Firebox fireclay lining repair: burnt brick and joints

I have been lining fireboxes for years, and every job starts with a look inside rather than with a hammer. I pull the grate, empty the ash pan and light up the side walls and the crown: burnt-out fireclay shows itself at once. The brick goes pale, loses its ring, crumbles under a fingernail, and the mortar runs out of the joints as dry sand. A web of hairline cracks along the joints is never cosmetic. Flue gases travel through them into the body of the stove and into the room, and they carry carbon monoxide, which has no smell, no colour and gives no warning at all. That is why nothing gets fired before I check the draught and the condition of the chimney: a cracked firebox with a reversed draught turns a stove into a source of poisoning, and before the heating season that check is not optional.

I replace only what has burnt away. There is no need to pull a whole stove apart for two courses of fireclay: the lining stands independently of the outer masonry, with a gap left for thermal expansion, and it comes out course by course. I clean the old joints back to sound brick, dust out the beds, damp them and lay on fireclay mixed with grog sand. The joint is kept thin, two to three millimetres. A thick joint burns out first, because mortar moves differently from brick under heat, and by the end of one season it simply pours out of the wall. Between the lining and the outer wall I leave a working gap filled with dry sand or a strip of card, otherwise the hot fireclay pushes the outer masonry apart from the inside.

I also reset the grate and the ash pan opening. A grate sitting crooked scorches one wall and warps its own bars, while a clogged ash pan starves the fire of air and makes the stove smoke. After the work the masonry has to dry: a week with no fire at all, then gentle firings with kindling and small splits, raising the load a little each day. A full fire comes no sooner than ten to fourteen days.

What is included: inspection of the firebox and a survey of every joint; removal of the burnt-out sections; fireclay laid on refractory mortar with thin joints; pointing and finishing; grate seated and ash pan cleared; draught and chimney check; written instructions for drying and the first firings.