Since 1876, instruction in the use of the blue
process has been given to the students of mechanical engineering of the
Massachusetts Institute of Technology, and they have caused its
introduction into many draughting offices. The proportions of the
sensitizing liquid, as originally given me by Mr Barnes, were as
follows:
Red prussiate of potash............. 8 parts.
Citrate of iron and ammonia......... 8 parts.
Gum arabic.......................... 1 part.
Water.............................. 80 parts.
_Results of Experiments._--In our use, it first appeared that the gum
might be omitted from the preparation when sufficiently hard papers were
used. Next, that a preparation containing
Red prussiate of potash........ 2 parts,
Citrate of iron and ammonia.... 3 "
Water......................... 20 "
printed more rapidly. This preparation I continue to use when much time
may elapse between sensitizing and printing; but, when the paper is to
be printed immediately after sensitizing, I use a larger proportion of
citrate of iron and ammonia. Before arriving at the conclusion that
these proportions were the best to be used, I made a series of purely
empirical experiments, beginning with the proportions:
Red prussiate of potash.......... 10 parts.
Citrate of iron and ammonia....... 1 part.
Water.........................
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