The Roasting Oven is a processing machine added by GregTech 6. It uses heat and oxygen to "roast" materials and oxidize them to obtain sulfur dioxide / carbon dioxide and often some solid byproducts. It is usually used as the first step of making sulfuric acid. It also provides the final step in purifying sulfide ores.
After version 6.07.09, there was a 20% chance that the process would fail to produce solid byproducts if air was used instead of oxygen.