The mantle project turned out to be not so daunting after all. Perhaps success simply follows from finally having the furniture arranged in a way I like and use. Perhaps success comes from just letting go, trusting my instincts, and not trying so hard.
I must thank Lisa over at Privilege for her comment on using paintings on her own mantle, as she prompted me to look at the small row of prints, paintings, and photographs that is in the guest room, awaiting placement. There were two photos there, taken by George, that had not yet found a home. They have now:
In front of them I placed a brass pharmacist's mortar and pestle and a tiny wind-up car, salvaged from a miscellaneous toy box. I like the scale of the very tiny car with the oversized mortar and pestle, and the brass, black, and gray pick up the colors of the painting above, its frame, and the mantle itself.
Tricia's comment about balance also made me think, as this is an issue I struggle with to some extent. I save photos of beautiful symmetrical arrangements even though it seems that I am more often attracted to balanced asymmetry. I prefer odd numbers, I prefer to play with scale. I would say I do like balance of a sort, and I don't reject symmetry outright, as evidenced by the symmetrical planter arrangements outside my garage, but it is not my default mode. I suppose part of my struggle with the mantle is that the room is already shaped by both the symmetry of the windows flanking the fireplace combined with the very real absence of any architectural detail in the room as it was built. I altered that sense of symmetry by having the bookcase built along one wall, and although I couldn't articulate until now, I believe this was part of my struggle and also a part of my solution. My first thought upon putting up the photos and objects on right side of the mantle was that I needed a single tall narrow object on the left to provide balance. It wasn't until after I stood back and looked at the room as a whole that I realized this mirrors the balance of the room (which I regretably cannot capture without a wider angle lens than I currently possess) with a single tall object on the right, and many smaller objects of varying heights on the left of the fireplace.
Of course all these thoughts and words may just be stuff an nonsense. Perhaps what I really need is to just sit myself down and study some of the basic principles of design. But I am happy with the final result and even happier to have found a solution using items that were already floating around the house. The simple truth is that I just don't have the patience required for hunting down perfection.