Flare, Solid Fuel or Battery: Three Ways to Heat Steel, Tested
Three ways to heat a steel plate so it shows in a thermal optic were run on the same range day. A road flare rated for 20 minutes burned for 15 and only warmed the bottom of the plate. A solid fuel firebox behind the plate was still burning at the end of the day and is good for about four hours on a load. A battery-powered heater gives about five hours on its large battery. The firebox and the battery unit both got hot, stayed hot, and took hits without changing.
Watch: We Test the Ultimate Thermal Targets (11:01) on YouTube. All three tests, the timing, the thermal camera close up and the optic at 100 yards.
Why heat the steel at all
A thermal optic shows heat, not light. A person or an animal glows against ground that has cooled, which is the whole point of the optic, and a plain steel plate at air temperature does not glow at all. The idea for a heated plate came from the same place as the moving hostage head: instructors asking for something to zero on and train on with the optic they carry.
Test one: the road flare
Lit at 10:20. Out at 10:35. The flare was rated for 20 minutes, and in fairness it went into the holder upside down, which cannot have helped.
At five minutes the bottom of the plate was warm to the touch and the top had no heat in it yet. Through a thermal camera you could see the heat had not reached the upper half.
The flare burned through the thin-metal prototype holder. Anything that lives behind the plate with a flare in it needs to be thick steel.
Verdict: it works for a short zero. It is not a full day, and every relight is a walk downrange.
The design problem nobody sees
AR500, AR550 and similar hard plate get their hardness from a grain structure that heat can undo. Hold the plate above its annealing temperature long enough and hard steel becomes soft steel, which brings back the splash, ricochet and pitting the hard plate was bought to avoid. So a heater has to put heat into the plate for hours without ever taking it over that line. Both of the fuel and battery designs are built around staying under it.
Test two: solid fuel
A firebox behind the plate burning briquettes, with a waxed starter in a paper sleeve that drops in and lights.
Expected burn on a load: about four hours.
It got hot, showed clearly through the optic at distance, stayed hot, and was shot while burning to see whether the impacts did anything to the fuel. Nothing unusual happened.
Test three: the battery unit
A large lithium-ion battery in an aluminum box behind the plate, feeding a heating element on the back of the steel. The box is big because the battery is big; that is what five hours at 100 watts costs.
100 watts is roughly the heat a person gives off, which is the contrast the optic is looking for.
The box ships with a long lead so it can be tucked behind a post or buried. Tie the lead up. On the first day of testing an unmanaged lead was shot. The lead is easy to replace, and easier not to shoot.
Result
Both the solid fuel and the battery units were lit or switched on at the same moment, got hot, were seen from distance, were shot, and were still hot afterwards. They were left running for the rest of the afternoon to see whether anything surprised us late in the day. Nothing did. The flare is the one to leave for quick zeroing sessions.
The battery unit is on the Thermal Optic Targets page. The solid fuel firebox is in Solid Fuel Thermal Target: Bolt It Behind Your Own AR500 Steel, and every target type compared is in Best Thermal Targets in 2026.
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