Editing Blueprint (section)

==The blueprint process==
[[File:Waldhaus Gasterntal Plan5.JPG|right|thumb|Architectural drawing, Germany, 1902]]
[[File:Joy Oil gas station blueprints.jpg|right|thumb|Architectural drawing, Canada, 1936]]

The blueprint process is based on a [[Photosensitivity|photosensitive]] ferric compound. The best known is a process using [[ammonium ferric citrate]] and [[potassium ferricyanide]].<ref>{{Citation | url = http://www.pslc.ws/macrog/work/blue.htm | title = Blue | publisher = PSLC | place = WS}}.</ref><ref>{{cite book |last1= C. Brown|first1= Walter|last2= K. Brown|first2= Ryan |date= 2011|title=Print Reading for Industry, 10th edition |publisher= The Goodheart-Wilcox Company, Inc.|page= 7|isbn=978-1-63126-051-3}}</ref> The paper is impregnated with a solution of ammonium ferric citrate and dried. When the paper is illuminated, a photoreaction turns the trivalent ferric iron into divalent ferrous iron. The image is then developed using a solution of potassium ferricyanide forming insoluble ferroferricyanide ([[Prussian blue|Prussian blue or Turnbull's blue]]) with the divalent iron. Excess ammonium ferric citrate and potassium ferricyanide are then washed away.<ref name="columbia">{{cite book |editor1-last=Bridgwater |editor1-first=William |editor2-last=Sherwood |editor2-first=Elizabeth J. |title=The Columbia Encyclopedia in One Volume |date=1950 |publisher=Columbia University Press |location=Morningside Heights, New York City |page=214 |edition=Second |language=en |type=hardbound |chapter=blueprint}}</ref> The process is also known as [[cyanotype]].

This is a simple process for the reproduction of any light transmitting document. [[Engineer]]s and [[architect]]s drew their designs on [[cartridge paper]]; these were then traced on to [[tracing paper]] using [[India ink]] for reproduction whenever needed. The tracing paper drawing is placed on top of the sensitized paper, and both are clamped under glass, in a daylight exposure frame, which is similar to a picture frame. The frame is put out into daylight, requiring a minute or two under a bright sun, or about thirty minutes under an overcast sky to complete the exposure. Where [[ultra-violet light]] is transmitted through the tracing paper, the light-sensitive coating converts to a stable blue or black dye. Where the India ink blocks the ultra-violet light the coating does not convert and remains soluble. The image can be seen forming. When a strong image is seen the frame is brought indoors to stop the process. The unconverted coating is washed away, and the paper is then dried. The result is a copy of the original image with the clear background area rendered dark blue and the image reproduced as a white line.

This process has several features:<ref>Ralph W. Liebing ''Architectural Working Drawings'', John Wiley & Sons, 1999 {{ISBN|0471348767}} page 576</ref>{{bulleted list |the image is stable|as it is a contact process, no large-field optical system is required|the reproduced document will have the same [[Scale (ratio)|scale]] as the original|the paper is soaked in liquid during processing, and minor distortions can occur|the dark blue background makes it difficult to alter, thus preserving{{bulleted list|the approved drawing during use|a record of the approved specifications|the history of alterations recorded on the sheet|the references to other drawings}}}}

Introduction of the blueprint process eliminated the expense of photolithographic reproduction or of hand-tracing of original drawings. By the later 1890s in American architectural offices, a blueprint was one-tenth the cost of a hand-traced reproduction.<ref>Mary N. Woods ''From Craft to Profession: The Practice of Architecture in Nineteenth-Century America'' University of California Press, 1999 {{ISBN|0520214943}}, pages 239–240</ref> The blueprint process is still used for special artistic and photographic effects, on paper and fabrics.<ref>Gary Fabbri, Malin Fabbri ''Blueprint to Cyanotypes – Exploring a Historical Alternative Photographic Process'' Lulu.com, 2006 {{ISBN|141169838X}} page 7{{self-published source|date=February 2020}}</ref>{{self-published inline|date=February 2020}}

Various base materials have been used for blueprints.  Paper was a common choice; for more durable prints [[linen]] was sometimes used, but with time, the linen prints would shrink slightly.  To combat this problem, printing on [[vellum#Paper vellum|imitation vellum]] and, later, polyester film ([[Mylar]]) was implemented.