b_j_w_79, I don't know if it would be of any use for burnt coins, but Bigfella (Shane) sells a concentrated solution that removes gunk from coins. I've found that if coins are left in the diluted solution too long, they tend to lighten in colour. From memory the solution was originally made by a chap on another forum who's ID was Niksfree.
You could send Bigfella (Shane)an email & ask him if he thinks it might work. He's a member here & has a coin shop in Newcastle. http://www.newcastlecoins.com.au
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