Corten steel planter Mia-SDesign: Petteri WiimaaMade from Cor-Ten steel which is almost pure carbon steel but processed in order to have a protective layer on the surface to make it weather-proof. The surface is originally greyish, due the weather conditions it turns reddish-brown (resembles the colour of rusting), which becomes darker within time.If used properly, Cor-Ten steel is expressive, durable and maintenance-free and can stand the test of time in its own peculiar way.