Yeah, a stupidly good one at that.
https://ftbwiki.org/Pyrotheum_Dust_(Thermal_Foundation)
A single dust smelts 120 items, which is x15(!) times the amount of its constituents (one coal and 1 sulfur for 2 pyrotheum, for a total of 240 items smelt compared to 16 if you used the sulfur and coal dust alone) which is why I thought it might be balance-related, considering that it basically has the same burntime as a block of coal coke.
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