Serge,
I did it a little differently. For my first spalting attempt, I cut up some dowels I got at a local hardware store into approximately pen blank sized pieces. Then I put those in a plastic zip-lock bag with some grass clippings, saw dust, Miracle Grow, water, and a beer (I don't have ready access to Russ's suggested horse manure). I let these sit for almost a month with the bag closed, checking on it periodically, and didn't see anything happening. Then, I moved to a new home, and in the process I remembered to open the stupid bag so the fungus could get the oxygen it would need to breathe. I then put this in my storage shed/shop, which is nice and dark, and left it there for another few months. I recently pulled a few pieces out to send to a friend, and it looks like the blanks are nicely spalted now.
If I were doing it Russ's way, I don't think I'd leave the log upright, but mostly because I figure I'll knock it over at some point (probably onto my foot). Others here have tried spalting stuff themselves too; perhaps they can give you more guidance for something that size.
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