This is an existing spring that was tempered too soft. First it was carefully heated to non-magnetic in the coal fire and then quenched in oil. That is vegetable oil.
Then it was degreased and carefully heated to draw a "Robin's egg blue" spring temper. I've also heard a Southern smith call this "Possum ear blue"! In any case it is probably around 630 degrees to impart a good spring temper.
Next a spring vice is used to re-install it.
The spring is now in place. Light oil is added everywhere it touches other steel to prevent it from binding on a burr. By the time the oil evaporates it will have worn in.
There it is!