Determining Base Exposure for Cyanotype

[bomzi]

Does anyone have any suggestions on finding the base exposure printing time?

I'm trying to find the minimum time for max black as the first step in a more controlled cyanotype process. The method I am following is outlined here:
https://www.timlaytonfineart.com/blo...contact-prints

The other method involves using a Stouffer step wedge which I do not have not do can I readily purchase one.

The print are made using Pictorico OHP film. Paper is Canson Montval with original chemistry.

I am not getting the results that I was hoping for. Adding exposure past 20 minutes does not result in a darker "black". And the black under the OHP film does not match the black without film.

A photo of the test strips is attached.

Any help is greatly appreciated!

[koraks]

So your base exposure is 20 minutes. I think you're confusing two issues: one is to get the highest dmax from the process itself, and the other is determining the minimum exposure to get that dmax.

Absolute dmax with cyanotype depends on a variety of factors, including (but not necessarily an exhaustive list):
* Exposure obviously
* The ratio in which both chemicals are mixed
* The time between coating and exposure / the humidity of the paper upon exposure
* The paper used (very important variable)
* The washing after exposure and any chemicals added to aid clearing of remaining iron salts
* The use of hydrogen peroxide as a quick oxidizer to immediately achieve dmax
* Drying time/aging of the print
If you follow the many recipes/manuals online to the letter, then look for a paper that will give you the best dmax and tonality. All papers respond differently to the process and even though classic cyanotype tends to work well with many papers, there still are differences particularly in dmax.
Some people use a final rinse with a few drops of hydrogen peroxide added to the water to immediately boost the tone to a deeper color. The same happens after drying the print and letting it rest for about 24 hours, but to shorten this time drastically, the peroxide route is sometimes chosen. However, I (and others have reported the same) have the impression that the print doesn't quite reach the same dmax when peroxide is used compared to a non-rapidly oxidized print. So when peroxide is not used, absolute dmax is only reached about a day after the print is made.

Note also that cyanotype never reaches absolute black. At best, you get a really, really deep blue that approaches black, but doesn't quite reach it.

[Randy]

As Koraks said - I just don't think it is possible to get a cyanotype exposure that will be as dark as the black you can get with a silver print.

[Andrew O'Neill]

I do the test strip printing through the overhead material. Take the time that gives the most density. Use that time to print your step wedge. Adjust the amount of ink being laid to get a good step wedge. I use QTR.

[bomzi]

By "black" I did mean max blue.

I've identified some other problems with my process.
1. It's the rainy season where I am which means that humidity is very high. Can't do much about it. Will have to wait for the rains to stop.
2. The water from my taps is very hard. I'm going to try a initial bath of distilled water with a little white vinegar.

[koraks]

Try a pinch of citric acid instead of vinegar. It's more effective in clearing the print since citric acid is a chelating agent, and I find the resulting hue more attractive than with vinegar. Vinegar yields more of a cool blue, whereas citric acid tends to make a slightly warmer, royal blue.