Unlike most inks, metallic inks are Opaque – that is, they are not transparent like process colours. Instead, they block out any other colours which printed before their run on the press, and are quite viscous and thick (have a low-tack rating) and take a long time to dry.
Before preparing artwork with metallic inks, please talk to your printer. This is because drop-shadows, black type/solids and objects/rasters over the top of metallic inks with effects such as multiply or darken may not print as they appear on screen.
Quite often to achieve the on-screen appearance, printers may use what is known as a “dry trap”, meaning the metallic ink will be printed in one pass, then left to dry, and once the ink is dry, the other colours are run in the next pass. This can also be done in reverse, but the end result is the same – two passes through the press, which not only takes longer, but means misregistration is more likely due to paper stretching as ink dries, and will cost more in press time.
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Thanks for your excellent treatment of this. I would only recommend that instead of using the word “transparent” you use the word “translucent” when referring to standard printing inks. Glass, for example, is transparent to all visible light. Translucent objects allow some light to travel through them. Materials like printing ink are called translucent. When light strikes translucent materials, only some of the light passes through them.
Metallic inks are, as you have rightly labelled them, opaque, whereas there are also other inks, such as “opaque white” which are really only semi-opaque, and require several hits to produce true opacity.
P.S. I have only been a prepress operator since 2000, but ran printing presses before that since 1985. I am currently the Prepress Manager for a regional printing company.