_--In order to be
efficient, such a frame must be capable of keeping the sensitized paper
_everywhere tightly pressed against the negative_. Again, such a frame,
being large, is necessarily somewhat heavy. It should be so mounted that
it can be handled with ease; and, in order that it may print quickly, it
should be so arranged that it can be turned without delay, at any time,
into a position that is square with the direction of the sun's rays.
Undoubtedly, if a sufficiently thick plate of glass should be used, the
ordinary photographic printing frames would answer the purpose, whatever
the size, but very thick plate glass is both heavy and expensive.
Commercial plate glass varies in thickness from one-fourth to three
eighths of an inch, and the thicker plates are rather rare. A large
plate of it is easily broken by a slight uniformly distributed pressure.
But the pressure that is required for the blue process printing,
although slight, is much greater than is used in the ordinary
photographic process. For the sensitized paper that is used in the blue
process printing is, comparatively, very thick and stiff, and it may
cockle more or less, while the paper that is used in ordinary
photography is thin and does not cockle. Now, it is easy to see that a
pressure severe enough to flatten all cockles must be had at every part
of the sensitized paper, and that, if the comparatively thin,
inexpensive, light weight, commercial plate glass is to be used, it is
desirable to have the pressure _nowhere much greater than is needed for
that purpose_, lest the fragile glass should be fractured by it.
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