In this paper, we present a new design for a 1 atm gas-mixing furnace using the gas mixture CO–CO2–SO2. This furnace can simulate disequilibrium processes such as magmatic and volcanic degassing. Here, we present the technical aspects of the design. The furnace can sustain temperatures of up to 1650 ∘C and has a hot zone that spans 200 mm vertically, where the hotspot is determined to be ∼ 32 mm below the midpoint of the furnace enclosure. The four mass flow controllers are individually calibrated and accurate to within 0.8 % of the specified value. The fO2 is accurately reproduced in the furnace within ±0.002 log units, as calibrated by the Fe–FeO reaction across the iron–wüstite (IW) buffer at 1300 ∘C. The furnace can reliably simulate dynamic conditions, where the fO2 can be modulated at a maximum rate of 2.0 log units min−1 by varying the gas mixture. A delay of 40 s is observed to attain the fO2 calculated from the gas mixture, at the hotspot. A series of safety measures to protect the user from exposure to the toxic gases are detailed. In our experiments, the furnace is used to determine sulfur isotope fractionation factors among melt, sulfide, and the gas phase, within a magmatic context, using either crystals of olivine or silica glass tubes. The furnace has the potential to investigate various other dynamic high-temperature reactions occurring on Earth.