Existing CMYK artwork needs separating into two colours

I’m working with existing illustrations which consist of only two colours, yellow and black. These were originally created as CMYK TIFFs.

This must be printed on a two-colour press, not CMYK.

The problem I face is that the printer is worried about the trapping of the two colours, i.e. as a result of inevitable paper distortions there may be white gaps between the solid areas of yellow and black. What the printer really wants is for the two colours to be on separate layers, one for yellow the other for black.

In order to achieve this, I could of course just use one of Photoshop’s selection tools to first select the yellow and create a new spot layers from this selection and then do the same for black then just dump the original CMYK layers, however this is not ideal. Whilst Photoshop’s selection tools are good, a lot of the detail will inevitably be lost where it will show up most, around the edges.

Is there any way to collapse a CMYK file into just two spot colours?

I have had a lot of problems like this before and had to spend ages reading PS help file to get my head around the problem but if I can remember rightly I had to keep it on one layer?
You have seen this:
* For spot color graphics that have crisp edges and knock out the underlying image, consider creating the additional artwork in a page-layout or illustration application.
* To apply spot color as a tint throughout an image, convert the image to Duotone mode and apply the spot color to one of the duotone plates. You can use up to four spot colors, one per plate. (See Printing color separations.)
* The names of the spot colors print on the separations.
* Spot colors are overprinted on top of the fully composited image. Each spot color is overprinted in the order it appears in the Channels palette.
* You cannot move spot colors above a default channel in the Channels palette except in Multi-channel mode.
* Spot colors cannot be applied to individual layers.
* Printing an image with a spot color channel to a composite color printer will print the spot color at an opacity indicated by the solidity setting.
* You can merge spot channels with color channels, splitting the spot color into its color channel components.

Cheers for that. Yeah it's a big problem.

The dificulty I have is that I'm working with an existing graphic which is yellow and black. In order to create a richer black, black is made up of black plus some cyan, magenta and yellow. The printer wants one layer of black and one of yellow. Creating a black layer is no problem, it's just black nothing else. The difficulty comes with the yellow layer. As areas of the image which are black also contain yellow (as well as cyan and magenta), if I create a yellow layer what aprears on that layer is all of the stuff that is meant to be yellow, plus all of the stuff which is meant to be black too.

Essentially what I want to tell photoshop to do is, "Now you've put everything that's black on a black layer, now put everything which LOOKS yellow on a yellow layer."

As things go, the job is going to print today and what we've done is to just discard the cyan and magenta layers and hope for the best. At 25,000 copies of a 16 page document this is a little nerve wrecking. I'd like to know how to get around this problem in the future.

I think of the wonderful examples of Czech lithography from the 20s and wonderful two-colour printing right up to the 70s and beyond and realise that now that four-colour more the norm I'm really missing some key facts.

If there is anyone who can provide any guidance on this I'd like to hear from them.

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