The Surface Is Not the Centre: Thermal Mass and Soak Time

Heat spreads by conduction, and the rate depends on the material's thermal diffusivity. Copper and aluminium are fast; steel is several times slower; ceramic, glass and stone are slower again by a large factor. The consequence is that time-to-centre grows with the SQUARE of the thickness. Double the bar and it takes four times as long, not twice. That single relationship explains most of what goes wrong here and it is worth internalising before any of the arithmetic. It is also why thin sections in the same piece reach temperature long before thick ones — the subject of a later rung, because it is the reason points and edges burn off while the body is still coming up.

Once the surface is at the colour you want, HOLD it there. The fire should be maintaining the temperature rather than still driving it up, and the work should sit in it without being pushed hotter. For hardening this matters most: the steel has to be fully austenitic through its section before quenching, and a piece quenched as soon as the surface reaches colour hardens only in a shell. The published heat-treatment blueprint depends on this being done properly. Turn the work and move it in the fire during the soak. A piece lying still has a hot side and a cold side, so it is soaking unevenly even while the total time looks right.

A kiln is fired slowly for exactly this reason. Clay and glass have diffusivities twenty to forty times lower than steel, so a pot wall that looks trivially thin takes minutes rather than seconds to equalise — and a thick foot or a heavy handle takes far longer than the wall beside it. Heat it faster than the inside can follow and the temperature difference across the wall creates stress. That is why ware cracks on the way UP as often as on the way down, and why a firing schedule has soak periods at particular temperatures rather than simply climbing. The same reasoning covers glass annealing, tempering a large casting, and warming a cold anvil before heavy work. Whenever a thing is thick and the material is slow, the schedule matters more than the peak.