Rich Black: Print
Black in Black
Rich black is a comparative term used to differentiate a process black (100% K) from one that is composed of black and combinations of other colors (C/M/Y/K), the added color giving a ‘richness’ to the otherwise pallid black (K). Although rich black commonly refers to printing, this technique has long been used in painting, where a deep red might be combined with black to produce a more rich color with added spatial depth.
The main difference between printing with process black (K) and a rich black (C/M/Y/K) is the rich black will have a higher density, due to using more ink, thus blocking the reflective quality of the paper white. There are many recipes for ink mixes depending on whether you are mimicking an RGB black in a software program, want a cool black or a warm black - we’ll discuss those later.
When you read posts of information online, it is important to determine if the writer is actually discussing a topic that is relevant to you - many posts I have read about Rich Blacks do not differentiate between offset printing and digital desktop publishing (Print-on-Demand publishing). Both of these processes CMYK, typically, but they put down ink onto the paper in different ways.
The Unclear Difference
Offset printing applies wet ink to rollers which transfer it to the paper to print an image, each color (including spot colors) having its own roller. If the ink oversaturates the paper it can cause all kinds of problems from pages sticking together, smearing, too much gain creating bleed in type or images, and so on. There is a limit to the overall level of ink, normally controlled in setting up your document to print through an ICC (International Color Consortium) color profile, like “U.S. Web Coated (SWOP) v2″, which limits total ink coverage to 300%. You do not have to use a total of 300% to achieve a rich black however - most ink mixes for offset start at around 140% (40% C / 100% K: cool black; 40% M / 100% K: warm black). In the high range 250% (50% C / 50% M / 50% Y / 100% K) coverage is the highest I typically see other people using/recommending.
Print-on-Demand and desktop publishing services use photocopier production machines to print material, and while these too use a CMYK color process, the process by which the ink transfers is slightly different. Most printers will use some model of Xerox or Canon high-output machine like a DocuColor or maybe an iGen3 - these machines use photoelectric drums to transfer dry colored toner and fuse it to the paper with hot oil. Oversaturation of toner on the transfer belt can gum things up causing machine jams or poor print quality due to streaking. In general, you are safe using the generic offset press ICC profiles with these machines as well.
Something to keep in mind, from an article by Charles Pickett, is that the iGen3 Xerox machine puts all the color down in one pass, and according to Xerox this process actually creates a richer black with process black than with a color mixture. You are welcome to experiment with this as you like, but my own experience printing on the iGen3 (used by Lulu.com outsourcers) does not confirm this.
Offset press requires consideration for trapping (keeping color back from the edges when overprinting black) and registration of rich blacks, which photocopiers may not require. For example, you would not use a rich black to print an 8pt font in offset, because any misalignment of the image between color passes would result in a blurred image or one where a fringe of magenta might show on one side of the font. In a photocopier where the image is transferred digitally and the color is put down in a single pass, this error in registration is eliminated - making it possible (though unnecessary..) to use a rich black for your tiny font!
Special Cases
Lastly, some words of caution concerning the use of rich blacks in your printing and file set-up. Do NOT use the registration black. Ever. It prints 100% of each color (including spot colors) and is only for crop marks and registration marks! (So, if you had a 4-color print with a spot color or varnish you would then be using 500% or more for the registration black!)
RGB black in conversion to CMYK will be different depending on which color profile you are using, and it will nearly always be tinted (ie- RGB (0/0/0) –> CMYK (75/68/67/90), these are real values assigned by Photoshop.) If you are matching a rich black in a page layout program, you should therefore check the values of the black you are matching to first - do not assume you know what color it is!
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Some Rich Black Color Recipes (C/M/Y/K):
- Cool Black : 40/0/0/100
- Warm Black : 0/40/20/100
- Dark Black : 75/50/25/100
- Process Black : 0/0/0/100
- Registration : 100/100/100/100 (registration marks only)
1 • Brix & Stone — December 15, 2008 at 4:27 pm
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