Cindy

Take a look at the panel Z clip,
http://www.monarchmetal.com/pages/panelclip.html

Best of luck with the shelf, rredogg

Your idea of upside-down metal brackets seems good. (FWIW, I've never seen a bracket designed specifically for hanging, though it might exist.)

If it was me, I'd try and use fasteners with wide heads, so you're not going to get pull through. (Pull through is kind of unlikely, but we're talking antiques, here...)

Here is a link that might be useful: Example of bolts with wide heads...

Have you considered a torsion box construction? This allows you to screw / lag a cleat onto the wall and will provide a lot of stiffness with reduced weight. My friend, Steve Shanesy, wrote an article on these (see link). While you may not need c-shaped ones, the same technique will work.

Here is a link that might be useful: floating torsion box shelves

If possible install shelf before sheet rock is applied (or remover sheet rock if necessary) your builder will know how to secure shelf and flat tape and spackle to the shelf. If done correctly it will look seamless withe no brackets and be very strong.

My initial thought was torsion box construction as mentioned by bobsmyuncle.

In the center span over the door can the shelf rest on the door casing?

Perhaps pocket screws to help support it and minimize the amount of brackets.

Another idea is threaded rod into the ceiling with a decorative cover.

In the center span over the door can the shelf rest on the door casing?

Perhaps pocket screws to help support it and minimize the amount of brackets.

The door casing is not useful as a structural support. It is attached to the wall and the door jamb with tiny nails meant only to support the weight of the casing itself. Piling a lot of weight on the casing could deform the jamb and cause interference with the doors.

Another idea is threaded rod into the ceiling with a decorative cover.

Much better idea.

Thanks for all the advise. The threaded rod into the ceiling won't work because the ceiling is vaulted and the rod would be pretty long. I found some very heavy "L" brackets at lowes that I think will work well as upside down shelf brackets. My hubby will tack weld a small gusset in the corner of the "L" so they will be more resistant to straightening out. Then I'll run a flat head bolt from the underside of the shelf into the holes of the L bracket and put a small nut on them. That should keep them from trying to pull through. To hide the nuts, I'll take a second sheet of shelving board and attach it to the bottom side, with holes drilled out for the nut heads and washers. This will allow the two 3/4" boards to be laminated together with some good wood glue. I'll put a decorative strip along the front edge and glue it on too, which should give it more strength. Where the L brackets are attached to the wall, I'll paint them to match the wall color. The shelf is pretty high up and will have pottery on it, so I don't think the brackets will be too noticable. I do plan to use plenty of brackets...probably one every 32" or so.

A torsion box would do the job very well.
You have complete freedom to use as many fasteners at whatever spacing is needed to anchor the ledger to the studs.

The torsion box completely covers the ledger (top, bottom, and ends) and the fasteners holding the torsion box to the ledger can be placed from through the top of the box so they would not show from below unless someone's head is higher than the shelf, and even then can be counter bored and plugged.