What is tone-on-tone printing?
Answered by Stephen Beals, a digital pre-press manager and has been writing for major print publications for many years. He is the author of A Practical Primer for Painless Print Production. He can be reached at stephenbeals@mac.com.
Tone-on-tone printing is a very subtle method of imaging.  The term refers to printing a color on top of the same or very similar color (i.e. the same "tone").  It could be black on black or any color printed on top of the same color. The extra layer of ink makes the second image visible, but only very subtly. It could also mean black ink printed on a black piece of paper or fabric, for example (or any color printed on the same color of paper or fabric). It's more popular in the screen printing industry where the difference in color tones is generally more noticeable. 


One of the problems with tone-on-tone printing is that it is difficult to proof. Most proofing materials simply are not designed to show a second hit of the same color.


Achieving a tone-on-tone effect requires some out-of-the-box thinking for digital print. Clearly you can't add black to black in the digital realm, and running a piece through the machine for a "second hit" of toner can be problematic, partly because of the nature of the heat and fusion process required in toner printing. But there are ways around it. For example you can print a plain black background with an enhanced black image on top by adding a percentage of the other three toner inks to the image. Caution: the word "percentage" is important! Printing all four toner inks as solid values can produce very poor results: that much toner can make the inks bubble and peel during the fusing process. 


You can also try using different mixes of the enhanced black to produce different subtle tones. Adding 20% magenta and 20% yellow will produce a different "shade" of black (warmer) than adding 40% cyan to solid black (cooler). Or use more than one mix on the same piece. Another warning: such a difference is very subtle and may not show up at all on some toner imaging devices. You also may need to turn color management off to get the result you are looking for, and that can effect other colors in your piece. We didn't say it was always easy!


For a tone-on-tone effect for different colors - say reds or blues - to work on a digital press, you can use different shades of the color to simulate the effect of double hits of ink, but you still have the problem of not being able to achieve the same ink densities permitted by other forms of printing. Although these methods create tone-on-tone images in one pass, your printer may want to charge you more because it takes more toner.


Non-toner digital print devices like ink-jet printers don't have the same limitations as toner-based devices, giving designers more freedom to create tone-on-tone effects, and many of these devices have a dozen or more ink cartridges to work with, allowing more leeway for tone-on-tone effects. Some even have varnish capabilities.


Which leads us to another method of tone-on-tone printing that works for many types of printing including special digital presses. Instead of using a layer of ink or toner for the image enhancement, a varnish can be used. Several digital printer manufacturers have released coating stations for their machines that can add a "varnish" hit to give the tone-on-tone effect. In addition, using different types of varnishes - such as applying a gloss varnish to an image and a matte varnish to the background - can produce a dramatic tone-on-tone effect. In offset printing, varnishes can often be applied in one pass as well. Varnish stations also allow for at least the perception of higher total ink densities.

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