[pure-silver] Re: shamefully off topic
- From: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
- To: <pure-silver@xxxxxxxxxxxxx>
- Date: Fri, 18 May 2007 02:53:37 -0700
----- Original Message -----
From: "Shannon Stoney" <shannonstoney@xxxxxxxxxxxxxxx>
To: <pure-silver@xxxxxxxxxxxxx>
Sent: Thursday, May 17, 2007 3:54 PM
Subject: [pure-silver] Re: shamefully off topic
I love that film. But I've switched to Velvia 100 because I
use it a lot in a pinhole camera and there are long
exposures, hence reciprocity failure with Velvia 50.
Is Cibachrome paper reversal paper? I have been wary of
doing color in my home darkroom because it's more toxic.
--shannon
There is no reason to get in contact with the chemicals.
Older color processes used some fairly toxic chemicals, I
think the current ones are a bit safer.
Cibachrome AKA Ilfochrome, is, perhaps, best described as
a direct positive process rather than a reversal process. It
is based on the idea that certain dyes can be made colorless
by the reaction products of development. Ciba/Ilfo chrome is
black when unexposed. The development bleaches out the image
where its light struck. I suppose this is a bit nit-picking
but it is a different process based on a different principal
than reversal. In a reversal process the image is first
developed to a negative. Instead of fixing the image and
removing the unused silver halide, the remaining halide is
used to produce the final positive image. For B&W, the
original silver image is removed by a bleach which does not
affect the halide. The halide is then exposed to strong
light or treated in a chemical which has the same effect.
The emulsion is then developed leaving a positive image.
Color processes are a bit different, the original silver
negative image does not have to be removed. Rather the first
development is carried out in a developer which does not
generate dye so that there is just the negative silver
image. Then the film is either exposed to light or to a
fogging chemical, as above, and developed again, this time
in a developer which produces the dye image. The dye image
is, of course, positive. The last step is to remove the
silver images, both positive and negative, leaving only the
positive dye image behind. There is a little more to than
this but not much.
Ciba/Ilfo chrome has the advantage that it can use a
different kind of dye than reversal (or negative) color
materials of the "chromogenic" type. The dye is generally of
a type which is more stable than that in chromogenic
materials. The colors are quite brilliant but the material
tends to be rather high contrast. By using a contrast mask
the overall contrast can be reduced. The mask is simply a
low density, low contrast negative printed from the
positive. The print is made by exposing through a sandwich
of the tranparency and mask. By making the mask slightly
blurred there is an effect of increasing sharpness as well
as reducing overall contrast. This is because the contrast
of small details like textures and edges is not reduced and
is interpreted by the eye as increased sharpness.
Enough already.
---
Richard Knoppow
Los Angeles, CA, USA
dickburk@xxxxxxxxxxxxx
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