Were you testing 172 because it was a problem ink? I'm wondering if it's the 072 base that's the problem, or the transparent white or both.
No. This was for a test of extended gamut printing (simulating spot colors with screen tint builds). The Orange was supposed to be PMS 021 (but measured as 172 in the press work) and I gave explicit instructions to the printshop as to what to request from their ink vendor. Either the instructions weren't passed on by the printshop or their ink vendor couldn't be bothered. The result was the splotchy halftone mess.
This problem with halftone screening (and wet trapping) of spot colors is not a new problem and it can happen with any spot color ink that is not used just as a solid. So, to be safe, I always tell the ink vendor exactly how the ink is going to be used.
AFAIK the ink formulation issue is pigment dispersion.
The ultimate particle size of most commercial pigments is less than 1 um, many are less than 0.5 um and the smallest particles will be as much as 1 to 2 orders of magnitude smaller than this. Due to their small size and surface energy considerations ink pigment particles have a tendency to form aggregates and agglomerates.
Aggregates are primary particles that are grown together at their surfaces. The total surface area of an aggregate is less than it would be for the individual primary particles (lower energy state). Aggregates cannot be broken apart with dispersion equipment (think of Lego blocks stuck together as normal).
Agglomerates are groups of aggregates and primary particles that are joined at their corners or edges but not grown together. These are the pieces that the dispersion equipment is meant to separate. Imagine those same Lego pieces separate and thrown in a cup.
Pigment is the single most expensive item in ink. As such the vendor will want to maximize the colour in every kilo and so will add the bare amount of pigment required to obtain the required colour. In order to do this the vendor will want to disperse each and every agglomerate possible. But not all company's control their dispersion as much as others. The clumping could be other things besides the pigment, such as wax, undissolved resin or even ink skin. Some pigments are harder to disperse than other colours.
(I'm sure D Ink Man could provide more detail and clarity)
Honestly, every printshop prepress/pressoperator should take a couple of hours out of their day to visit an ink vendor and take a tour of the facilities and ask every question you can think of (e.g. pigment grind/disperion, the effect of OBAs, fade resistance, etc, etc). The smaller "boutique" shops will usually be happy to do this and they will usually have all their ink manufacturing done under one roof (unlike the big vendors).
