Hot foil needs a flat, supported sheet. Heat, dwell, and the depth of the plate all act on that sheet while it is still a wrap, not a finished lid. The foil is in the paper. When the paper is then glued onto the greyboard, the mark goes with it and reads as part of the plane.
If you “add gold” after the box already exists, the metal has nowhere to bite. It sits on the object. In a picture that looks like a bright rectangle hovering above the lid, with no change in the surface around it.
The order is the craft: board first, so there is a lid; foil on the flat wrap; then the wrap onto the lid. Name the board’s caliper and the wrap’s weight before you name the metal.
How to check it
- Can you point to the flat sheet the foil was stamped on?
- Does the mark share the plane of the lid, including across a curve or an edge it was wrapped over?
- In a side light, is there a bite, or only a brighter patch?
Usage
.lid {
/* Foil belongs to the wrap, so it follows the plane. */
background:
linear-gradient(#8a6a32, #8a6a32) 50% 42% / 38% 16% no-repeat,
var(--paper);
}