Definition of "clinker"

She left the road at the little shed where he whom she still regarded as her father used to keep his tricycle, and walked up the clinker path towards the house.

1938, Xavier Herbert, chapter XXXII, in Capricornia‎, New York: D. Appleton-Century, published 1943, page 581

The coal was terrible stuff—Indian, Abdul told me. The "dart" was used often and I saw some monster clinkers.

1942 July-August, Philip Spencer, “On the footplate in Egypt”, in Railway Magazine, page 208

Cold and grim sat that malevolent brute the furnace, greedy, bottomless—its grate bars clenched over clinkers which no shaker could dislodge.

1944, Emily Carr, “Dew and Alarm Clocks”, in The House of All Sorts

This wall of rock, which had no doubt once formed the lip of the crater, was about a mile and a half thick, and still covered with clinker.

1886 October – 1887 January, H[enry] Rider Haggard, She: A History of Adventure, London: Longmans, Green, and Co., published 1887

Nobody could pretend that a huge slope of clinker is aesthetically pleasing.

2004, Richard Fortey, The Earth, Folio Society, published 2011, page 10

This burning has baked and clinkered the adjacent strata, producing a very resistant formation, which rises with conspicuous abruptness from the flat terrace underlain by the soft Lebo shale member.

1923, United States Geological Survey, Bulletin 748, page 125

The use of coal with a low ash fusion temperature (1204°C, or 2200°F) caused frequent clinkering on the grate during initial tests. The clinkering stopped when the coal was replaced with one having a higher fusion temperature […]

1981, David W. Schultz, Municipal solid waste, resource recovery: Proceedings of the seventh annual research symposium at Philadelphia, Pennsylvania, March 16-18