Hair foil and heat
Foil does not add heat. It encloses the section, so the warmth from the scalp and from the lightening reaction stays around the hair, and the product stays moist and keeps working. That is why hair in foil usually lifts more evenly than hair left open. We have no test data on processing speed and make no claim about it.
How foil retains warmth
Three things happen when a section is closed in foil.
The product stays moist. Lightener works while it is wet. In open air it dries from the surface and slows down. Folded in foil, it cannot dry out.
Warmth stays in. The scalp is warm, and the lightening reaction gives off a little heat of its own. A closed foil keeps that warmth around the hair instead of letting it escape.
The section is isolated. The product touches only the hair it was put on.
None of this is heat supplied by the foil. Foil is a wrapper. Its effect is to keep the conditions around the hair steady, which is why foiled sections tend to lift evenly from root to end.
Foil compared with other wrappers
| Aluminium foil | Plastic film | Open air | |
|---|---|---|---|
| Keeps product moist | Yes | Yes | No |
| Keeps warmth in | Yes | Less | No |
| Conducts heat along the wrap | Yes | No | Not applicable |
| Lets you see the hair | No | Yes | Yes |
Aluminium is a good conductor of heat. In practice that means a foil evens out the temperature along its length, and it also means a foil near a heat source warms up fast. The comparison of materials is taken further in foil vs meche vs highlighting film.
Thickness and heat
Buyers sometimes ask whether a thicker foil “holds more heat”. A heavier foil has more metal in it, but foil is so thin that the difference between two salon gauges is small beside the effect of simply closing the section.
Choose thickness for what it does change: strength, body and resistance to tearing and to the comb. Those are measured, and the figures are on hair foil thickness and alloy.
Foil with heat processors
Added heat is a decision about the lightener, not about the foil. Three principles apply.
- Follow the lightener’s instructions. Many lighteners are not meant to be used with added heat. The product’s manufacturer decides, not the foil.
- Expect the foil to get hot. Aluminium conducts heat, so foil under a dryer or a processor warms quickly, including where it touches the scalp.
- Check more often. Heat shortens the margin between lifted and damaged hair. Open a foil and look.
Printed and coloured foil raise one more point. Our ink test is run at room temperature, and its page says so; it does not prove how the ink behaves under a heat processor. See the limits section of printed hair foil ink vs bleach.
What we don’t claim
We make the foil; we do not formulate lightener, and we have not run tests on processing time. So we do not claim that foil speeds up lightening by any percentage, that one thickness processes faster than another, or that any foil is safe under heat with any product.
What we can state is what we test: the thickness of the foil, how it folds wet, and how its ink behaves in lightener at room temperature. Those are on how we test hair foil.
FAQ
Does foil make bleach work faster?
Foil does not make bleach work faster by itself; it has no heat of its own. It encloses the section, which keeps the lightener moist and holds in the warmth from the scalp and the reaction. Lightener that stays moist and warm keeps working for longer than lightener left to dry in the open air.
Does thicker foil hold more heat?
Thicker hair foil has slightly more metal to warm up, but the difference in warmth around the hair between two gauges of hair foil is small. What thickness changes noticeably is strength: a heavier foil resists tearing and holds its shape. Hair Foil Factory has not measured a heat difference between gauges.
Can foil be used under a heat processor?
Foil can be used under a heat processor only if the lightener's manufacturer allows heat with that product. Aluminium conducts heat well, so foil under added heat warms quickly and can become hot near the scalp. Follow the lightener's instructions and check the hair often.
Is coloured foil different in heat?
Coloured or printed hair foil is the same aluminium with a coating on the surface, so it behaves like silver foil in ordinary processing. Added heat is a different matter for the coating: Hair Foil Factory's ink test is run at room temperature, and does not cover heat processors.