I still don’t know where the residue is left. These temperatures 770C and 804C are fire polishing and full fusing temperatures, so I guess you are casting in these moulds. Others have done this kind of thing successfully, so maybe you need to get back to the manufacturer to see what their recommendations are.
If the residue comes off easily, then you are in luck, if it sticks then I wonder if you are renewing the kiln wash each time. When kiln wash is taken above about 720C, it changes composition, making it stick to the glass in the next firing.