Date: Tue Nov 2 15:38:04 1999
Posted By: Larry Lear, Staff, Registered Architect w/B.A. in Exp. Psychology, Hobbs + Black Assoc.(Architects)
Area of science: Chemistry
Hi Jimmy Wu:
My answer to you is based on my experience as an architect, not a chemist,
so please bear with me and forgive me if I extrapolate.
One can reproduce a multitude of exact
reproductions, off this single carefully drawn image. In architecture (as
in other disciplines), it is important that many different people recieve
these exact representations so that detailed work can be coordinated
across disciplines. These images also need to hold up to the rigor of the
construction site and the dirty hands of many construction workers, etc...
Numbers cannot be smeared or flake off, and the blueprint performed very
well for this task.
- Blueprint paper begins white (heavy bond paper, sometimes cloth). The
paper is then "sensitized" by coating it with (as you mention) a mixed
solution of ammonium ferric citrate and
- Next, a
translucent sheet (in which the actual architectural/mechanical diagrams
have been drawn) is placed over this sensitized paper.
- A strong light is
then directed through the translucent sheet onto the sensitized paper. The
markings on the translucent paper obscures the light and the image is
reproduced to the exact "scale" or proportions, onto the blueprint paper.
- The coating on the light-exposed portion of the paper is "reduced" by the
light to insoluble blue ferroferricyanide, or Prussian blue.
- When the
paper is then washed with water, the copy emerges as a white-line print on
blue paper that is stable in light.
Blueprints were commonly used 50+ years ago, but are not seen anymore.
Current forms of reproduction include "blueline" ammonia (diazo) prints
(these are a dark blue line on light blue paper) and most recently,
electrostatic printing. Some people feel that the latter catagory will be
the standard in a few years. The electrostatic type prints are first
digitized for accuracy (to prevent distortion)- a translucent sheet is not
needed. The print is then produced from the digitized information in the
quantity required. Although similiar, the standard office "xerox" should
not be used due to the distortion that may occur through the optical
Today, 95% of all architectural "drawings" are plotted onto translucent
paper (for blueline reproductions) or bond paper (for electrostatic
reproductions) from a Computer Aided Design (CAD) station. Hand drawing is
rapidly becoming a thing of the past, however, early in the design process
most architects still use free hand techniques.
Hope this answers your question and provides some perspective, thanks for
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