What is foiling in printing?

The distinction from metallic ink matters. A metallic ink contains reflective particles suspended in a binder, and it looks like a grey or ochre approximation of metal. Foil is an actual mirror-finish layer. Side by side there is no comparison, which is why foil remains the standard for anything that has to look genuinely premium.

It needs a die — a metal block engraved with the shape to be foiled, heated and pressed against the sheet with the foil ribbon between them. That die is a tooling cost per design, so foiling carries a set-up charge in a way that a coating does not, and small quantities feel it more.

Two design constraints follow from the process. Very fine detail and small type can fill in or break up, because the foil transfers under pressure and heat rather than being printed at plate resolution — bold, simple shapes foil beautifully and hairlines are risky. And because foiling is a separate pass on a separate machine, its registration tolerance to the printed image is wider than colour-to-colour register on press, so a design where foil must sit exactly inside a printed outline will show variation.

Cold foil is the related alternative: applied inline using an adhesive rather than heat and a die, which makes it cheaper on long runs and viable for larger areas and finer detail, at some cost in brilliance. Foil over soft-touch or matte lamination is the combination that reads most expensive.