This idea is a simple one - cut some clear plastic acetate to the exact measurements of a playing card. Have a card 'signed' when of course they really sign the acetate (I suppose you would have to hold it on top of the deck whilst they did it).

Then transpose the signature to a dupe by palming the acetate.

Anyone come across anything like this? Is it practical?

That's good intuition. In practice there's some lighting risks.

Market item. Also used as locator card introduced to borrowed deck. The transparent cards are sold by the deck at magic shops.

Ah yes, it would shine if it caught the light, I hadn't thought of that.

Crystal Method by Darius was exactly this... well it was a whole project with lots of ideas by different people, on how to use them. And yeah, the problem is the light.

//Dan

There is no problem with Fax by Loki Kross. Or "sign it and fake it " by me. my effect won't be released but if you get Fax you will have all you need, and will get the joke.