Corten is steel that is covered with a characteristic rust-colored patina. The rusty coating on the steel is both a decorative element and anti-corrosion protection. The oxide layer forms a barrier to further corrosion. It is said that "corten is alive" - that is, it constantly changes its color depending on factors such as humidity, sunlight, time, etc. The new corten sheet is gray, after it starts to cover with oxides, e.g. when using Cortenol, it changes color to light red - rusty. Over time, it darkens to brown and eventually may even turn black.