First we will tackle the exposure time or better times. I use a double exposure, because in my opinion the best results are reached this way. For the first exposure a stochastic screen is used, and for the second we use the positive film of the image we want to print. Some people combine the image and stochastic screen in photoshop (using the bitmap / diffuse dithering function) to end up with only one exposure. This will not give you the best results, you'll get a more grainy look.
The stochastic screen;
a halftone screen based on pseudo-random distribution of halftone dots, first used in the seventies. For a more technical explanation go here.
The stochastic screen eliminates the moiré effect in multiple colour printing. It allows us to print photographic images without the annoying effect of the dotted screen. If well performed you will end up with a print very close to the original picture, but with the advantage that you can digitally work with the image. You will be able to choose papers and colours, and when printed with traditional oil based inks on the right paper it will be an everlasting image.
The positive film;
The selected image will be printed on an inkjet printer. It will be printed as a positive image, and it will be the same size as the resulting print. Because it will be a contact exposure. There will be more on the positive film later.
Why we need the double exposure? The stochastic screen will prepare our polymer plate so it will be able to produce a wide tonal range, from deep black to subtle whites, without losing detail.
In the graphic below you will notice that the polymer plate exposed without the stochastic screen isn't able to produce the dark parts. The ink will only stick to the sides of the black volumes in your print. In the intaglio etching process the aquatint technique is applied to create this result, this results in holding the ink in place as the plate is wiped.
Once you are familiar with the polymer process, you can try out different screens.
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