An excerpt from “Making Photogravures With Polymer Plates: A modern technique of historical photo-mechanical printing using steel-backed polymer plates, etched with water and printed by hand with traditional intaglio processes“.
The base exposure time is the minimum time needed to achieve 100% black. The time is different for each person’s workflow because of several variables that affect the intensity of ultraviolet radiation.
Strength (and type) of UV radiation
Different exposure units emit different levels of ultraviolet light. They will also emit different wavelengths of ultraviolet light. Most commercially available exposure units are calibrated to emit between 340 – 370 nm, but you should check the bulb type to make sure.
Distance from UV source
The farther away the polymer plate is from the ultraviolet source, the less UV radiation it will receive in any given time. The plate will need more exposure time the farther away it is from the UV source.
UV absorbing layers
Glass and plastic block varying amounts of ultraviolet light. Thicker glass or plastic will block more ultraviolet light than thinner glass or plastic. The image transparency is made of plastic, and will block some ultraviolet light before it has any ink printed on it. The glass in the contact frame will block some ultraviolet light. Some UV exposure units have glass that protect the bulbs. That glass will also block some ultraviolet light. Caution: while these layers block small amounts of UV light that will make a difference in your pictures, they do not block enough to make the work environment safe for your eyes. Protect your eyes by making sure your system does not have any places where light can escape, and by wearing UV filtering safety glasses! Photogravure is an intaglio printmaking process. Traditional copper photogravure uses an aquatint pattern (made with rosin) to create a very fine dot matrix in which the artist controls the depth of each dot. The deeper the dot, the more ink it will hold. More ink means deeper tone. Less ink means lighter tone. KM73 polymer plates are fine enough to hold a similar dot matrix. However, the way the “aquatint” is created is different. Instead of using a traditional aquatint process, polymer plates must be exposed to a film that has a random dot pattern (preferably using a stochastic screening process that appears random to the human eye). This “stochastic screen” creates the “aquatint” on the polymer plate. The terms “stochastic screen” and “aquatint screen” are used interchangeably in many printmaking circles. Each artist must experiment to discover how long to expose the KM73 plate, with the stochastic screen, to make a good enough aquatint pattern to print 100% black. Once the base exposure time (minimum time) is found, a simple calculation can be made to find the optimum exposure time.
Depending on the exposure unit you have, times can vary from just over 1 minute to 16 minutes long. If you do not know the intensity of your system, you may want to make many marks, allowing you to make many exposures.
The marker will wash off the plate during processing, but the ink will have blocked enough UV light to leave visible marks. You can also scratch the plate instead of using a marker. The plate needs to be marked so that the times can be found later.
The experiment shown in this chapter exposed this plate from 2 minutes to 16 minutes, in 1 minute
Being slow and methodical during this entire process will save you a lot of headache in the future. The goal is to get very useful information that is going to be the basis of future calculations. Make sure that the plate does not move during this experiment.
The printed side of the screen must face the emulsion side of the plate. The KM73 plates have steel backings with greenish polymer emulsion on the front. The printed side of the stochastic screen has a matte appearance to it. The other side will appear glossy.
Leaving a small part of the plate uncovered will give it full exposure to UV light, curing it. Any writing on that part will be readable. This is the part where you will have made marks by scratching (or with some kind of marker) that will allow you to track the different exposure times.
The acid etched side of the glass (matte) must go against the stochastic screen.
Here is the order for the stack you have created:
- UV exposure unit, (ultraviolet light shines through the glass, through the screen, onto the plate)
- contact frame glass,
- stochastic screen,
- KM73 plate,
- contact frame back
The light-blocking bag goes on top of the stochastic screen. If it went between the screen and the plate, it would create gaps through which light could leak. Make sure that the edge of the light-blocking bag lines up with the mark that was made on the plate and contact frame. This is how you are tracking the exposure times.
This exposure process is additive. The initial exposure of 2 minutes plus any additional time that the section of the plate is left uncovered. 2 minutes, +1= 3minutes, +1= 4 minutes, +1= 5 minutes,
+1= 6minutes, +1= 7minutes, +1= 8minutes, +1= 9minutes, +1= 10minutes, +1= 11minutes, +1= 12minutes, +1= 13minutes, +1= 14minutes, +1= 15minutes, +1= 16minutes. If you have some data that gives you an idea of how long your exposure needs to be, you may choose a smaller
time range for your experiments, and work with shorter intervals, to achieve finer results.
This entire washout process only takes 2 minutes: 1 minute soaking, 1 minute gently brushing.
The brushing process is not a scrubbing process. The brush should move back and forth across the plate in a varying pattern with no more weight on it than the weight of your resting and relaxed
hand. (see chapter 1)
Caution: wear gloves for this part of the process. These commercial plates were not designed for handling
with bare hands.
Wet polymer plates are still not 100% cured, and are vulnerable to over-handling. Remove the water quickly and gently. Do not scrub the plate or you will ruin the dot matrix that has been created by the stochastic screen.
This part of the process is tricky. Applying too much pressure with the paper towels will leave an impression on the polymer plate. Any water that is not removed from the plate quickly will leave marks as well. Set up paper towels before beginning the exposure process so that they are ready to go as soon as the plate comes out of the washout tray.
The polymer plate must still be post-exposed (after being fully heat-cured) before printing. The post-exposure time is at least as long as the base exposure time. Since this experiment is about finding the yet-to-be-known base exposure time, then it is better to err on the side of caution. Let the heat-cured plate sit in a sunny window for an hour. After that exposure to sunlight, post-expose
the plate for the same amount of time as the longest exposure time in the experiment (in this example: 16 minutes) More is explained in chapter 1.
Note: Some steps are skipped, with the assumption that you have read other chapters about preparing
ink, inking and wiping a plate, and printing. These selected steps have been left in this chapter so that you can see the overall work flow involved with finding the base exposure time.
chapter 1 Main Workflow: (includes all missing steps for inking and printing)
chapter 9 Preparing Paper
chapter 11 Preparing Ink
chapter 12 Folding Tarlata
Under such high printing pressure, ink blurs and blends a bit. This blending compensates for some imperfections in the aquatint pattern on the plate. But, it only compensates for 100-90% black. The imperfections start to show at 80% and lighter. After experimenting with a few different exposure units, having run this base exposure time experiment several times, I have developed a formula that provides an optimum exposure time. The optimum exposure time is one that produces an aquatint pattern that most accurately matches the pattern of the stochastic screen.
minimum exposure time x 1.6 = optimum exposure time
I use the optimum exposure time as the time for both the stochastic screen exposure and the image exposure.
For the experiment in this chapter the minimum exposure time (a.k.a. base exposure time) is 10 minutes. To find the optimum exposure time, multiply 10 minutes by 1.6 to get 16 minutes. I’d use 16 minutes for the stochastic screen exposure, and 16 minutes for the image exposure.