Silver density vs silver mass

Discussion in 'Silver Gelatin Based Emulsion Making & Coating' started by Photo Engineer, Feb 25, 2007.

  1. Photo Engineer

    Photo Engineer Subscriber

    Apr 19, 2005
    Rochester, NY
    Multi Format
    Ok, here is another myth to be busted.

    There is no absolute correlation between silver mass and silver density. Some people have said here and elsewhere that there is always a 1:1 correlation between mass and density but this is just not so.

    The developer used, and the type of silver halide used to produce the silver image are what determines the actual density of the image, not the quantity of silver produced.

    Here is an example. Let us assume a square transparent window 1 meter on a side. Now, stick up to it a 10 cm square opaque cube. Measure the light visually coming through this window compared to the window without the cube. Not a huge difference.

    Now, break this 10 cm square cube up into 1 mm square opaque cubes and spread them evenly over this same window and measure the light that gets through. You will find in this latter case that virtually no light gets through.

    This is an extreme, but this is similar to what happens with different emulsions and different developers. Developed silver can form as platelets, filaments and spheres and can range in size from less than 0.1 micron up to as large as 10 microns depending on developer, emulsion type and emulsion grain size.

    If that were not enough, silver metal, as developed or produced by reduction, can vary in color from yellow, through brown to blue and then black, and even the black formed in most cases is not uniform. It has a particular spectral signature for each product and developer and this influences how we view a negative, how a negative is viewed by a given paper, and how the print is viewed by the human eye.

    To give an example, about 10 milligrams / ft square of CLS (yellow) silver in color films, or gray silver in AH layers in color films can give us a density of about 3.0 whereas the developed silver is so much less efficient in absorbing power, it may take 300 mg / ft square to achieve the same density. This is a 30X difference in mass to achieve the same relative density. It is due to the size and shape of the silver grains in the two cases.

    That is how we can get fine grained or coarse grain developers from one film, and why we can get warm tone and cold tone developers from the same paper. This does not address staining developers in any way. They function by a different method.

  2. colivet

    colivet Member

    May 28, 2004
    8x10 Format
    Mind blowing! Keep them coming...