carbon transfer and fogging

[timgray@rogers.com]

Been trying the carbon process as per the Bostick & Sullivan PDF. The most pressing issue is fog - I can't get a totally white white. I'm guessing the most likely cause is the sensitizer - potassium dichromate. I mixed a batch of 3% and dilute it down to 1% on a print by print basis - but at 1% I need a really contrasty negative. I made 1 litre but after the first week it's turned more of a coffee colour rather the the fresh "cherry" colour when I first made it - stored in the dark, but room temperature - 72 - 75F. I originally was storing in the refrigerator but it seemed to precipitate out. In any event I'm going to make a fresh batch @ 2% and see if that helps - certainly the first couple of prints I made had less fog than recent ones. Any other ideas? Thanks.

 

[Vaughn]

Did the sensitizer turn dark after use, or without use.

If you tray sensitize, the sensitizer will not keep after using it. Any organic material (paper, gelatin, dust, etc) will react with the dichromate, turning it brown.

I have had Ammonium dichromate react with rubbing alcohol and turn dark -- from impurities/additives in the rubbing alcohol most likely. It is one of the reasons I use acetone when brush-sensitizing.

The sensitizer when stored in the fridge can have some of the dichromate fall out of solution -- usually will go back into solution once warmed back up.

Odd that you need a more contrasty neg for 1% -- the weaker the solution, the lower the contrast you need in the negative -- 1% should be good for negs of relatively low contrast -- one that makes a decent silver gelatin print with perhaps a #2 VC filter.