Antique Garden Furniture - 1Earth Antiques and Appraisals Magazine

Condition
One of the great things about iron is that it's weatherproof. Up to a point. Rust is an issue, and will affect value. By its nature cast iron is brittle, and if it's heavily rusted, a piece can just snap off.

Most iron garden furniture was originally painted (black, white and dark green were the most common colors). Flaking paint is normal, and part of the appeal.

What you really want to look for is the structural soundness. Are all the pieces there? You want good condition, minimal rust and hopefully only one or two paint jobs. Original paint is the most desirable — but since most pieces were kept outdoors, that's pretty difficult to find. You'll probably only see original paint on pieces kept in a garden house, not exposed to the elements.

Rare cast-iron garden settee
with Gothic quatrefoils and
honeycomb seat, marked
"Calla a Paris," ca. 1850

A piece with extensive repair or damage will decrease in value by about 50 percent.

Restoration is possible, though not always practical. Recasting significant portions of a piece could cost much more the total value. But with smaller damage, a simple thing like recreating a missing splat is fairly reasonable. And you can sometimes find replacement pieces at salvage places.

References

Auction price guides like Miller's and Kovel's have sections on garden pieces, usually under the category of architectural antiques.
Miller's Antiques Price Guide 2002 by Elizabeth Norfolk
Kovels' Antiques and Collectibles Price List 2002 by Ralph Kovel, Terry Kovel
Antique Garden Ornament: Two Centuries of American Taste by Barbara Israel
The Golden Age of American Gardens : Proud Owners, Private Estates, 1890-1940 by Mac Griswold
Furnishing the Old Fashioned Garden: Three Centuries of American Summerhouses, by May Brawley Hill