A good indicator of embedded metal in a wood turning blank is a dark or grey stain on the side of the blank as seen below.
If these stains show, be very careful when starting to hollow the piece as its possible that metal could embedded inside. An example is shown below, the nail has be lifted to show it more clearly.
When turning a piece which has the possibility of an embedded piece of metal, the first option is to scrap the blank. If you want to continue with the piece, wear full face protection and take very shallow cuts taking great care. Listen for sound changes when turning, sometimes a ticking noise might be heard which could indicate an embedded object.
There are many reasons for embedded objects in our woodworking blanks. Purple staining on Yew sometimes indicates metal embedded as farmers used to nail barbed wire fences to Yew trees to prevent cattle picking up the Yew’s poisenous berries. Many trees sourced from cities contain shrapnel from explosiones during the second world war.
Given these examples together with cracks, loose bark an unstable knots in wood, the wearing of eye and full face protection should be normal practice.
Take care
Alan