David Krut Publishing,
Africa’s No 1 Arts Bookstore and Publisher

The pic above is a Heidelberg 5-colour press, and that’s not me walking next to it!

This is for those of you who have the faintest interest in book printing, presses, noisy machinery. Personally I love the sound of a large Heidelberg printing press, which is a good thing since I have to spend hours standing next to one when I finish a book. This is what’s called “being on press”.

By the time I am on press I am usually satisfied that all the things that could have gone wrong with the book have gone wrong, been spotted and are fixed. Well last week I was thrown a curve ball. I was finishing overseeing the printing of a book that we are not publishing but only seeing through production and then distributing: Dina Prinsloo’s Points for Departure (see this website for more information on the book). Dina had spent thousands having proofs made of all the images in the book so that my job on press could be made easier. Also, I was working with a designer who has been in the business for years – Louise Heckl from Ex Libris Press – and she had supplied me with a machine proof (done by her husband the famous Wolfie who is a master printer and has been operating a one-colour press for decades) so that I could match the colours of the two blocks of colour that Louise had incoporated into her design.

It took me over an hour to get the colours right for section one of the book, and even then I was not happy with the sheet – it was muddy, had lost all the detail in the images and worst of all we were running the press at very low values (which the machine minders get nervous about because they have very little control over what happens – just too little colour on the sheet). I stopped the job and went off with the sheet to see Louise, Wolfie and Dennis da Silva of Sivertone who did the proofs for Dina.

Wolfie and Dennis scrutinised the sheet and both concluded that thte trouble was being cause by a stochastic screen, instead of the usual conventional screen. This all starts to get quite technical, but the easiest way to explain it is that when you print you need a screen to hold the colour. IF you look closely at a printed sheet through a loupe you will see a collection of circles or hexagons – that’s the screen. Stochastic screen is different to conventional screen in that the dots on the paper are far more densely clustered – it looks a little like static on a tv screen. As a result you get very little white coming through the colours, which accounts for the muddy result that I was getting on press.

The two experts advised me to ask the printers to run the same section on new plates that had a conventional screen. The differences between the two sheets I now had in hand were startling. We were now able to run the sheets at pretty much standard values and this time match them almost perfectly to the proofs I had.

There’s a pretty good description of printing and screens on Wikipedia: http://en.wikipedia.org/wiki/Color_printing for those of you who want some more technical detail.

Another site that gives some more information about stochastic screen has this to say:
Stochastic printing, also called frequency modulation (FM) screening, uses small (10, 20 or 25 Micron), same size dots in a random pattern and varies the density of the dot to create an image that is closer to continuous tone.
Source: http://www.allgraphicdesign.com/stochastic.html

And then there’s this, and if you look at the two lists showing the advnatages and disadvantages of stochastic screen you will see what the possible problems are with it. I had all of them with the Prinsloo book:

The best way to define stochastic printing is to compare it to conventional printing.

In conventional printing, images are printed using grid-like screens that separate the image into evenly space dots that are larger in size in the darker areas and smaller in size in the light areas. In four color process printing, separate screens are used to reproduce each of the four colors, black, cyan, magenta and yellow.

In stochastic printing, images are printed by dots spread randomly throughout the image area. The dots are not equally spaced and aligned in a row or grid and they vary according to the tonal value to be reproduced. The lighter areas have few dots, the darker areas have more dots.

There are significant differences, advantages and disadvantages of both stochastic and conventional screening. Printing with stochastic screening offers the advantages of more detail, less ink on the sheet, no moir pattern, increase tonal values, crisper fine lines and small type and cleaner reverses. Conventional screens have the advantages of greater latitude of changing color density on press, more forgiving halftone reproduction, better printing of large amounts of ink on paper and better production of one and two color printing.

The charts below offer a comparative review of the advantages and disadvantages of each screening technique:

Stochastic Printing
Advantages

* More detail
* Less ink on the sheet
* Film of ink more evenly distributed on sheet
* More light reflects back to the eye from the printed image
* Ink dries faster on the sheet
* Eliminates moir patterns
* Easier to print more than 4 colors in one pass
* Better for touch plates
* Eliminates sawtooth edges in screens
* Best for printing textiles and other patterned images
* Shadows are more open and details are crisper
* Precise plate production
* More consistency from start of printing to end of printing
* Simulates continuous tone photographs
* Fine lines, small type and reverses print better

Disadvantages

* Can reproduce too much detail; grain of image’s photographic film can become visible, imperfection in flesh tones are more visible
* Holding a small dot is a great challenge on press
* Very unforgiving on press
* Tighter controls have to be implemented
* 20% more dot gain than conventional
* Proofing is difficult
Source: http://www.neenahpaper.com/articles/articles.asp?ft=Home&articleType=printing&article=7

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