Nothing is more frustrating than spending the day in the darkroom, doing battle with a negative to produce the "perfect" print, and returning the next day to find that all the prints have a fine fog over the entire paper. There can be several reasons for this: a light leak in the darkroom, outdated paper, a light leak from the enlarger or (as is usually the case) a bad safe light. Its a simple fact of a printers life that safe light filters go bad with time, and usually much more quickly than you might imagine. But the real problem is that they go bad without your notice. The deterioration of the safe light is a gradual process that slowly adds more and more fog to your prints with each subsequent printing session. If you use your darkroom regularly, you should be checking you safe lights at least every 3-6 months. It only takes a few minutes and at most two or three sheets of paper.

1 - Place your Zone VIII or blank negative into your enlarger and set the enlarger at a height that will completely cover an 8" X 10" sheet of paper.      

2 - Set the lens at ƒ/ 16 and run a test strip using 2 seconds ( 2,4 ,6, 8, etc.)    

3 - Develop and fix the sheet fully before turning on the room lights.       

4 - Once the lights are on, inspect the sheet and look for the shortest time that begins to show some density. You are not looking for a full density, only an exposure that will just begin to  activate the paper  emulsion.    

5 - Get yourself ten coins (or similar objects) and go back to the enlarger and expose a new sheet of paper at the established time. When the enlarger's exposure is complete, leave the sheet of paper in the easel and set the coins in different areas on the paper. Then one at a time remove the coins at 30 second intervals, for a total of 5  mins.               

   6 - After the last coin is removed, develop and fix as before. When you turn on the room light and inspect the sheet. You are looking to see where the coins were laid on the paper. If you see a density  difference in the form of a circle, then there is a chance that your safe light filters have gone bad OR that your safe lights are placed too close to either the enlarger or the developer tray. (Safe lights should be no closer than four feet from the paper at all times.) OR that your enlarger is leaking light.  If you suspect the former then run the coin test again after moving the safe lights. If you suspect the latter, turn off the safelight and repeat the test 

   Another safe light test is related to the Dragging Development technique on page 76. To test for using this technique repeat the above test on 5 or 6 sheets on paper and mark each sheet on the back for a different development time. (Example - 4, 5, 6, 7, 8, etc. minutes) Develop each sheet at the time marked, fix completely and then inspect. Check for density increase in the covered areas on the sheets when compared to the original sheet. If you see an increase you should shut down your safe light when using this technique or move it farther away from your developer tray to insure that the safe light is not fogging the paper when extending the development time.

   Another quick method of testing for paper fog is to mix up a dilute solution of potassium ferricyanide and after the print is fixed, place a drop of the potassium ferricyanide solution on the print border for a few seconds. Rinse off w/ water and return it to the fixer. If that area turns whiter than the rest of the border then your paper is fogged.