One of my favourite passages from the movie A Christmas Story (1983) goes like this:
The Old Man (Ralphie’s father): It’s a clinker! That blasted stupid furnace dadgummit! [He walks down a few stairs to the cellar and slips down the rest.] Damn skates! [Coughing.] Oh, for cripes sake, open up the damper will ya? Who the hell turned it all the way down? Again! Oh, blast it!
Ralphie (narrating): In the heat of battle, my father wove a tapestry of obscenities that as far as we know is still hanging in space over Lake Michigan.
The clinker the Old Man referred to was a mass of fused ash that remained after coal was burned in their furnace.
Not everybody, especially some aggregate producers, are as upset as the Old Man when they find clinkers. In some parts of the world, clinker is used as aggregate and is a welcomed material. For example, in the plains of eastern Montana, western North Dakota and northeastern Wyoming, traditional sources of aggregate are extremely limited and clinker is often used as a substitute. In fact, clinker makes up more than one-third of the crushed stone produced in Wyoming.
The presence of clinker has been noted for some time. When American pioneers Meriwether Lewis and William Clark were wintering over at Fort Mandan (North Dakota) during their famous expedition along the Missouri River, Clark wrote in his journal (21 March, 1805): “Saw an emence [sic] quantity of pumice stone on the sides & feet of the hills … with evident marks of the hills having once been on fire.”
On 16 April, 1805, Lewis wrote: “I believe it to be the strata of coal seen in those hills which causes the fire and birnt appearances frequently met with in this quarter. Where those birnt appearances are to be seen in the face of the river bluffs, the coal is seldom seen, and when you meet with it in the neaghbourhood of the stratas of birnt earth, the coal appears to be presisely at the same hight, and is nearly of the same thickness, togeter with the sand and a sulphurious substance which usually accompanys it.” [sic]
Lewis and Clark were spot on. Natural clinker is a result of softer rocks being “fired” by burning coal. Natural clinker deposits are often found where down-cutting of streams exposed coal beds to the air. Lignite and sub-bituminous coals in the western US have high moisture content, and the drier air promotes the loss of inherent moisture where the coal outcrops. The coal oxidises and self-heating accelerates until spontaneous combustion occurs.
The heat from the burning coal rises so most of the clinker develops above the burning coal bed. Sandstone and mudstone bake to a bright red brick-like rock and shale may be fused like a ceramic in a kiln and look like hardened lava. Clinker beds range from a few metres to as much as 30m thick.
The coal that fuelled the process burns to an ash that takes up only a fraction of the space it originally occupied. The overlying baked and melted rocks slump and collapse into the void left by the burnt coal. The spaces resulting from the collapse serve as convenient homes for a variety of animals, including rattlesnakes.
Come to think of it, I probably would weave my own tapestry of obscenities if I stumbled upon a rattlesnake den while prospecting for clinker. Oh, fudge!