Darkened oak

My son-in-lw has an old building where some oak beams have been darkened by ammonia vapour. Has anyone any idea for reducing the dark colour and soallowing him to add new wood without it being too obvious. He does not want to darken his new wood, which was mu first thought. Thanks in anticipatioon if anyone can help!
G.H.Ireland
--
igor
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Both bleach and oxalic acid are used to lighten wood and remove stains. Don't know how well it would work on fumed oak. Try it out in an inconspicuous place.
Cheers,
Frank

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