The process of hot-stamping is very similar to type-set printing. Regular type is used, but instead of ink, the type is heated and then a ribbon of foil is pressed against the chip. The heat does 2 things, it melts the chip and thermally transfers the foil color to the chip.

This process is usually done, one chip at a time and if the operator goes too fast, then the type losses a few degrees of temperature, thereby allowing a weak hot-stamp. But, if the process is too slow, the type stays hotter and will cause a deeper melt and also the transfer of foil color to the sides of the type indentation. Moreover, if the temperature is set too cool or too hot, regardless of a well paced printing process, the same results will happen, i.e. a weak hot-stamp or a heavy one.

Now, the foil used is not just a strip of the color material being transferred to the chip (and BTW foil comes in many different colors). The color is affixed to a base material, which provides a degree of durability to the foil strip. It can withstand the heat (of properly heated type) and can stretch so that the type face does not tear, which would result in pieces being left stuck to the chip, or breakage of the strip, which requires re-threading to the release and take-up reels.

I believe, that in some types of foil, there is an additional coating, which may be a binder. That coating is on the surface of the foil color, but when stamped on to the chip it is then between the foil color and the chip. Usage of the chip (wear) will result in the foil color wearing off and that binding material can then be seen. That binding material is silver in color, sometimes white and with more usage, it waers away all together.