Metals in the Kerosene/Paraffin Lamps
Brass is the only metal (actually a metal alloy) used in all lamps, as the burners are made solely from brass sheet. Another metal often used is zinc, which is the main component of all zinc castings. Other metals such as iron, tin, etc., occur much less frequently.
Brass
Brass is an alloy of copper and zinc, with quite different proportions of the two main components. The copper content is usually higher than the zinc content; an approximate composition of 60-65 parts copper and 35-40 parts zinc might apply to most brass pieces in our lamps. The alloy of copper (red) and zinc (silver) gives the typical light yellow shiny colour of brass. With more copper in the composition, brass gets a reddish colour, with increasing zinc content, the golden yellow colour becomes brighter. In addition to these two metals, brass can also contain small amounts of other metals, such as lead, arsenic, etc., in order to specifically change the processing properties of the alloy.
Brass is not magnetic; melts at approx. 900-1000°C depending on how much zinc is in the composition. With higher zinc content, the melting temperature decreases. With 30% zinc content, brass acquires excellent formability. It can be very easily rolled into plates and wires. This is also the reason why all burners and also almost all shade holders are made of brass. With 37% zinc in its composition, brass can be cast very well. Many base parts and other decorative elements of our lamps are made of cast brass, rather than bronze (see below), unless they are made of cast zinc.
Advantages for kerosene/paraffin lamps:
a) Brass can be polished excellently. Both soft polishing wheels and polishing creams can be used to clean somewhat dirty or tarnished brass surfaces and polish them to a high shine.
b) Brass is relatively soft. Bent pieces of brass, even thicker castings, can be bent back again.
c) Brass is very good for soft soldering. With hard soldering you have to be careful with the soldering temperature. Even below the melting temperature, brass changes its internal properties.
Disadvantages for kerosene/paraffin lamps:
a) Brass tarnishes over time. The copper content reacts with the oxygen in the air and forms two different copper oxides, which are red and dark brown. The final state of almost dark black-brown colour is reached when copper sulphide (formed from the sulphur in the air) is also added. However, these dark-coloured layers are only on the surface and can therefore be removed mechanically (sanding) and chemically (acid bath).
b) The copper oxides formed by tarnishing brass can, under certain circumstances, react with the carbon dioxide in the air and form the dark green copper carbonate. The intense green crystals of copper carbonate are insoluble in water and can only be removed mechanically by grinding.
c) Brass is attacked by acids. Acids (hydrochloric acid, sulphuric acid, nitric acid, phosphoric acid, even in very low concentrations) dissolve the zinc atoms from the brass surface in a short time. The remaining copper atoms turn the surface copper-red. This layer can be ground off until the yellow, intact brass surface appears again.
d) Old brass sheets that have been cold-formed under pressure (e.g. into kerosene/paraffin tanks) develop hairline cracks over time. Many old fonts made of sheet brass have these cracks, from which kerosene/paraffin leaks out. You either have to solder them from the outside or seal them from the inside with a suitable epoxy resin.
Bronze
Bronze is actually very rarely (if ever) found in petroleum lamps. Since the cast brass pieces used are very often mistakenly called bronze, I must briefly introduce this alloy here.
Historic bronze is an alloy of copper and tin with about 90% copper and 10% tin. Today, bronze is used as a collective term for all alloys consisting of copper and another metal (with the exception of brass made of copper and zinc). Therefore, there are different types of bronzes with very different properties, such as aluminium bronze, phosphor bronze, arsenic bronze, etc. For the casting of church bells, a bronze with 80% copper and 20% tin is used, as it is precisely this composition that produces excellent resonance in the sound.
Brass or Bronze?
The problem I hinted at above is that brass and bronze are easily confused. Both have the same colour, melt in the same temperature range, are non-magnetic, solder well and tarnish with the same colour. But since they are chemically quite different alloys, it is best to tell them apart. And that is problematic! We laymen (and also I as a chemist) cannot distinguish between brass and bronze without chemical-physical aids and measuring instruments.
Since tin bronze cannot be processed into thin sheets, it is quite clear: all lamp parts made of sheet metal, such as burners, globe or shade holders, pillars, fonts and much more, are inevitably made of brass. This clear dividing line disappears, however, when it comes to cast parts: Both brass and bronze can be cast well! That I nevertheless have the courage to say that bronze is very rarely used for kerosene/paraffin lamps is due to the fact that firstly, bronze is much more expensive than brass, and secondly, the casting process is more difficult to control with bronze than with brass. If it is cheaper and easier to use brass than bronze for relatively unimportant items like kerosene/paraffin lamps, why choose the more expensive and difficult alternative?
Well, unfortunately, it has become customary in the art world to dub all cast articles made of a shiny yellow metal as bronze. This is probably due to historical reasons, because in past centuries, high-ranking art objects (sculptures, statues, etc.) and valuable utilitarian objects (candlesticks, clock cases, etc.) were cast in bronze, as it was the higher-quality, more prestigious choice for making these objects.
As a result, a lamp base cast in brass, for example, is automatically dubbed bronze, but this is not correct, because despite their striking similarity, brass and bronze are chemically quite different materials. You may even come across this fact in the lamp catalogues of renowned producers such as Wild & Wessel, because there they also speak of bronze for brass parts! This is particularly interesting when, for example, catalogues are available in two or more languages. In the German text they mention bronze; in the English translation stays "brass" instead of bronze.
As a logical and understandable conclusion, I give brass as the casting material for the kerosene/paraffin lamps, even though the object could theoretically be made of cast bronze.
Zinc and Zinc Casting
Zinc is the most commonly used metal for cast lamp bodies or parts. However, pure zinc alone was never used, but always in an alloy with other metals, because only the specific mixture with other metals such as lead, antimony, etc. ensures an alloy that melts at low temperatures and has a low viscosity. Zinc is also very inexpensive.
Zinc is a base metal with a bluish-white appearance. Its low melting point of 420°C and its easy alloyability with other metals has made zinc a much-used metal in industry. At room temperature, zinc is very brittle in its pure state. This brittleness is still partly retained in its alloys, where zinc is the main component.
For us lamp collectors, zinc has become most important in its alloys with lead. By adding lead and other metals such as antimony or copper in small quantities, one obtains alloys which, with their low melting point and low viscosity, are ideally suited for casting very detailed lamp bodies and sculptures with the finest ornamentation. It is therefore not surprising that a very large number of lamp vases and figure lamps were made from cast zinc. Cast zinc was the celebrated material for the exuberant ornamentation of historicism in the petroleum lamps produced by the millions.
Régule is the French name for a zinc alloy that was used to make all kinds of sculptures, art objects, clock decorations, vases, and of course the figures for lamps. Interestingly, the term "régule" was originally the name for another alloy, namely of tin or lead with antimony as a kind of bronze imitation. But only the replacement of tin by zinc brought the hoped-for, great success. This is the material from which most of the colour patinated or metalised statues of the late 19th and early 20th centuries were made. Régule became the "poor man's bronze".
Spelter is the English name for an alloy of zinc with lead. This alloy was used - very similar to régule in France - for the cheap production of all kinds of decorative objects such as candlesticks, clock cases, table decorations and for sculptures in Art Nouveau and Art Déco.
Zamak is another zinc alloy that was used for similar purposes. Zamak contains zinc with small amounts of aluminium, magnesium and copper.
Advantages of zinc castings for kerosene/paraffin lamps:
a) The surfaces can be easily cleaned mechanically. Fine abrasive attachments or steel wool are sufficient for cleaning and polishing. Cast zinc surfaces can be polished very well.
b) Objects made of cast zinc (lamps, figures, clocks, etc.) are often much cheaper than corresponding pieces made of real bronze.
Disadvantages of zinc castings for kerosene/paraffin lamps:
a) Surfaces of cast zinc objects tarnish over time! A whitish-grey layer forms, consisting of zinc oxide and zinc carbonate. Well cleaned and highly polished cast zinc surfaces lose their beautiful appearance within 1-2 years, becoming dull and greyish if not protected by a protective coating. For this reason, cast zinc lamps were always galvanically bronzed or painted in the past. Lamps with a bright zinc surface did not come on the market.
b) Zinc castings can only be soldered with extreme caution because the melting temperatures of the alloys can be very low.
c) Zinc is soluble in acids and alkalis. Prolonged treatment with caustic soda to remove layers of paint adhering to lamp surfaces will therefore also affect the cast zinc surface.
d) Zinc castings are quite brittle and can break or crack when struck by hard objects. Bent zinc castings can rarely be bent back without provoking cracks.
e) The vast majority of cast zinc lamps have lost their former bronzing as a protective layer through years of use and cleaning with harsh cleaning agents. They now come on the market as dark grey, completely unsightly objects and need extensive cleaning and re-bronzing, which is labour-intensive.
Tin
Tin is also a silvery metal and is very often confused with zinc. Therefore, I would like to introduce this metal here as well, although it has not been used much in lamps.
Tin has a melting point of 232°C, which is unusually low for heavy metals. It is non-toxic both in its pure state and in most of its inorganic compounds. This is why tin was often used in the past to make tableware and drinking vessels. Due to its low melting point, tin was also used for tinning toxic (copper tableware) or easily oxidised (sheet iron) metals. Most tin cans made of sheet iron used to be tin-plated ("white metal").
The pure tin is quite soft and can be scratched with a fingernail. Therefore, tin has been made harder with about 5% antimony. Modern European tin has a composition of 94% tin, 5% antimony and 1% copper. The commercially available soft solder for soldering metals today contains approx. 95% tin, as lead may no longer be used.
Pewter is the English name for tin alloys with antimony and copper. It contains 85-99% tin, 5-10% antimony, 2% copper, with small admixtures of other metals.
Britannia metal is a specific alloy of 92% tin, 6% antimony and 2% copper. This alloy was used very often, as it has a colour very similar to silver and can also be polished very well.
Pewter was only sporadically used in petroleum lamps because, firstly, it is much more expensive than zinc and, secondly, because of its softness, it cannot be polished to a high gloss. On the other hand, pewter oxidises much more slowly than zinc, which is why it is still used today to make organ pipes, which retain their pleasantly soft shine for many years.
Pewter objects, including lamps, can be quite easily identified as such because they almost always bear a stamp with the pewter content. My only pewter lamp (L.306) bears the sign of the foundry.
Iron
Iron is one of the most important metals for mankind, not only in history ("Iron Age") but also in today's industry. Without iron and steel, we would not have been able to achieve our mobility and standard of living today.
After zinc and tin, iron is the third metal in our consideration that has a grey-silvery colour and could therefore theoretically be confused with the other two metals. However, iron is the only one of the metals (or alloys) described here that is magnetic and can therefore be identified very easily.
Iron is used - historically - in two completely different forms of processing: Wrought iron contains only traces of carbon and is therefore very malleable and workable. Cast iron, on the other hand, contains 2-4% carbon, is harder and much more brittle than wrought iron and therefore cannot be further deformed like the latter. Cast iron breaks under hammer blows.
Iron is used in different ways and for different purposes in kerosene/paraffin lamps. Threaded rods with nuts, washers and larger sheet metal washers to fix lamp parts together are almost always made of iron. The hidden metal fonts are also mostly made of tin-plated or brass-plated sheet iron. In addition to these mundane uses, which remain invisible from the outside, iron has also been used as a design element, in the form of cast iron (e.g. the pyramidal square base of many British kerosene/paraffin lamps) and wrought iron (e.g. on many floor lamps). Decoratively shaped and black lacquered bands of wrought iron were also used for a while together with copper in the representative table lamps, as this composition allowed charming colour contrasts (see lamps L.222 and L.251).
Lamps with sculptures or figures made of cast iron, on the other hand, are rare. Obviously, cast zinc, which is much easier to produce, was preferred to cast iron. The very pronounced tendency of iron to oxidise (= rust) probably also contributed to this. My only lamp with cast iron figures (L.300) has quite a few superficial traces of rust from earlier times.
Advantages for kerosene/paraffin lamps:
a) Iron can be soft-soldered as well as hard-soldered.
b) Bent iron can be bent back easily, but only with a lot of effort.
Disadvantage for kerosene/paraffin lamps:
Iron rusts quickly. Rust is a mixture of iron oxides that have stored water in their crystals. The rust formed takes up more space than the metal and therefore cracks the surface, causing more and more intact iron to come into contact with the air. Rusting will inevitably continue if this process is not interrupted. Bare iron surfaces that have been cleaned from rust must be immediately protected from further air contact with suitable layers of paint.
Copper
Copper is also a much-used metal in kerosene/paraffin lamps, but almost only as the main component of brass. Non-alloyed copper, on the other hand, is used rather rarely, as copper is harder and less flexible in its working properties than brass. When copper is used as part of a kerosene/paraffin lamp, it is usually in the form of copper sheet and only to create colour contrasts with other lamp parts made of brass or black-painted iron. The fact that brass is preferred to copper in every respect can also be seen in lamp L.086, whose copper vase is actually made of sheet brass that has subsequently been copper-plated.
Copper can be distinguished from all other metals described here because it has a typical red colour.
The properties and advantages and disadvantages of copper that interest us are almost identical to those of brass, so I will not describe them again here. The only difference is that there are probably no stress-related hairline cracks in copper sheets.
Galvanic Finishing of Metal Parts
Most of the metals described here can be intensively changed in their appearance by coating them with another metal that looks much nobler. The deposition of one metal on another metal is done with a physical-chemical technology called electroplating.
In electroplating, the piece of metal to be refined is connected to the negative pole of an electric battery with direct current, and the more noble metal is then connected to the positive pole. Both pieces of metal are placed in a saline solution that is electrically conductive. The applied electrical voltage causes positively charged atoms (ions) of the more noble metal to detach from its surface, migrate to the other, negatively charged metal, get neutralised there in terms of charge and precipitate as metal atoms on the surface of this metal. Depending on the length of the procedure, very thin to thick layers of the more noble metal can build up on the surface. The bond of the nobler metal coating on the other metal is permanent.
In this way, metals can be electroplated with silver, gold, nickel, chrome, copper, brass, zinc, tin, etc., either to give the original metals a more noble appearance or to protect them from tarnishing and corrosion.
In high-end, prestigious British lamps, the duplex burners are sometimes silver-plated. In continental Europe, there are often nickel-plated (or later even chrome-plated) burners or vase parts made of sheet brass. Sometimes even brass parts are copper-plated just to change their colour.
Burners with different metal finish (top row: the corresponding burner logos)
From left: 8‘‘‘ Kosmos burner of Hugo Schneider, brass
Duplex burner of Lampe Belge UK, brass copper-plated
8‘‘‘ Kosmos burner of Thiel & Bardenheuer, brass nickel-plated
8‘‘‘ Kosmos burner of Kauffmann & Cie., brass chrome-plated
Duplex Bijou burner of Richard Evered & Son, brass silver-plated
As the lamps made of cast zinc absolutely had to be protected against tarnishing, they were galvanically coated with a brass layer. This is called galvanic bronzing. This is a somewhat complicated process in electroplating because now, instead of a single "nobler" metal, two different metals, namely copper and zinc as the components of the brass, have to be precipitated on the zinc surface. As far as I understand it, this means that there is no metallically closed surface of the coating, but there are tiny crystals of the individual metals that adhere to the zinc surface. This thin coating layer is not as mechanically resistant as the real alloys, especially when it is applied very thinly for reasons of cost or time (electroplating takes time!). This also explains why most cast zinc lamps have almost completely lost their brass coating over time due to abrasion and other types of mechanical stress.
Tarnishing of Metals
Tarnishing of metal surfaces is produced by the chemical reaction of metal atoms with air components such as oxygen, sulphur, etc. The chemical substances formed by such reactions often have a completely different colour than pure metals themselves. Over time, this affects the appearance of the metal, even though some people consider tarnishing to be patina formation and thus "valuable".
Silver forms the black silver sulphide with the traces of sulphur in the air and tarnishes accordingly to a brownish-blackish colour.
Copper (and thus also brass and bronze) reacts with atmospheric oxygen to form two types of copper oxides, which are coloured red and brown. The accumulation of copper sulphide, which is also formed, on these oxides produces the dark brownish tarnish colour.
Zinc tarnishes by reacting with the oxygen and carbon dioxide in the air. The resulting molecules, zinc oxide and zinc carbonate, are whitish-grey in colour.
With iron, we do not speak of tarnishing, but of rusting, because the iron oxides formed with atmospheric oxygen form the rust. In contrast to other tarnish layers, which only remain on the surface as a closed layer and prevent the rear metal layers from coming into contact with air, rust in iron removes the upper metal layers from the surface, as the iron oxides take up more volume. This continuously exposes new metal layers and rusting continues.
Nickel and tin tarnish only very slowly and not noticeably. They hardly change their colour; at most they lose some of their shine.
Gold and chrome do not tarnish.
To prevent the metals from tarnishing or even rusting, they can be electroplated with suitable metals or coated with a suitable protective lacquer. The silver-plated and also original brass British burners are almost always protected against tarnishing with such protective lacquers. The fact that they nevertheless become unsightly discoloured over time is due to the fact that these protective varnishes themselves become porous over time due to the effects of air and light, and the tarnishing of the underlying metal surface then begins in spots.