Top - transparent carrier sheet
Middle - micro-thin metallic foil
Bottom - hot melt adhesive coating

The application principle is that when the foil is laid over pre-printed Laser or photo-copied data with heat and moderate pressure applied to it (use a domestic iron), the adhesive will stick the printed area and not to the substrate. When peeled away from the substrate, the stuck element of the foil remains firmly attached, giving an extremely accurate and attractive metallic finish.

To prevent contamination of the work piece, mask any areas that are not being foiled with a sheet of paper (baking tissue is heat resistant, but almost any clean paper will suffice). Masking is recommended where small pieces of foil are being used on larger work areas.

Why does it only stick to Laser or photo-copied print?
The carbon content in the ink (which is actually a polymer/carbon mixture) reflects the heat more than the substrate, causing the hot melt adhesive to fuse on to the carbon printed area. Other types of ink do not have this property and will not give the same results.

Can I apply to much, or to little heat?
Yes - the foil adhesive fuses around 180 deg. Celsius, which is usually about the halfway setting on the average domestic iron, although such implements vary widely due to utilitarian thermostatic controls. The adhesive will fuse at lower temperatures (from 130 deg. Celsius) with variation in dwell time and pressure.
Inadequate heat will fail to fuse the adhesive and result in patchy foiling with small areas of foil failing to release from the carrier sheet. Excess heat may not only damage the foil, but melt the polymer that binds the carbon in the print. This will release the mixture on the underside of the foil, as well as degrading the print itself. Evidence of overheating can be seen with patches of carbon sticking to the underside of the foil.
However, there is quite a wide tolerance between the two states and a little practice can prove to be very gratifying.

What should I consider before starting?
1 Substrate - the smoother and firmer, the better the result. Cheap hairy paper = moderate to poor finish.
2 Print density - the greater the density, the greater the accuracy of application required.
3 Heat - covered above
4 Pressure - moderate, firm and even.
5 Cushioning - the surface beneath the substrate. Requires flat, smooth surface with low heat absorption.
6 Dwell time - duration of contact between foil and iron. One smooth, even stroke should suffice.
Remember you are transferring heat. If you put your finger on to a hotplate - the longer you dwell and the greater the pressure, the greater the burn.
7 Cleanliness - remove any contamination from your tools.

Do not iron directly on to the printed matter. The print is a polymer/carbon mixture and the polymer content will melt, stick to your iron and transfer elsewhere. Use baking tissue to mask the work and prevent iron marks.

If I under heat the foil, can I re-apply it?
Yes - if applied correctly, it only sticks to the printed area, so there is no problem in re-applying virgin foil to the same area.
If you have patches due to overheating (check the underside of the applied foil for carbon deposits), the print area itself will have been damaged, so re-application cannot work.

Can you use glue instead of an iron?
Yes - the foils will adhere to most types of glue (hot or cold). Just lay down your glue pattern and allow to become tacky. Cover the area with a piece of the foil and lightly press, or roll to make contact. Allow the glue to dry, as per the glue supplier's instructions and peel away.

Conclusion Like most craft projects, using the foils requires a moderate level of skill, which can be easily acquired with a little practice and patience. The rewards are well worth the initial effort and the use of foils brings a valuable addition to the crafters kit.