'Four colour', you can't talk about printing for long before someone uses that phrase, but what's it all about? Well, it is as simple as that, a printed sheet that has four different colours to produce the finished result. But if it was that easy then there wouldn't be tons of literature on the subject. As a press operator there are a few things you should know about 4-colour work before you can guarantee perfect results every time. Four colour is more precise than spot colour work and because I only have a two-colour press I cannot rely on a visual check for colour accuracy. When we print the first pass on a four colour set, for instance black and cyan, it is impossible to asses if the colour density is accurate for the final print. If it is a landscape print that has a lot of greenery then the cyan is important for the green of the trees and grass but if it's too heavy then they will look blue and if they are faint then they will look yellow.

The reason we use four colours is because we start with a white base. We therefore need to print a subtractive set of filters to reduce the white base to give the colours needed for the final job. The subtractive primary colours are cyan, magenta and yellow. Only three colours you will notice. In a perfect world these three colours would produce solid black but due to difficulties with pigments it is only possible to get to darkish brown. It's because of this we use black as the fourth colour. Sometimes it's referred to as K, this stands for Key, where the equipment that produced the film labels it as such.

For accurate colour matching we must use additional tools, in my case a Gretag D19C densitometer. It measures the density of the printed layer of ink by shining a light at the ink and measuring the amount of reflected light. When matched to a pre-determined set of values the printed image will be accurate to the customers specifications. The D19C has many other features but at the end of the day it is just a densitometer and not a spectrophotometer. The spectrophotometer can measure any colour where the densitometer can only measure CMYK.

This is a copy of the Gretag values for CMYK printing. It lists three values for each colour that cover the three most popular papers used in the industry. The ink will give a different reading on the densitometer for each paper it is printed on. It is important to determine exactly what kind of paper you are using and run the job to those values. If you print the set too heavy then you set yourself up for drying problems and if you print the set too faint then the colours might not be up to strength. It is advisable to only trust the measurement of wet ink. Once the ink starts to dry it will fall into the valleys of the paper fibres, this will give a darker/higher reading.

On closer inspection you will see that the values listed are all relevant to each other. That being that black has the highest reading, cyan has a slightly higher reading than magenta and yellow is the weakest. This is the main consideration when printing a four colour job. Only when all four colours are in the correct range will the job look correct. This is possibly as important as getting the readings spot on. If you are printing on a paper that is not listed then it is down to your own judgement to select a set of values that are correct for the paper, as long as all the colour are relative to each other then the colours will be correct. Remember that all the press operator can do to influence the job is print the colours wrong. If they are printed correct and the colours are in fit then the minder has done the best job he can do and if there are any problems with the finished print then the problem lies elsewhere.

On the press there are a few other considerations when printing. Yellow ink generally lays back in the duct and will require a stir at regular intervals. Magenta is a high pigmentation ink and is very strong. This makes it sensitive to changes. Shackell-Edwards have mixed us a special reduced tack magenta to help with this problem but it still needs to be kept in mind. Black and cyan are generally pretty good. When printing one colour over another the total value of ink has an effect of the resulting colour. Generally you should not print more than 300% ink coverage in a wet-on-wet situation. As we only have a two colour press we can only achieve 200% so this shouldn't be a consideration. However, for some reason, solid magenta has trouble laying over solid yellow. This is a problem when we come to print our company logo as the red dot is solid M & Y. This is called trapping. The D19C can measure for trapping but don't concern yourself with the figures it gives, as said before it's really only for 4-colour presses. Just keep an eye on the logo and other reds.

Another problem is ink tack. All litho inks are tacky, it's the reason the blanket is there. Once on the run the ink tack has a pulling effect on the sheet. At best you will have to clean the blankets ever few thousand sheets to remove build up and at worst the ink tack will pull the sheet out of the grippers and you'll loose the sheet. The problem with CMYK printing is blinding of the blanket during the run. The ink tack gradually pulls paper fibres from the sheet and they become trapped on the blanket. After several thousand sheets the build up starts to protrude above the surface of the blanket and forms a ridge. The area immediately behind and in front of the ridge (in the direction of rotation) becomes a valley that the plate cannot reach. This becomes a non-printing area. It happens gradually but can become a problem within a very short time. The way around this is to reduce the tack of the ink, usually maganta, by adding tack reducer and clean the blanket. The low tack Fourmost ink combats this but you should be aware that it could happen on the run.