SILICIOUS DEPOSIT ON FLUE-SHEET.
Certain kinds of coal deposit a hard, silicious substance upon the back flue-sheet, which gradually accumulates till the draught is seriously impeded. This, of course, prevents the full benefit of the hot gases being obtained; and consequently the steam goes down. Flues stopped up with cinders produce a similar effect. The flues getting choked up with cinders is not always an indication that the petticoat-pipe is performing its duty improperly. Stopping up of flues is often caused by wild, unskillful firing. A shovelful of coal pitched high, deposits part of its load direct in the flues; and some pieces that are a close fit do not go through. They stick half way; and small cinders soon follow, that quickly close up the entire passage.