Geograph - photograph every grid square

This fist-sized chunk of material has some smoothly curved faces, and some sharp edges. It looked, at least after I had cleaned it up, very much like a piece of obsidian. In terms of chemical composition, that is more or less correct, but I do not believe that the material has a mineral origin.

It was located at the foot of the near (that is, the northern) edge of the platform-like feature shown in NS3976 : View to the site of Mains of Cardross Farm. For views along the top of that northern edge, see NS3976 : Mains of Cardross: northern edge of the platform and NS3976 : Mains of Cardross: northern edge of the platform.

In the same area I came across several lumps that were more easily identifiable as clinker (the word "clinker" can refer to different things, but I am here discussing the kind produced as waste when burning coal). This dark, translucent piece, with its smooth faces, is almost pure silica (hence its likeness to a lump of obsidian); I take this dark siliceous material to be one extreme of the continuum of clinker produced in coal furnaces (perhaps from the bottom, where the containing vessel could impart its shape, giving some smoothly-curved sides).

At the other extreme (perhaps from the uppermost part of the waste) is the most calcareous clinker, much less glassy in appearance, and with pebbles of the most refractory material embedded in it.

Somewhere between those two extremes are the pieces of clinker that, while much more opaque and varied in colour than this obsidian-like piece, and less glassy, still have a fused appearance; compare NS3977 : Clinker, from nearby Dalquhurn Point.

As noted at the first of the links given above, I take this to be material that was used to fill in the pits after the mid-twentieth-century extraction of sand from the ground around the farm buildings.