In Offset printing it is theoretically possible to print up to four colour layers with a 100% coverage above each other. However, this incredibly high overall colour application leads to the fact that the smallest mechanical stress can result in scratching, the product takes a long time to dry, the paper soaks through and the colour, in the worst case, can no longer stick to the paper. Elements which consist of 100% cyan, 100% magenta, 100% yellow and 100% black, that means which have an overall colour application of 400%, are not producible. The overall colour application for Offset printing should remain, if possible, under 320%.
As there can be a excessive overall colour application , there can also be the opposite problem. An insufficient overall colour application is also known as breaking off grid pattern. This is given if the ink coverage per colour channel is below 5%. If a colour is made up of 3% cyan, 3% magenta, 3% yellow and 3% black, all colours together would have an overall colour application of 12%. However, every single colour channel has far less than 5% ink coverage, that means the grid is too weak and can´t be reproduced.
To make black surfaces particularly deep and brilliant, a trick is used. The surfaces are not only created with black colour but are also highligted with a surface in a chromatic colour.Polished black can be created in two different ways. If (up to) 50% cyan are mixed into the black colour, we call this a cold polished black, if magenta is used, we call it warm polished. Mostly, cold polished black is used. Warm polished black is used for instance in jewellery catalogues (gold jewellery) to make the metal look “warmer”.
warm polished =
cold polished =
Texts should not be polished!
In fine elements or small text (body text), which are to be black it is necessary to pay special attention to the fact they may only be made up in the black colour channel, as otherwise register differences can occur by the overprint of the single colour layers. The typeface thus looks blurred and duplicated.
This text is black!
This text is 4-coloured!
If black texts are positioned on coloured surfaces, it should be ensured that the option „overprint“ is activated. If the text is set to "overprint" the text is printed flat on the background image and it can not produce any white gaps. If “overprint” is not activated, the background image will be cut out in the spots where the black text is situated and white gaps/register differences can occur while fitting in the black text.
The text is set to "overprint".
The text is NOT set to "overprint".
You should set both black texts and graphics to “overprint”. Most layout programs (e.g. Adobe InDesign) do this automatically. You should change this option only with much caution, if you want to avoid, for example, a colour cast in the black elements. One should always observe the effect on the register accuracy - which is more severe? A register diference or a slight colour cast? Other colours than black can´t overprint. If, for instance, a coloured or white element is set to „overprint“ on a coloured surface, it will disappear or the colour will be distorted because print colours are not opaque but translucent.