[Drone] tipped us off about [Joachim]’s efforts to alter a crystal’s frequency. Through a process called penning, a crystal’s resonant frequency is lowered by painting the crystal with an indelible ink marker. Our curiosity piqued, we went off and found more information about penning crystals. It turns out this technique has been around for nearly as long as there have been amateur radio operators. Outside of your local oscillator, and radio jammer, how might you best use a hacked crystal?