Re: SG emulsion query

From: Liam Lawless <liam.lawless@blueyonder.co.uk>
Subject: RE: SG emulsion query
Date: Mon, 14 Feb 2005 19:58:42 +0000

> Thanks, I suspected something of the sort. I have made simple
> chlorobromide emulsions quite a few years ago, but now wish to make
> something *better* than I did then. My working conditions are
> primitive but I already have a reasonably good and fairly fast
> bromide formula, and wish to refine it by increasing contrast and
> D-max.

In order to improve Dmax, without coating too much silver, things you
can do are:

- to minimize dead grains (grains that won't develop even after exposure)
- to make them fine grained
- to adjust hardening of the gelatin (to about 300% swelling)

I think what they do is combination of these, by making thin tabular
grains.

Do you keep track of about how many grams of silver you coat per
square meter?

In order to improve contrast, you'll have to have a good control to
trim the toe of the sensitometric curve. There are many ways to do
this described in patents using double jets. It seemed that single jet
technology was never updated after mercury/cadmium/lead and all other
useful dopants were abandoned due to environmental regulation, so
there is not a lot written in this area after 1960s. I've found
acceptable replacements for cadmium (used in chloride and
chlorobromide) and other dopants for bromide emulsions.

If you need to make contrast harder than grade 4 with a single jet
method, rhodium doping is the only way I can think of.

Since you didn't describe your emulsion formula, I might add that good
sulfur sensitization increases speed and contrast. Gold adds more
speed but makes toe longer. So I use gold for negative emulsions but
not for print emulsions.

Also, with bromide emulsions, just right amount of iodide
concentration can sharpen the grain size distribution and helps
increase contrast.

If you can send me your formula, either on or off list, I'll send you
suggestions.

> Maybe mixing with chloride emulsion is at least worth trying...

Blending emulsion is more likely to reduce overall contrast gradient.
This is good for negative emulsions but is not what you said you wanted.

> Or would they behave as two separate emulsions in one solution,
> i.e. the chloride component being so underexposed so that it has
> little or no effect on the image characteristics?

Just blending will behave like two emulsions, especially if the
emulsions are blended after stabilization process. But if the
emulsions are blended in primitive stage (before sensitization) and if
you give good ripening (preferably with a thioether compound,
thiocyanate, ammoniun sulfate, or other ripener known in the art), you
can dissolve and mix. In order to do this, one kind of emulsion must
be much much finer grained. Especially if you want to convert AgCl
host crystals by depositing AgBr, the AgBr has to be extremely fine
grained.

Halide conversion techniqus are seen in patents in 1960s and some
1970s. They seem to be unpopular today because they leave bad
pressure marks when processed in automated machines. But in my hands,
with tray processing, this is a very useful technique.

> as I remember, CB emulsion is more prone to problems
> with fog and black specks?

Black specks are different issues, I think. I've never seen pepper
fogs in my emulsions, but that's a problem in emulsion making. Same
for yellow fog (well, this is in part a developer problem as well,
especially with high chloride emulsions). Chloride/chlorobromide
emulsions are a lot more susceptible to chemical fogging, due to, for
example, some junk contained in paper. In my hands, no chloride or
chlorobromide emulsions made satisfactory results when coated on paper
directly. Most cases, they resulted in Dmax fog everywhere. Careful
sizing with hardened gelatin, preferably with poly(methyl
methacrylate) and other copolymers to make the surface durable, make
them work well, but I have to make absolutely sure to stay away from
cut edges if I cut sized paper. What a pain in the azz. Chlorobromide
emulsions will respond to lith printing techniques much better than
bromide, but I've been getting very nice blacks with bromide
emulsions. If I want to make it warmtone, it'll have to be slower, but
ISO P50-100 is not bad. (For neutral black emulsions, P300 and up.)
That slow warmtone bromide emulsion developed in DS-14 responds to
toning very well, just as good as, if not better than, Fortezo.

--
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)