Ormolu (from French or moulu, “ground or pounded/powdered gold”) is strictly speaking the technique of applying a gold amalgam (gilding) to a metal object, typically one of bronze. The finished product was not only beautiful, but sturdier, practical, and cheaper… comparatively speaking.
Making ormolu was an arduous process involving complex stages even before it was ready to apply. To begin, the bronze object had to be cast using specific methods that resulted in a detailed surface primed for gilding. Then there was the work of finely grinding 18 or 24-carat gold, before blending into an amalgam with copper, zinc, tin, and/or mercury. Once perfected and cooled, the gilder applied it to the bronze object. This too had a wide variety of techniques, from gluing or hammering gold foil or gold leaf, to “fire gilding” by using a mercuric nitrate solution and then heating the gold amalgam until fused.
Ormolu, also called “gilt bronze” or in the French bronze dorée, was wildly popular throughout the 18th and 19th centuries. As noted above, it was cheaper than a pure gold object, but far from inexpensive. The gold required to create the gilding, on top of the other metals, the painstaking process, and skilled artistry, added up to a hefty sum.
The finished item would have a brilliant gold-like appearance easy to differentiate from the real thing by a trained eye, of course, but incredibly beautiful and rich. Ormolu design was used on just about anything imaginable, although the most common was for the decorative mountings of furniture, clocks, lighting devices, and porcelain.
By the mid-1800s the negative effects of mercury were becoming well-known. In fact, in 1830 France outlawed the use of mercury in ormolu (or anything for that matter). This did not completely halt the process with mercury, naturally, but over time better gilding techniques were discovered and the highly-ornate style fell out of favor.
For a thorough, concise article, visit this page on Mayfair Gallery — Ormolu: As Good as Gold