Just thinkin' out loud (keyboard style)...
The actual moisture content to cause hissin'/boilin' must be relative to to several things... including wood density, structure and the temperature of the fire (or temperature increase rate of the hissin' wood). I almost never see American Elm hiss 'n' boil, even though I know it ain't as dry as desired... and if I do, it's near always an un-split round. On the other hand, I'll get the random split of oak that hisses even though it's 3 or 4 years seasoned. I've burned green (split) Silver Maple in the fire pit without any hissin'/boilin'... but water bubbles out'a not-so-dry Black Cherry.
As I understand it... as the water heats it expands and/or converts to steam (further expansion) building pressure inside the wood. When the pressure builds to a certain point (relative to water temperature) it prevents the conversion into steam. The pressure forces the water out'a the wood where it instantly boils and/or converts to steam as it's no longer under pressure (causin' the hissin' sound).
OK... so with that in mind... a less dense wood, with a more open grain structure, will allow more of the water to expand/convert into steam, and provide a less resistive escape path (i.e., steam at lower temperature, lower pressure... meanin' no hissin' and exterior boilin'). What you get is a lazy escape of steam indiscernible from smoke unless you look really close. But... if you increase the temperature increase rate actin' on the less dense wood (or add more water), it would be possible to (at least in my hypothesis
) build enough pressure to cause hissin' 'n' exterior boilin'.
So... the answer is... there ain't a "minimum" moisture content... it all depends.
And I didn't even need Wikipedia
And... and... I only need 34 more
*
...ehh whatever
... need pics of the hissin' wood to formulate the hypoten... err... hypothesis.
