Substituting Potassium Dichromate for Ammonium Dichromate?

[Tom Taylor]

I'd say maybe a little light, and the contrast seems a bit off to my eye, but it's really good for a first (?) try. Where is it? Doesn't look like any I've seen on the OR/WA coast.
--Greg

Thanks Greg. It’s Battery Cranston constructed in 1897 to guard the entrance to the Golden Gate and can be found a hundred or so meters behind the Golden Gate Bridge on the ocean facing side. I shot the image a couple of months ago on an overcast and foggy day. Fog covers the top of the image down to and including the top of the Marin Headlands visible across the bay in the distance. This was my 2d try (the first is posted below) exposed immediately after the 1st below. I had to crop a small portion off the bottom due to a registration snafu. I awaiting a 11x14 contact frame as my 8x10 frame is perfect for 4x5 but too small for 8x10.

Thomas

One of the challenges of mixing traditional and cyanotype prints is that cyanotype usually needs a longer scale negative. I get good punchy cyanotype prints from negs in the Dr of 1.5 to 1.8.

With cyanotype experiments and new papers or sensitizers or sizings I always print a 1/2" x4" 1/2 stop series step wedge. It actually is taped to the side of contact frame glass, so that it is there ready to go. Slide the neg to expose first time next to it, and slide it into the UV box.

Thanks Mike. A step wedge is on my wish list. I placed a AA battery on the top left of the frame of the image below (the small circle). After about 30 minutes in the sun there was no change in the blue color so I assumed it had enough exposure. I had to tilt the framed for the 2d image and therefore couldn't use the battery but exposed it for 30 minutes as well.



Thomas

[gmikol]

Man...I guess I have carbon on the brain...I was seeing dichromate, and I thought those were carbon prints. Now that you explain the taking conditions, the first image makes a lot more sense. Like the second one, too.

This is traditional cyanotype? The results look quite dark, and slightly green. Maybe just a camera/scanner/monitor thing?

I really should get back to cyanotypes...

--Greg

[alanrockwood]

It may or may not be a significant factor in this case, but generally speaking if you substitute an ammonium ion in place of a sodium ion it will shift the acid/base properties of the solution toward the acidic side. The reason is that the ammonium ion (NH4+) is the "conjugate acid" of the weak base, ammonia (NH3). Therefore, the ammonium ion is slightly acidic, whereas sodium ion has almost no effect on the acid/base properties of a solution.