Green Timber is also known as Fresh Sawn and it means that the wood hasn’t had time to air dry and that it hasn’t been put into a kiln. Usually the wood is supplied in beam form however some species can be cut down, while still fresh, and machined to be used for cladding.
Some hardwoods, in particular Oak and Sweet Chestnut, have a lot of tannins in the Oak which react with metal and you get a blue stain forming on the wood from where it meets the saw. In larger section sizes that are used for beams it is often less obvious than on smaller sections like cladding.
So, what can I do to remove the Blue Stain?
If left outside the rain will, eventually, wash the stain away for you. However, if you want to remove the stain straight away you can use oxalic acid. You can buy the acid in crystallised form which you then mix with water to form a solution which, when applied to the timber, removes the blue stain.
You can buy the oxalic crystals from a company called AG Woodcare and it can be used to remove tannins from surfaces like slabs and bricks but it is important to test a small innocuous area of the surface first to make sure it doesn’t ruin the surface.