Cleaning the Zinc Parts
The cast zinc lamps offered on eBay Germany are unfortunately not in a nice condition. They usually have a dark grey appearance and their surface is dull. You can't even guess how different coloured and shiny they once were when they were newly manufactured and marketed. Unfortunately, all catalogues of German and Austrian lamp manufacturers have been printed without colours and do not give today's collectors the opportunity to admire such lamps once in their original appearance. Please look at the detail photo of the lamp L.156 by Wild & Wessel in the picture below (3rd photo of the bottom row) to see the exceptionally beautiful galvanic bronzing of the time in the original.
Lamps with cast zinc parts were originally always galvanically coated with another metal, as the zinc surfaces form zinc oxides and zinc carbonate over time due to exposure to air and thus lose their initial silvery shiny beauty. This coating is usually made of brass or copper to give the lamps a favourable colour. This procedure is called "bronzing". However, since electroplated bronzes were quite thin, these metal coatings have very often not survived the test of time. Decades of wear and tear have caused zinc castings to lose their bronzing almost completely (see also Metals in Info Boxes).
Some zinc cast lamps in found condition (upper row: bronzing and patination rubbed off - lower row: original bronzing and patination well preserved).
Upper row, from left: L.272 completely rubbed off and oxidised
L.110 heavily rubbed off, with traces of earlier bronzing
L.104 zinc cast parts rubbed off; black lacquer still intact
L.191 rubbed off, with traces of former green patination
Bottom row, from left: L.196 with original bronzing still in good condition
L.202 with original copper plating still in good condition
L.156 with excellent preserved original bronzing
L.301 with very well preserved original green patination
Another way to protect cast zinc parts, especially the figures and sculptures made of cast zinc, from tarnishing and to give them a bronze-like patina was to paint them over with appropriate colours, which were partially or completely rubbed off analogous to galvanic bronzing.
The cast zinc lamps, which had become unsightly and looked blackish, were painted over with a bronze paint at some point. These layers of paint have also developed a patina over time; they have become darker and duller, and have even peeled off in places. The cast zinc lamps offered on eBay have been abandoned to their fate for decades; their surface is completely darkened and also heavily soiled. In some cases, you have to be clairvoyant to guess the beauty of the fine cast zinc surface underneath. Mild cleaning with dishwashing detergents may remove the surface grime, but cannot conjure up the former shine. These lamps now come up for sale and are unfortunately quite unattractive due to their appearance.
Before we start thinking about how we can help these zinc cast lamps, which have become completely unsightly, to regain a new, completely satisfactory appearance, we should first clean them.
If a cast zinc lamp has not been painted at all and only has remnants of the original galvanic bronzing, then it is sufficient to clean the surface with the familiar abrasive attachments and then polish it a little with steel wool. Zinc cast surfaces cleaned in this way are then ready for "re-bronzing" by hand.
Removal of the Old Lacquer
If, on the other hand, the lamp has been repainted at some point to conceal the dirty, dark, "weathered" surface underneath, then these layers of paint must inevitably be removed. This procedure is unavoidable, especially with lamps where the fine details of the cast surface - for which zinc castings are famous - have literally been buried under thick layers of paint.
Since the zinc cast lamps primarily have very ornamental, strongly reliefed surfaces with many heights and depths, and especially the deep areas still contain a lot of paint, cleaning them with abrasive attachments or even with steel wool is an almost impossible undertaking. With such lamps, only a bath in caustic soda will help.
Caustic soda is sodium hydroxide dissolved in water (chemically: NaOH) and has the property of dissolving the binding agents of old lacquers so that the lacquer can be removed from all surfaces. The well-known paint strippers for painted wooden furniture also work on this basis. To do this, prepare a ca. 10% sodium hydroxide solution and leave the lamp parts to be cleaned in it for a few hours (possibly overnight). 10% caustic soda is prepared by dissolving 100 g sodium hydroxide in 900 g water while stirring. This makes 1 litre of 10% caustic soda. If this quantity is not sufficient, more caustic solution can be prepared so that the lamp parts to be cleaned are always covered with caustic solution. Sodium hydroxide is usually sold as "pellets". These are white granules to avoid accidental ingestion of dust. Sodium hydroxide is available from chemical dealers, pharmacies or online mail order companies on the Internet.
| Attention! Caustic soda is very corrosive to the skin! Whenever you work with the caustic soda, starting with the preparation of the solution, and later when cleaning the metal parts, you must wear good rubber gloves (not the thin latex gloves, but the much thicker ones for dangerous liquids; available in well-stocked DIY stores) and protective goggles that close all around! If you have, a work coat would also be highly recommended to protect your clothes from splashes. Do not wear open-toed shoes, sandals, slippers, etc., but all-round closed shoes. Do not take all this lightly! I am a chemist and know what I am talking about. Caustic soda leaves very ugly, very slowly healing holes on the skin, eats through all organic materials (i.e. also through textiles made with cotton, silk, linen, etc.). Synthetic plastics, on the other hand, are not attacked by caustic soda. Therefore, a plastic bucket is recommended as a container for working with caustic soda, and plastic brushes for cleaning metal surfaces. After work, clean your gloves and brushes under running water without splashing around wildly. Even if you feel that you have avoided any splash on your skin, I recommend washing off your open skin areas with water. You should be careful not to even get the dry pellets (granules, as sodium hydroxide is supplied) on your skin; skin moisture is enough to dissolve some NaOH, which you unfortunately notice much later. |
The metal parts of the lamp need some time in the caustic soda bath so that the lacquer dissolves well. This can sometimes take a few hours. Of course, this depends on the thickness of the lacquer layer. Higher concentrations than 10% speed up the dissolving work, but are even more dangerous than described above and even attack the zinc surface. Try to work with the low concentration, even though it may take a long time to remove all the lacquer. You have to scrub the metal parts from time to time with a strong plastic brush to remove dissolved layers of lacquer and to allow the sodium hydroxide solution to penetrate better into the layers of lacquer that are still intact. Normal kitchen brushes or toothbrushes with stiffer bristles are suitable for this purpose.
When the bath has completed its effect, the zinc parts freed from the lacquer must first be carefully but thoroughly washed with plenty of water so that no residues of the caustic soda remain at all in any hidden recesses. The result after the bath is a matt, grey metal surface, but this can now easily be cleaned uniformly with the Dremel® sanding attachments.
In the past, I used to polish such cleaned zinc castings with fine steel wool to a high gloss and leave them that way until I discovered how quickly zinc can tarnish. Since then I have been "bronzing" these zinc surfaces with a method I developed myself, which I describe in the next subchapter.