our silvering work

About Silvering

Brass barometer plates and clock dial components have been chemically coated with silver for several centuries. The purpose is partly decorative in that the light silver with its matte texture makes a pleasing contrast with highly polished brass, but the intent is also to make the numerical and graphic engraving contrast sharply with the silvered field. This permits easy and accurate reading of the position of pointers and hands.

The silvering process itself is not electroplating, as is used to form a relatively heavy layer of silver over another metal substrate, but instead is a chemical transfer of silver using a catalyst to suspend the silver in a paste and then precipitate it out onto the base. Electroplating forms a fairly thick and durable coating with a high sheen, whereas chemical silvering lays down a coat of silver that is only microns in thickness. Attempts to clean or polish chemically silvered surfaces will immediately result in total removal of the silvered layer, exposing the substrate metal.

In the past, silvered brass barometer plates were often coated with shellac or just wax, both of which are porous, and atmospheric corrosion can set in, blackening the thin silver coating. Small drops of errant mercury also amalgamate with the silvered surface and blacken it. Over time, this blackening turns to deeper corrosion, pitting the underlying brass. The black engraver’s wax in the engraved lines and numbers also cracks with age and mercury can lodge in it, causing even new silvering to erode in a short time. The remedy is to heat the brass plate to vaporize the mercury, then heat it further to burn off most of the old wax. The remainder of the wax is removed, then the dial can be prepared for the new coat of silvering.

“Traditional” silvering, which we provide, is done with a hand-applied paste containing silver nitrate, cream of tartar, salt, and whiting. Before silver nitrate was commonly available, a silver arsenate compound was employed, using an arsenic compound to break down a solid silver object. Today we do all silvering removal in a water bath to prevent release of the older silver dust, for obvious reasons. Preparation and finishing for these processes are methods used for hundreds of years in our trade. We then hand-coat the plate with a low-sheen removable lacquer developed for the restoration industry.

Before silvering:

Dial and ancillary plates from a mid-19th century five-dial barometer.

The main dial has turned slightly brassy from deterioration in the old silvering, and has several black blotches from spilled mercury. It also appears that the dial has been attacked with a solvent that washed away most of the wax in the engraving. Nearly every stroke of the engraver’s tool has been exposed as bare brass.

The other three plates, hygrometer, thermometer and level/maker’s name plate, also show some mercury staining and deteriorated silver surface. Since all the plates are seen closely together on the front of the barometer, we burn off the mercury then clean and resilver all of them at the same time so that they match.

After silvering:

The engraving wax has been renewed on all the plates, the surface was grained consistently, and then the silvering applied and lacquered over. The engraver’s work is now completely visible, without the distractions of flaws and corrosion, and the plates have been preserved for many more years of use.

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