Hi,
I'm new to the board, (found it when trying to change my mailing address with the AWS) but have been checking it out for a few days now, and am impressed with the amount of knowledge I've seen displayed.
I could be wrong, but from your description and an assumption that this chair is in fact a casting, I would venture to say that the material is a magnesium alloy. I use a fine file to get some small shavings and put them under my oxy-acetylene torch - if it burns it is magnesium. I have never brazed magnesium, only TIG, but I have read that it can be done. There are companies that sell flux specific for magnesium brazing.
This article may help:
http://www.key-to-metals.com/Article35.htm
Good luck,
Brian