Thanks for the interesting image. I have already seen it somewhere else here in the forum.
Primarily I am investigating for contemporary historical sources of such charcoal pans or fire vessels for heating up the hot irons, either as written form, illustrations or archaeological finds. The image seems to be a 19th century or later one and most of such "modern" illustrations are carrying slight modern interpretations.
In the "Marienburger Tresslerbuch" book of accounts of the Teutonic Order from 9th August 1409 is an entry which is probaly can serve a hint for such a fire carrying vessel:
4 scot vor 4 polfermesechen von bleche gemacht und vor 4 roren, do der bochsenschocze fuwer mag inne tragen
My rough German transcription (I am not a linguist):
4 Scot für 4 Pulvermaße aus Blech gemacht und für 4 Rohre, darinnen der Büchsenschütze Feuer mag inne Tragen
4 Scot [currency] for 4 powder measures made from metal sheet and 4 barrels/pipes/tubes in which the handgonner may carry fire
I guess the four roren may be a small vessel either for carrying fire for the hot iron or for lighting the matchcord.
The mentioned experience that the hot irons are rapidly cooling down was also described by Ulrich Bretscher on his Black Powder Homepage
but he only used a relatively thin metal rod, he probably shoud have tried a thicker metal bar with only a small tip. A friend has experimented with a forged hot iron and he also experienced that it cools down, but with improved personal experience and handling it is possible to start a handgonne. As soon as our new home page will go online we hopefully can present more details.
At present I wonder if also small handgonnes such as the Tannenberg, Danzig or Mörkö types were started by hot irons. In all historic illustrations showing hot irons absolutely certain for lighting guns, the pieces are large cannons or tiller guns.