Not sure what temp it takes to 'fix' the oxide.
Main problem with brushing it on is that it can be 'spotty'. That can be good or bad. One time I wanted it heavy so I put it on greenware, bisqued and then reapplied on the bisque. It did come out heavier than usual (on bisque only). My intent was a very metallic no glaze finish.
This was just applied haevily to bisque:
Putting it on greenware helps it fix some, but it can still be rubbed off. So the temp required for a full fix is higher than cone 6.
This was put on bisque then rubbed off with sponge just to highlight: