Could you cut metal with a jet of hot sulfur vapor in a way analogous to oxy-fuel cutting of iron?
Sulfur boils at under 450° and reaches several hundred kPa of vapor pressure by 600°, and it reacts exothermically with most metals, with adiabatic flame temperatures in the 1000°-2000° range at atmospheric pressure. Sometimes the metal sulfides melt lower than the metals, and invariably much lower than the oxides; ferrous sulfide, for example, melts at under 1200°, much lower than iron’s 1538°, wüstite’s 1377°, or hematite’s decomposition at 1539°. Nickel sulfide melts at 797° and boils at 1388° (though I’m not sure you can just react nickel with sulfur), while metallic nickel doesn’t melt until 1455°, and its oxide not until 1955°. Chromium sulfide melts at 1350°, but metallic chromium doesn’t melt until 1907°, and its oxide (chromia or viridian) not until 2435°. Tungsten disulfide melts at 1250°, lower than tungsten trioxide’s 1473°, while metallic tungsten doesn’t melt until 3422°.
Even in the cases where the sulfide is higher-melting than the metal, it’s usually not by much. Cuprous sulfide melts at 1130°, slightly higher than copper’s own 1085°. Aluminum sulfide melts at 1100°, considerably higher than metallic aluminum’s 660°, and boils at 1500°; by contrast, sapphire doesn’t melt until 2072°. Even titanium yields a bit: its sulfide melts at 1780°, while titania holds out to 1843°. Tin(II) sulfide melts at 882°, while its principal oxide (the tetravalent) doesn’t melt until 1630°.
So maybe you could use sulfur-jet cutting to flame-cut not only iron and steel but also low-chromium stainless, tungsten, copper, brass, bronze, and aluminum.
Generally the metal sulfides transform to oxides when heated in air, not vice versa, so we can’t expect the jet of hot sulfur vapor to be very good at removing the oxide. Still, though, it might be good enough; no oxygen atom stolen from the oxide by the sulfur vapor is going to stick around long enough to reform the oxygen.
Unlike oxygen, you could maintain your sulfur in solid form when not using it, and you don’t need a separate ignitor or fuel; upon heating past 450°, you would get a jet of vapor which ignites spontaneously in air, though with a low adiabatic flame temperature of something like 1200°, and producing vitriolic fumes. You could perhaps heat the vapor to a much higher temperature by passing it through a heated ceramic nozzle, or perhaps a heated chromed metal nozzle, if it turns out that viridian is sufficiently resistant to sulfidation. If you can convert the sulfur vapor into a plasma (for example, an inductively coupled plasma), you might be able to increase its reactivity and attack oxide coatings that would normally resist it; maybe even just burning a bit in air will be good enough for that. Also, the hot SO2 in the flame will try to reduce oxide coatings.
If you really want to increase the temperature of the flame, mixing some metal powder into the hot sulfur would do the trick. The easiest way to do this would be something similar to a solid-fueled rocket engine made of solid sulfur with substoichiometric quantities of metal powder mixed in, ideally aluminum. As the reaction proceeded, it would expel superstoichiometric sulfur through the nozzle along with sparks of metal sulfide. Ignition temperatures are in the 350°-550° range.
This has the unfortunate feature that, as with solid-fueled rockets, there’s no way to turn it off once it’s started, which is less than ideal for flame cutting of metals. Maybe you could do something similar to a hybrid rocket motor, using a combustion chamber containing solid metal fuel in a mostly-sulfur atmosphere. As you pump more sulfur into the chamber, it heats up and expands, traveling out the nozzle; some of it also reacts with the fuel to produce more heat. The chamber can’t have an all-sulfur atmosphere, and the sulfur can’t be forced to travel through the hot fuel to get to the nozzle; in either case all the sulfur will be consumed instead of producing the desired metal-cutting stream.
In terms of health hazards, the whole system is kind of nasty. You’re producing a stream of vitriol as you cut, and the slag that melts out of the kerf is a metal sulfide, which will produce stinky and poisonous hydrogen sulfide in moist air thereafter, possibly for years. And iron sulfide, at least, has been well known for its pyrophoricity since antiquity, but a thing that surprised me is that sometimes it’s so pyrophoric that it ignites when exposed to air.
It lacks some of the safety hazards of oxy-fuel systems, though. Unlike with methane or acetylene, it’s impossible to have a leak of sulfur vapor that builds up an explosive gas mixture over time before finally exploding; if the vapor is concentrated enough to burn, it’s also hot enough to do so spontaneously, because the auto-ignition temperature is 230°, and boy did that surprise me the first time it happened to me. In rare cases, sulfur dust can make air explosive. Aside from the fire risk, cold sulfur and even molten sulfur are relatively safe materials; a ruptured tank would not produce an explosion, asphyxiation, or render nearby objects highly inflammable the way oxygen does. There are no high-pressure bottles in the system that can explode or act as rockets.