Looking at a colorized photo of a long-dead historical figure can feel like stepping through a time portal. You’ve likely seen the famous portrait of Albert Einstein clasping his hands, for example, but the image is given new life when you can make out the tone of his skin and the shade of his sweater.

Colorization is a time-consuming process, and the work begins long before artists pick up their graphics tablets. As Vox reports, each hue is obsessively researched using diaries, government records, and the expertise of historians. So when you look at a professionally colorized photo, you aren’t merely looking at an artist’s interpretation of the past—you’re getting an image that’s as close as possible to the real thing.

Get an in-depth look at the craft in the video below.