When a designer is creating a piece, what is driving him to use a particular order (or I assume combinations of)?
George and I may disagree on this point, but I think at their root level, the orders suggest a certain structural capability when applied to furniture. I think its also true, and maybe George will chime in on this, that the use of moldings almost always punctuates structural transitions, describing load transfers, explaining it, suggesting mass etc.
So when we think of a Tuscan Order table, we think of that strong Italian man, with thick legs and a strong back. There's capability there. Go ahead and put something heavy on it. There'll be no artistic tension, no unease. The design will be balanced.
Conversely, we might seek a design that elongates and soars, like an Haute Couture model on a runway in Paris. A thin, sinuous cabriole leg, curving up, lifts the eye (mass increasing with altitude). Such a form almost appears as if it is in motion, leaping from the floor.
So we choose the orders to convey a sense of capability, but also motion, and to some extent gender. Mack Headley (like many before him) anthropomorphizes the orders, referring to them as the "maiden, young man, adult man", etc.
Do certain periods base their designs on a particular order?
It seems that way. Queen Anne furniture seems to use the lighter orders. Jacobean furniture uses the heavier. But it can also vary piece to piece or maker to maker. Some argue that the orders were unknown to furniture makers and that any similarity is mere coincidence. We've had the same fruitless debate about golden section. Its very difficult to know what period furniture makers knew or were thinking. My view is that we can see golden section or the classical column orders represented, at least to some extent, so its smart of us to embrace their use. Not everyone agrees. I just want you to know that there are different views of this than what I'm writing.
My advice is to at least know what the orders are algebraically. That way, you can check your own designs and see if what you are making is overly thick and blocky and will have a Fred Flintstone look or will look too flimsy (or be too flimsy). Here's something I wrote a while ago that may be helpful:
http://www.popularwoodworking.com/blog3/Classical+Column+Orders.aspx
It includes a downloadable excel spreadsheet that defines the columns algebraically as a function of their shaft diameters. It can be useful. I've found it so.
Lastly, what I think is funny about the orders is how noticeable their use and misuse really is. And this isn't something one needs to go to art school to recognize. Precisely because the orders are based on the natural world and not arbitrary esthetic principles, we can usually see it when something "just looks wrong". Our untrained eyes are accustomed to what "right" is. Right is the proportion of our heads to our bodies, the diameter of a tree trunk etc etc.
Adam