Hello, I came across this information about the history of ATCs - hope you find it interesting:
ATC's have been around almost as long as art has. They were very popular in the 16th century, when men would have portraits of their mistresses painted on small cards. They were also given often by families when they arranged marriages between chidren.
In the mid 1700's the English began using them as advertising.
ATC's have always been used by artists, especially during the Impressionist years, to trade with their fellow artists so they could study each others techniques.
Artist trading cards, also known as ATCs, are 2 ½ x 3 ½ inch (64 x 89 mm) miniature works of art which artists trade with one another, similar to the way people trade sports cards. They can be any medium: pencil, watercolor, acrylic, oil, collage, scratch board, mixed media - anything the creative mind of the artist can think up. Artist Trading Cards are produced as originals, as limited editions, or as a series.