Re: Dry Plate Speed & Shelflife

From: Richard Knoppow <dickburk@ix.netcom.com>
Subject: Re: Dry Plate Speed & Shelflife
Date: Tue, 01 Mar 2005 15:55:26 -0800

> Mees talks about
> Wratten's early dry plates and states that the speed of
> plates from c.1880 was on the order of 40 times that of wet
> plates.

Silver gelatin emulsions in 1871 and 1880 have several important
differences in the techniques used in preparation and I would not be
surprized if their speeds were drastically different.

Writing this from memory, but Maddox's emulsion was pretty slow. But
the speed picked up pretty quickly in early 1870s. I think they had
ASA equivalent of 1/10 or on that order. But that decade had a very
steep improvement in speed to reach ASA up to around 10 by
c. 1880. Years from 1880 to maybe around 1920 had much slow
improvement in terms of emulsion speed.

> He also states that wet plates had rather uniform
> speed so early dry plates were often rated in the
> comparitive exposure to wet plates. (see chapter 2)

One major reason that necessitated sensitometric study was the
variation of sensitivitty in silver gelatin emulsions depending on the
material and technique. You know those Schiener's and Warnerke's
sensitometer designs.

> The early plates were hand coated using
> tea pots to hold the emulsion for pouring.

I've tried to pour emulsions with pipettes, and they make decent even
coating, the coating is too thick and not very good. I'd rather use
pipette as a coating rod to spread the emulsion for thinner coating...

> While he does not
> go into detail this does suggest that early dry plates could
> have reasonably good keeping qualities even before machine
> coating.

Again c. 1880 is a decade ahead from "early years" of silver gelatin
and you can't pick one to talk about another. Mees's statement you
mentioned is irrelevant to the instability of early emulsions. Until
Johnston and King reported the benefits of desalting (washing)
emulsion in 1873, the emulsions had poor speed and poor stability.

> This is a non-technical book where there is often a
> blurring of dates but the above seems to be reasonably
> clear.

The book you referred puts focus on Kodak Research Labs. But in order
to understand early years of silver gelatin process, you'll have to
look at books by Eder, Baier (in German) or maybe other similar
ones. Not a lot of history literature gives good treatment on this
process, perhaps because each historical event and its technological
significance are poorly appreciated by non-emulsion makers.

--
Ryuji Suzuki
"Well, believing is all right, just don't let the wrong people know
what it's all about." (Bob Dylan, Need a Woman, 1982)