
**Author:** ![TNTruth](https://avatars.discourse-cdn.com/v4/letter/t/d9b06d/32.png) [@TNTruth](https://boards.straightdope.com/u/TNTruth)\ **Post date:** [October 5, 2000, 10:38pm UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/8 "2000-10-05T22:38:19Z")
Yeah, bluelines. I havenā€™t even seen a blueline since college. Does anybody still use this process pinnr described? Every company I know of now uses CAD, with E-size plotters and E-size xerox machines. ---
**Author:** ![KevinB](https://avatars.discourse-cdn.com/v4/letter/k/c0e974/32.png) [@KevinB](https://boards.straightdope.com/u/KevinB)\ **Post date:** [October 5, 2000, 11:09pm UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/9 "2000-10-05T23:09:03Z")
Now youā€™re bringing back memories. When I started working as an engineer back in '82 we still drew all our schematics on velum with pencils and plastic templates. I still have a logic template in my desk drawer. No one knows what it is. The first system I designed had 78 pages of D-sized schematics, all hand-drawn. Running a set of copies meant feeding the 78 pages of velum through the blueline machine, one pass to expose the sheet and another pass to process it. SLOW, and extremely stinky. Thank goodness for CAD. Kevin B. ---
**Author:** ![Gunslinger](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@Gunslinger](https://boards.straightdope.com/u/Gunslinger)\ **Post date:** [October 6, 2000, 12:33am UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/10 "2000-10-06T00:33:03Z")
> [@](#): > > \*Originally posted by TNTruth \* > \*\*Does anybody still use this process pinnr described? Every company I know of now uses CAD, with E-size plotters and E-size xerox machines. \*\* We had one in my high school drafting class. We mainly used it to make blue ā€œedge cover thingiesā€ (goes over the staples on the edge of a set of papers) by copying a clear plastic strip with all **except** the lettering blacked out, for a result of white lettering on a blue background. Iā€™ll post a pic of it in a few minutes. ---
**Author:** ![Gunslinger](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@Gunslinger](https://boards.straightdope.com/u/Gunslinger)\ **Post date:** [October 6, 2000, 1:00am UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/11 "2000-10-06T01:00:47Z")
[See the aforementioned blue border](http://albums.photopoint.com/j/AlbumIndex?u=135174&a=9226593). I love the little picture on it. ---
**Author:** ![Gunslinger](https://avatars.discourse-cdn.com/v4/letter/g/4491bb/32.png) [@Gunslinger](https://boards.straightdope.com/u/Gunslinger)\ **Post date:** [October 6, 2000, 1:06am UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/12 "2000-10-06T01:06:46Z")
We did the actual drawings with AutoCAD 14 and an E-size HP Designjet plotter, BTW. This was last year. Oh yeah, the little blue and yellow cards attached? My ā€œdistrictā€“qualified for stateā€ and ā€œstateā€“first placeā€ rating cards šŸ˜ƒ (99-00 SkillsUSA/VICA State Competition, Display, Architechture) ---
**Author:** ![Wrath](https://avatars.discourse-cdn.com/v4/letter/w/ee7513/32.png) [@Wrath](https://boards.straightdope.com/u/Wrath)\ **Post date:** [October 6, 2000, 1:48am UTC](https://boards.straightdope.com/t/why-are-blue-prints-blue/35802/13 "2000-10-06T01:48:03Z")
Because ā€œred printsā€ sounds funny. > [@](#): > > \*Originally posted by TNTruth \* > \*\*I havenā€™t even seen a blueline since college. Does anybody still use this process pinnr described? \*\* The printing industry does, quite a lot. In addition to a color match proof (actual color as it should look when printed), a blueline is made with each of the CMYK plates at a slightly different ā€œtintā€ of blue, and trimmed & folded exactly how the final piece should look. With the 2 proofs, you know what to expect.