On 20 Jan, 20:00, cynic wrote:

Yes. They're bedded in mortar, which has a similar heatsinking effect.
If they're not, they can crack too.

Quite the opposite to heat sinking, the refractory retains the heat so
the cast iron attains a higher temperature.

Rubbish. Whilst it's going to "retain the heat", all that's going to
do is to keep it warmer in the morning, after the fire has gone out.
Peak temperature when lit for a piece of thin cast iron is going to be
higher for a plate with an air gap insulation behind it, compared to
masonry in reasonable contact. It's about the dynamic equilibrium of
heat flow in vs. out when burning (i.e. conduction), not about heat
storage (i.e. heat capcity).

Solid fuel (coke) boiler
firebars and fireplace grates are also made of cast iron and attain
red heat (1000C) on occasion.

Firebars are made of two different metals: some are a simple grey cast
iron, others an austenitic stainless. Parkrays and similar use a mix,
with the better steel in the middle where it's hotter. If you put them
in the wrong place under a coke fire, the plain grey iron doesn't last
at all long.

Mostly though, firebars are just that: bars. The reason they're
installed as separate bars is to stop cracking and warping like this.
The bars are free to move (compared to a one piece grate) so that they
don't crack (they'd warp first) and any slight warping is absorbed in
the rattle space around them. Although solid one-piece grates were
used for a long time, they didn't work for the higher temperatures
when we switched from coal on open grates to coke in enclosed box
stoves.

Also, "red heat" is around 600 to 800 C. 1000 C is a pale yellow.
Although you can achieve orange heats within the coals of a domestic
fire, you won't get the ironwork to 1000 C.