Coal seams show as black or dark-brown bands in stream banks, road cuts, quarry faces and landslide scars, usually inside layered sandstone and shale. A lump is probably coal if it is light for its size (about half the weight of the same volume of rock), leaves a black-to-brown streak when scraped on unglazed pottery, and burns steadily leaving little ash.
Part of Coal Surface Mining
Prospecting for coal is a matter of reading where layered sedimentary rock is cut open, then confirming a candidate with three cheap tests before any digging starts.
Look Where Erosion Has Already Cut The Ground Open
Coal is a sedimentary rock, so it lies in flat or gently tilted layers between other layers, mostly sandstone, shale and clay. You will not find it by digging at random. You find it where water, a landslide or an old quarry has already sliced through the layers: the cut bank of a stream, the foot of a cliff, a hillside scar. A seam is a dark stripe that runs sideways along the exposed face, often only a few centimetres to a couple of metres thick.
Old workings are the easiest clue. Spoil heaps of black shale, shallow ring-shaped hollows, and abandoned rail beds and pit-head structures all mark places where someone found coal before you. Treat the site itself as a hazard (see the danger block below), but use it as a map.
What You Need
- A spade and a hand pick to clean the face and expose fresh material
- A piece of unglazed pottery (a broken tile or pot base) for the streak test
- A bucket of water and a hot fire for the burn test
- A tape or marked rod to measure the seam thickness (Measuring Rods)
Method: Three Tests For A Suspect Black Layer
Step 1: Weigh it in your hand. Coal has a relative density near 1.3 against about 2.5 for typical rock, so a coal lump feels about half as heavy as a rock of the same size (1.3 ÷ 2.5 = 0.52). A black stone that feels heavy is not coal.
Step 2: Streak it. Drag a corner across unglazed pottery. Bituminous coal leaves a black streak; lignite leaves a brown-black one. Black shale also marks, but it is harder and splits into thin flat flakes.
Step 3: Burn it. Put a lump in a hot fire. Coal keeps burning and leaves a small ash residue. Carbonaceous shale glows, smoulders and leaves most of its bulk behind as grey ash, so judge by how much is left, not only by whether it burns.
Worked Example: Is The Seam Worth Following?
Inputs: a seam 1 m thick, traced for 10 m along the bank, with the digging reaching 2 m back into the hill; relative density 1.3 t/m³.
- Volume = 1 m × 10 m × 2 m = 20 m³.
- Mass = 20 m³ × 1.3 t/m³ = 26 t.
The parent article’s household figures are 4-8 t a year for bituminous coal, so 26 t is about 3-6 household-years of fuel, not the 50-80 t per metre of advance quoted there for a seam 2 m thick and 10 m wide: that volume gives 2 × 10 × 1 = 20 m³, which is again 26 t. Work from volume, not rules of thumb.
Common Mistakes
- Trusting the surface. Outcrop coal is weathered and weaker than coal a metre behind it. Dig back before judging quality.
- Confusing coal with black shale, jet or bitumen. Run all three tests; one test alone misleads.
- Skipping the thickness measurement. A thin seam under thick cover costs more rock moved per tonne of coal (see the overburden arithmetic in the open-cut article).
Safety
Old workings and cut faces collapse without warning
Never walk or dig beneath an undercut bank, and stay off ground that dips in ring-shaped hollows or shows sunken lines: these can be old shallow shafts or tunnels that have lost their support. Old workings can hold bad air and flooded passages. Look from the edge, never enter.
What’s Next
- Learn next: Geological Indicators — the rock sequences that make a seam likely
- Combine with: Extraction Methods — choosing how to get it out