How well do the holtzmann shields work against extreme heat? Although the shrapnel and stuff might not damage him, he might suffer some extremely bad burns.
Uhm yeah, that one is a bit tricky. It's said that the Holtzman field is usually set on a speed of 6 to 9 cm per second, but no explicit mention is made of the effect on temperature. As nearly all molecules in the air have a speed exceeding that by far, I would assume that said figure applies to the net diffusion speed and not to individual atoms, which would mean that
heat can propagate trough the shield, though at a reduced rate compared to normal. A reduced diffusion speed would mean a lower net energy transfer per second. This means that hot air will burn Duncan but it has to be a significant amount and of a significant temperature. In essence, explosives can burn him, but they'll need to explode relatively close to him. This is further supported by a passage in the book taking place during a training fight. It is said that the air becomes stale and Paul (or one of his trainers) became quite hot due to the heat build-up. This again points towards present, but slow diffusion and thus a slower and slightly dampened response to temperature changes.
It should however be noted that artillery has fallen out of grace by the time Dune comes around as it is mentioned that the Harkonen used several very old pieces of equipment on Dune since shields can't be used there, lest they attract the sand worms. No mentions are made of grenades or other explosives being used during combat.
This probably means that the radius at which an explosive would cause severe burns is too small for them to be effective weapons.As for the gas grenades, they would work, but again lowered diffusion speed would give Duncan some time to get away before they come into full effect (unless the used gas is of a very lethal kind)
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