Ah dam, forget my reply, I thought you were asking about the bruises.

The hand stayed because he didnt burn her that far back. the stronger the use of balefire, the more the person is burnt back. I believe a few weeks at most, with the cheodan kal. not enough time to save the hand.

Not enough strength to burn her back that long

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Interview: Nov 6th, 2009

TGS Signing Report - Paul Grow (Paraphrased)

Question

Another fan asked if the balefiring of Graendal could bring back Asmodean, if in fact Graendal was the person who killed him.

Brandon Sanderson

Brandon essentially said no, because it happened much too long ago. At best balefire burns someone out of the Pattern three days to a week at the absolute most. But usually more like five minutes.

Tags

balefire, graendal, asmodean,

I think that Since Semirhage had injured his hand using the fire it might have hindered the balefire's ability to erase Semirhage's attack. But I also agree with @Mat Farshaw that he could have not burned her back long enough.