Carbon Journal, Day 8
Day 8: Variables & Controls
Nothing too new and exciting to report on. Over the past four days, two new sets of tissue have been made, tested, and now more tweaking is on its way. If it's not already apparent, carbon transfer printing is a process riddled with variables, all which need to be meticulously controlled to obtain consistent results, let alone a decent print. The variables that effect the overall appearance of a carbon print are:
- Pigment Loading - This is where you control the color and opacity of your gelatin tissue. Each tone has its own "sweet spot" of pigment % that produces a fine carbon print.
- Tissue Height - How "thick" you pour your tissue can directly correlate to the contrast and relief present in the final image. Typically measured in a
- Sensitizer Concentration - This controls how sensitive to UV light your carbon tissue will be. Typical percentages range from 2% - 5% either potassium or ammonium dichromate. Higher percentages translate to more sensitivity, used for "thicker" negatives, like those used for Platinum/Palladium printing.
- UV Exposure - Much like exposing a negative under an enlarger, UV exposure will control the density of the final image. One will always want to, at very least, test how long it takes the rebate of their negative to achieve "maximum black".
While it may not seem like four variables could control so much, the truth of the matter is that there are many combinations of these four variables that can get out of hand quite quickly. Example time; I pour a lamp black tissue that's comprised of 10% gelatin, and 1.5% pigment. Each tissue is relatively thick, 120ml of glop per 10x12" piece of x-ray film, correlating to 1ml glop, and 0.015g lamp black per square inch of tissue. Assuming every negative I use to print on this tissue is of a uniform, relatively high contrast, sensitizer strength would need to be ~3% potassium dichromate to get a nice, long tonal scale in the final print. This also assumes that UV exposure is constant (thanks to the plate burner, it is!).
What if I like a particular negative, but it prints way too flat with the above combination? The first check I always perform, before messing with unnecessary controls, is to vary sensitizer strength. Reducing the dichromate from 3% to 2%, perhaps even 1.5 or 1% may be necessary to get this negative to print. If this doesn't solve the problem, a higher contrast tissue, is needed. To do this, pigment load needs to increse, from 1.5% to the 2-3% range, and potentially even a decrease in tissue thickness. From here, new tests for sensitizer % and UV exposure must be calculated. Typically, this would result in shorter exposures at higher sensitizer percentages.
Should someone like their carbon prints to have a little more noticeable relief, they'd have to do the opposite, and make a low contrast tissue. Generally, this is accomplished by decreasing pigment concentration, perhaps < 1%, and increasing tissue height. From there, these thicker, less opaque tissues are sensitized with a much lighter dichromate %, then exposed for much longer than a regular tissue. Again, only proper testing for each variable changed will give the printer the knowledge to accurately predict their results.
What I've been talking about isn't a large batch of images, folks. This is ONE image at a time. Carbon transfer printing is a laborious, yet rewarding process. Here, the patient, careful photographer whose negatives are consistent is rewarded with predictable results, and a streamlined printing process. Finally, for the experienced carbon printers out there, I'll leave you all with a list of each variable, just the above four, that have an effect on the final image:
- Water purity
- Presence of Air Bubbles in Water
- Relative Humidity
- Ambient Temperature
- UV Ouput of Ambient Light
- Tissue Support Medium
- Gelatin Bloom Index
- Gelatin % of Glop
- Amount of Sugar in Glop
- Type of Pigment & Tone
- Tissue Height
- Sensitizer %
- Sensitized Tissue Age
- Sensitized Tissue Dryness
- UV Exposure Time
- Mating Bath Temperature
- Time in Mating Bath
- Drying Time
- Development Temperature
- Development Time
- Sizing of Final Support
Still not terrified to try carbon printing? Check out my Carbon Transfer Workshop going on early next month.