Hi, Kathy I'm about to do this floor myself, in our new house. Never having even heard about "cobblewood" flooring, I've been going on the advice of a dear friend who did this in her house years ago. She sent instructions in minute detail for me. Her "grout" is plain ol' white glue & sawdust. Over the years and through the seasons, this floor has become more and more beautiful, as the fir pieces age. Occasionally, with changes in humidity, some of the pieces "open up", sometimes pieces "close", sometimes they "separate". She doesn't fret, just mixes a little glue & sawdust & spreads it in the cracks. No big deal. Personally, this appeals to me a lot. It's not what is considered perfect by today's somewhat homogenized, generic standards for that spit & polish manufactured look, but each floor, done in one's preference of size, pattern, wood type, etc., makes for, in my view, a floor with lots of character. Soooooo, when I finish mine, I'll let all know how it went. P.S. My friend put hers down over standard sub floor, with forced are heating. Mine will be over gypcrete and radiant in-floor heat.....don't expect a big difference.
P.S. II My first big obstacle is finding a saw big enough to cut 6x6's - am thinking of asking a local sawmill........anyone have other ideas?