I am attending a 3 day course on photo etching at City Lit over 3 weekends. Last weekend we were introduced to the principles of photo etching and the three different techniques to get source material onto transparent acetate in readiness to expose to a prepared photopolymer plate. I am interested in using photo etching to print drawn images.
A Little On Traditional Etching & Aquatint
In traditional etching a metal plate after being worked on with resists and acid will have grooves and furrows that the ink sits in. When the plate is passed through a printing press the paper is forced by the pressure into the grooves and furrows picking up the ink. If the acid has eaten away a large area of the plate, this is referred to as an open bite. An open bite because of its large area will not hold ink, as the ink gets wiped away from the surface of the plate easily. To get a dark tone over a large area you need to create many ‘pits’ or ‘dots’ in this area for the ink to sit in – imagine the many dots in a newspaper image. To get these ‘pits’ a powdered resin is applied to the area, heated, and then the acid eats away around the hard powdered resin resist and introduces ‘pits’ for the ink to sit in. The powdered resin is called an ‘aquatint’.
Photo Etching Day 1
Photo etching is where through a photo-mechanical process you transfer a negative of the source image onto a photo receptive metal printing plate; and after processing can print from the plate a positive image. The source image must be grayscale and it is transferred onto transparent acetate. The acetate is laid on top of the photopolymer plate and exposed to light in a special unit. The light penetrates through the acetate where there is no image blocking its way to the photopolymer layer on the plate. When you remove the photopolymer plate from the light unit you wash it in ‘developer’ which removes the areas of the photopolymer layer where your image blocked the light, so your image is revealed as bare metal on the plate. The plate is then submerged in acid which etches, ‘eats’, away at the bare exposed metal, these areas will hold the ink for printing a positive image.
The three ways you can prepare a source image for photo etching are:
1) A prepared digital image printed onto acetate.
2) A photocopy onto acetate direct from source image.
3) Drawing with indian ink directly onto True Grain.
It is important to understand that when you transfer an image photo-mechanically onto a metal printing plate that you will need to introduce ‘pits’ to your image where you require tone; so that your metal printing plate will print the tonal values of your image.
To help us understand how to print tone through photo etching, our first task was to produce a test plate by photocopying onto acetate direct from a source image. The source images I used were an image cut from a newspaper, black pen drawing, and part of a photograph I had.
The newspaper image was made up of many tiny ink dots; after photo-exposure to the metal printing plate these dots were acid etched and became ‘pits’. These ‘pits’ held the ink and so a tonal image was printed. The tonal image is quite dark, this is because many dots made up the newspaper image creating a strong tone; in fact too much of a strong tone.
The black pen lines after photo-exposure to the metal printing plate, and after acid etching became ‘thin grooves’. These thin grooves held the ink and so printed as a line.
The photograph is a tonal image, and a smooth image unlike the newspaper image which is made from dots. After photo-exposure to the metal printing plate, the dark areas of the photograph were acid etched. On the printing plate you can clearly see the image; BUT after printing this section printed as a messy blur. Why?
Well, the dark areas of the photograph were acid etched as an open area of plate – an open bite. You wipe an etched plate’s surface to remove excess ink from the surface but to leave ink in grooves and pits; if you are wiping an area that is ‘an open bite’ then the ink is easily removed, the ink does not sit neatly like it does in a groove.
As I wiped the surface of my plate I found all the ink was wiping out of the area where my ‘photograph’ was etched. So I under wiped trying to leave ink in that area. But the ink wasn’t sitting nicely in grooves or pits, it was just messily sitting on the surface which is why it printed as a mess.
To photo-etch and print a photograph successfully I would have had to introduce digitally ‘dots’ to the image (like the newspaper) so that the image photo-exposed and acid etched these ‘dots’ as ‘pits’ to hold ink and so print a tonal image.
Photo Etching Day 2 will be focused on using Photoshop to manipulate a tonal image I want to print.
(© Catherine Cronin)