How do they work?
The cards are printed with a specially engineered heat sensitive ink. So when the cards are warm the ink is invisible, as they cool down to room temperature the ink visual fades in.
How do you keep them warm?
That’s easy… keep a hand warmer in your pocket, business card holder, or wallet. You can either purchase an electronic hand warmer or use disposable ones. Disposable hand warmers cost about $0.35 each and last up to 10 hours.
What if its really hot outside?
The ink is fully invisible around 92°F (33°C), and will start to transition sooner. So they are best used at room temperature. If you are a frequent outdoor performer these cards may not be the best choice for you.