Heat up the body in a furnace with no oxygen and you’ll get a desiccated mummy. There won’t be any ashes. You’ll drive off the water as vapor (65% - 85% of body weight), but that’s about it. All the original solids will be largely unchanged (except for maybe some calcining-type reactions).
You’d probably wind up with a shrunken head.
All organic matter can be reduced at high enough temperatures to gasses and elemental solids. Without O2 heating a body above 2000 degrees C would break all the double C=C and single C-H bonds of our proteins , lipids and kerogens what you would get in a reducing environment without O2 would be hydrogen gas, solid carbon , CH4 gas, and H2O gas. Our bones are calcium phosphates which in a reducing environment would turn to calcium oxides stripping the O2 from H2O as Ca has more affinity to the O2 than H2 yielding more H2 gas and solid CaO with traces of PO2. Ultimately you would end up with a lot of CO gas, H2 gas, powdered elemental CaO, PO2, Fe in trace amounts also a solid due to red blood cells. At high temps a human would be reduced to basic elements by thermal destruction of the chemical bonds holding the molecules together 2000C would do the job really 1000C would leave a pile of powder but 2000C plus would melt that into a slap like glass and gasses.