Bog iron was discovered during the pre-Roman iron age. Most Viking Iron was smelted from bog iron. In a bog, iron is concentrated by two processes. It is now rarely mined as more concentrated ore deposits are used as primary sources of iron ore. The first is chemical. In the acidic low oxygen environment of a bog, a chemical reaction forms insoluble iron compounds which precipitate out. The second is Biological. This is due to, anaerobic bacteria (Gallionella and Leptothrix) growing under the surface. These excrete concentrate iron as part of their metabolism. The presence of these bacteria can seen where there is an iridescent oily film on the bog water. ( In Iceland this is called jarnbrak or iron slick). When a layer of peat in the bog is cut back pea sized nodules of bog iron can be found and harvested. They are reasonably pure. As the process of iron production is ongoing the same section of bog can be harvested for iron about once every 40 years. Iron made from bog ore will often contain some silicates. These form a glassy coating on the end product that helps prevent rusting.
