What is a Folienschlüssel (foil key), and what kind of locks does it open?

A foil key is a soft-metal key blank with a deliberately weak blade. You insert it, push hard, and the pins themselves stamp their cuts into the soft blade — creating a working key on the spot.

How it works:

1. Insert foil key blank (made of thin, soft metal)
2. Apply heavy pressure on the bow + slight tension
3. Pins press into the soft blade, creating dents at their
   actual height positions
4. Withdraw, examine the dents, polish/cut to match
5. Re-insert → working key (or use a "shaped" version directly)

After one or two insertions, the soft blade shows visible dimples where the pins pressed in; polishing and trimming those dimples produces a working key.

Why "Folie" (foil)?

The blade is much thinner and softer than a normal brass/steel key — typically aluminum or pot metal. The thinness is intentional so that the pins make a visible imprint with hand pressure alone.

Where it's used:

• Mailbox / cabinet locks with shallow pinning depths and weak springs.
• Bicycle locks with low-quality pins.
• Older car door locks before sidebar systems.

Why it doesn't work on modern home cylinders:

• Modern springs are stiffer than the foil's structural strength → blade bends instead of pins moving.
• Security pins (spool, serrated) catch on the soft blade and break it off in the lock.
• The foil leaves obvious traces — soft metal flakes inside the keyway, visible to forensic examination.

Tip: Foil keys are essentially a manual impressioning technique — same idea as classic impressioning, but with the blade itself doing the recording. The two techniques rhyme: one uses the blade as a recorder, the other uses a separate key blank and hand-files between attempts.

Go deeper:

• Wafer tumbler lock (Wikipedia) — the wafer/jiggler-vulnerable locks foil keys target and why flat wafers are easy to manipulate.