Kaj Franck is a Finnish designer best known for his work for Arabia Finland. He studied furniture design in 1932 at the Central School of Industrial Design in Helsinki. Kaj Franck designed ceramics, glassware and my favorite, enamelware. He went on to design glassware for Iittala, introducing the idea of individually sold glasses rather than the traditional set.
There are many patterns associated with Kaj Franck's enamelware. The most often found and widely produced are the mushroom pattern bowls. I have seen salad bowls and serving bowls in this pattern, it subsequently also ends up being the most affordable. Harder to come by is the elusive lobster pattern. I was lucky enough to come across one which is pictured below.
There are also hearts, vegetables, little blue clovers, the red daisy pattern (pictured below on mugs), chevril, cherries, there are about 20 patterns in all.
Many Arabia Finland aka Finel pieces you find will be stamped on the bottom as the mug shown above, but many are not. If you want to start collecting the best thing to do is identify the patterns associated with the designer and brand. I for one was hooked the first time I got my hands on a mushroom bowl! Stop by my etsy store as I almost always have some Kaj Franck in stock.