5/16/2021 - Update - I left the cards double sleeved and packed tight in an enclosed space with dehumidifier packets. They warped again. The uncurling was not permanent.
I'm careful with my cards, always have been. I keep them in double sleeves. Temperature controlled. Low moisture. I store my cards in a Pelican case with desiccant packets and in a safe with desiccant packets.
I'm one of those people.
Over the years, I occasionally get a foil card that is already curled. The foils I've opened from packs or that are already flat when I get them stay flat. But the foils that are already curled, they are a different story.
I've tried everything to uncurl the foil cards. Pressing between stacks of books for weeks on end doesn't work, even when I use tissue paper to "draw in the moisture." Physically curling the cards the other way doesn't work, at least not for more than a few minutes. Double sleeving helps to physically hold the cards flatter, but it's still obvious that there's a curled foil in the deck. I've stored cards in sleeves and out of sleeves in sealed containers with desiccant packets. I've tried everything, that is, except putting them in the oven.
Guess what finally worked?
First, a couple of disclaimers. Don't do this to your cards. This worked for me, but maybe I got lucky. Maybe my set up is entirely different from yours. Maybe it's pure coincidence. You've been warned.
Here's the deal. I've had a few cards in one of my Commander decks that drive me crazy. The deck is for casual play, but it's still annoying to have a few cards that are obviously curled foils in the deck. If one gets shuffled to the top or shows up in a cut, it feels bad because everyone knows what it is. They know I'm not cheating, but it can still ruin the moment. I've stored these cards in a tight deck box for years and the curl has never come out.
In other words, these cards were technically unplayable. I had nothing to lose.
My assumption is that the foil layer of the card on the face is plastic or something similar that doesn't absorb moisture. The cardboard layer is paper and does absorb moisture. Too much moisture and the paper layer expands. Since the plastic layer cannot expand, the card curls "up" toward the face. Presumably the opposite happens if the card is too dry.
Like I said, I've tried other methods to remove moisture from the cards with no success. This time, I decided to try putting the cards in the oven.
I started by heating the toaster oven to 150 degrees. Then I turned it off. I only used the residual heat on the cards.
The first plate I put the cards on didn't allow them to lay flat, so I switched to a bigger plate.
I took them out of the oven, then pressed them in a big book between sheets of tissue paper as they cooled.
Nope. Still curled. I decided that I had everything out already, so I might as well give it another try. This time I turned the oven up to 175 degrees and left it on with the door open.
I also put the cards on the edges of the plate so that I could see clearly if they started to bend. The card on the left is starting to bend slightly across the back. I didn't take my eyes off the cards. Once I saw the bend forming the other way, I took the cards out and pressed them in the big book again between sheets of tissue paper.
Success. It's difficult to see in this picture, but the cards are completely flat. I put them back in double sleeves, back in the tight deck box, and left them overnight. When I took them out the next day and shuffled the deck, I could not tell which cards they were in the stack. These cards are as flat as any of the other cards.
All told, this process took about 20 minutes. Now that I know what I'm doing, I could flatten dozens of cards using the same process in less time. The biggest thing is to watch the cards in the oven closely and pull them as soon as you see the curl flatten. You can always put the cards back in if you need to.