This is accomplished by specially sensitized paper by which a fac-simile
of the original drawing can be made; that is, black lines upon white
ground. It also avoids the objection to the blue print paper of shaded
drawings which show light and shade reversed.
The prints made by the process are said to be absolutely permanent and
can be altered, added to or colored the same as original drawings.
The sensitized paper is sold ready for use, but it can be prepared by
dissolving two ounces of citrate of iron and ammonium in eight ounces of
soft water; keep in a dark bottle, also, one and one-third ounces of red
prussiate of potash in eight ounces of water; keep in another dark
bottle; when about to use mix an equal quantity of each in a cup and
apply in a dark room with a soft brush or sponge to one side of white
rag paper, similar to envelope paper, let it dry and put away in a dark
place until required for use.
[Illustration: DRAWING OFFICE RULES]