Fogging Test

It is important to work with light sensitive materials in conditions which do not adversely affect those materials.  Of the most common is fogging from stray light or an inappropriately chosen work light.

Testing for fogging is straight forward and should be performed whenever a new work light or new material is put into use.  It is especially important to have conditions which have passed the fogging test when running a clearing test since fogging could be mistaken for incomplete clearing.

Before having an opportunity to write up a testing procedure for fogging, a great procedure was mentioned by a fellow Pt/Pd printmaker.  The following fogging test description and explanation is presented with permission from its author,
Tom Ferguson.
Web site:  http://www.thefstop.com/tf.html
 

I'm a big fan of people doing tests themselves, rather than simply taking other peoples "word".   One learns soooo much more that way!  There are simply too many variables in this world.  Does my light bulb enclosure block more UV than yours??  Are South American light bulbs different in output than USA bulbs??  Is a Philips 75W bulb different than a GE??  So I do suggest that anyone coating paper do a simple fog test.

Coat a piece of paper (in the dark) and find the exposure without a negative (in your usual UV light source) that gives the first barely visible gray in a processed and dried print.  Next coat another piece (in the dark) and give it this same exposure, but
don't process it.  Now cut this paper into 3 pieces.  Put a large heavy coin on each piece of paper and leave it in your room with the desired lights on.  Label and remove one after 15 minutes, another after 30 minutes, the last after an hour.

Process and dry the test prints, and see if the coin's shadow shows as a lighter tone than the rest of the test print.

Why go to the trouble of pre exposing to "first barely visible gray"?  I know, a few books don't bother with this step!  Most, if not all, light sensitive materials have a "threshold" that must be passed before they start to react.  What follows will not be correct to a knowledgeable scientist, but the ideas are sound.

It takes some number of photons hitting the emulsion to turn it on.  Say that number is 10.  If your safe light gives 9, then you don't see fogging.  But then you put it, with your negative, into the UV and expose.  That highlight you wanted as paperbase gets 5 photons threw the negative,  It has now gotten 14 photons, and prints as a slight gray, rather than paperbase.  Opps, fogging where you didn't think it would be.  If we pre expose the test paper to a "first barely visible gray", then any additional photons affecting the paper will be visible.

 
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