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In Ant-Man,

Scott Lang breaks into Hank Pym's house and discovers a lock that requires Hank's finger print to open. So Scott presses tape in a door knob to extract a finger print, places a ring around the finger print on the tape, fills the ring with a chemical, heats it over a flame, and then peals off a rubbery product containing the finger print which he then uses to open the lock. Each time I see this, I wonder why the tape did not melt, how he knew where on the knob to find the print, how he knew he was not getting a chiral mirror of the print, what chemicals he used, etc.

This seems like an important scene. Has Marvel documented anywhere how they explain what he did in detail?

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Caveat: I'm not sure that what Scott did would actually work as well in practice as it did the the movie.

Having said that, he's using a variation of a very common practice in forensics called "superglue fuming". It's well known that SuperGlue - which is primarily made up of a chemical called cyanoacrylate - adheres very well to the oils that our fingers excrete. The adherence is so precise that it can be used to lift fingerprints from surfaces where fingerprint powder otherwise wouldn't give a clear image, particularly organic tissue.

What Scott did in the movie was to create a "mold" of Hank's thumbprint, using superglue. The process was:

  • Use a piece of packing tape to lift a fingerprint from the door. This basically means lifting the oil residue left over from when Hank touched it. Note that this would create a mirror-image of the print (a front-to-back reversal).
  • Using a large metal ring as a container, pour a glob of cyanoacrylate overtop of the finger print. The glue would have "molded around" the oils on the packing tape, creating a second mirror image of the print.
  • Heat the cyanoacrylate to make it more fluid. I guess this was supposed to get the glue to "mold" better.
  • Remove the object from heat and let it harden.

The reason I'm skeptical that this would work is because, in real-world applications, the cyanoacrylate is never applied directly to the fingerprint. Typically, an open container of cyanoacrylate is placed in a seal environment with the object to be printed, and heated until it evaporates. The vapor in the air clings to the finger oils more readily than the "dry" tissue, revealing the print. Just dumping a glob of glue onto the packing tape would most likely destroy the print by smearing the oil.

I'm also skeptical (as you point out) that the packing tape would have peeled off as easily as shown in the movie. The stove wasn't turned up very high and the tape wasn't directly in the flames, but I would have at least expected some of the glue from the tape to stick -- sitting in the hot sun for a few hours does this, let alone being held over a lit stove.

As far as how Scott knew how to do this successfully, though, that part's obvious: he's an experience cat burglar; his crew even recruited him partly because of his experience cracking advanced safes. It's almost certain that he's run into fingerprint scanner technology before, as it's been around for decades, so he would have figured out on his own what substances in the house he could use to fake one out. Superglue is also pretty widely associated with "fingerprint lifting" -- though not the way Scott uses it -- so it would have been a pretty easy choice.

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