Green vs. Dried Wood
Some items were made from green (recently harvested) wood and some were made from wood that had been sawn into boards and dried. For the purpose of this discussion, we are talking about furniture scale work. Small items (bowls, spoon, etc.) could be made from literally anything green or dry.
Working green wood is easier than working dried wood. The wood fibers are softer and sever more easily so your tools don't dull as fast. But because of that pliability, the edges need to be very sharp. It's much easier to remove material in large chunks.
The process yields boards where the fiber runs the length of the board so resulting boards are stronger than sawn boards which sever the long grain fibers if the cross the cut line.
On the downside, your tools will rust if you aren't careful and fine work isn't possible until the wood dries at least a little. Also, your wood choice is constrained to ring-porous woods (see below).
Splitting boards from logs is called riving. This is a process where you take a section of log that's as large a diameter as possible which is straight and has no branches. Using this process, the boards will be no more than about 40% of the width of the log you are starting with. So, say you have a 24" diameter log. You can expect to get, at best, 10" boards. Probably closer to 8" depending on the quality of the log.
You start by splitting the log in half. This is the hard part and is done with wedges and mallets. Then you split it into quarters. Split again. Continue to split until you get to the right size.
What's the right size? It's the smallest wedge that will give you the board you want. The pieces you have split out are wedge-shaped. However, you don't want the pith or the sapwood and bark as shown on the right. So you start by splitting or hewing those off. You are left with a trapezoid which you need to turn into a square.
Pick a face, we'll call that one face or your board-to-be. Now mark towards the other face the thickness you need and split off the rest. That part should have been an acute triangle.
This operation is done with a froe or a hewing hatchet. As you gain experience with the process and the tools, you'll find you will be doing more with the hewing hatchet and less with other tools. Since the grain is very straight, you will be able to split off small parts in a predictable manner. This means quite a bit of dimensioning can happen at the stump with the hatchet. Leaving less material to plane or saw off.
Now you have your rough board ready to trim and plane to final dimensions. As noted above, this will be a strong and dimensionally stable board. And, if this is oak, it will have a very attractive ray fleck like quartersawn but better. In fact, it's the best wood there is and you can't buy it. You can only make it with sweat equity. However, it's not very economical of wood. You are making a lot of scraps and firewood to get some premium pieces. This is no big deal if you live in a forest, but this isn't practical if you have to have logs of suitable size transported in your pre-industrial society.
As mentioned above, riving only works well with ring-porous woods like oak, ash, elm, and black locust. Ring-porous means there is a significant difference between the fibers grown early in the season versus late in the season. They are less dense and have open pours as view end on. The late-season wood is tight and dense.
We're no sure when and where riving wood started, well before SCA period anywhere there are forests would be a safe bet and it was continued as common practice in the "backwoods" into the 20th Century.
The other kind of wood is sawn wood. It's a very efficient use of the tree and if you saw the tree into boards near where it was felled, it's a whole lot easier to transport. The downside is that it must be dried before you can use it. The rule of thumb is that it must dry about 1 year per inch of thickness. Newly felled and sawn wood is about 30% moisture. You want it to get down to 10-12% before attempting to make anything nice out of it.
Why is sawn wood not used green? It will warp, cup and twist as it dries. You've severed the fibers and they shrink as they dry, since this is necessarily uneven, this force warps the wood. Riven wood fibers are not severed and it shrinks uniformly as it dries making it very stable.
We have evidence that commercial sawmills are in operation by the middle of the 14th Century (I'll add the reference when I can find it again). More commonly, logs were pit sawn, a practice that continued until the advent of steam power. It's a lovely thing where the log is positioned over a pit or on to a trestle as shown to the right. One man on top, one man down below. They operate a two-handed saw. You can see being the pit guy sucked.
Boards were then stacked with air gaps to allow drying. Anything that wasn't structural probably started life as a 1" board. Once it dries (and shrinks some) and then is planed, it will come in around 3/4" a common thickness to this day.
From early 17th Century Newfoundland, there is a mention of sawing:
6 October 1610 John Guy to Sir Percival Willoughby:
“…we have digged a saw-pitt hard by the sea side, and put a timber house over it [co]vered with pine boardes; there are two paire of Sawyers workinge in it, the pyne trees make good and large bordes and is gentle to saw, they be better than the deale bordes of norway, there is now a pine tree at the saw-pitt, that is about tenne feete about at the butt, and thirtie feete longe is eight feete about…” (from Gillian T. Cell, English Attempts at Colonization, 1610-1630 (London: The Hakluyt Society, 1982) pp. 61-2.
Deale is the English term for the pine trees that grow around the Baltic and up into Scandinavia. They are still ill-considered to this day (think IKEA).
How Do You Tell?
So, how do you tell if a piece was made from sawn wood or riven wood?
There's a saying about 17th Century furniture in the historical furniture community, that it was "bark on the inside and varnish on the outside".
I've seen that on frame and panel chests from period. The outside corner of the posts on a chest are the reference surface the maker bases the construction on. Anything inside the groove that retains the panel will not be seen and therefore can be whatever. So it will either show saw marks or hatchet marks. Or it might be missing the inside corner altogether.
If the post is square and smooth on all 4 faces, a clear indication that it's almost certainly out of SCA period and possibly a modern reproduction.
If the grain is visible, and the wood is oak, it will be clear from the grain if it's riven: it looks quartersawn only more so and will have clear ray fleck on both faces. Quartersawn will often have more better grain on one side.
If the item is a turning, it will be round if turned dry and somewhat oval if turned green.