Alphabet der Heimatkunde
Zinc, an entirely new metal,
commonly used for all sorts of everyday objects.
Until the end of the 18th century, zinc could not be made in its pure, metallic form and was not even known. As late as in the 1830s the metallurgists succeeded in producing zinc by using calamine, a special zinc ore which had previously been used in the brass making process for centuries.
At the beginning of the 19th century it was possible to smelt pure, metallic zinc on a large, industrial scale. In the meantime however, the ore deposits in the upper part of the local rock formations had mostly been exploited for brass making. Consequently, the miners had to go deeper to find sufficient quantities of ore. Upon reaching a certain depth, however, the characteristic of the ore changed from the calamine type to what we call “blende”, a sulphurous ore type mainly consisting of sphalerite.
Before processing this type of ore, it had to be roasted. This was necessary to set free the sulphur content in form of sulphur dioxide, which initially just escaped into the atmosphere. Together with the humidity of the air, this sulphur dioxide was transformed into sulphuric acid. At those times, even small patches of grass – let alone bushes or trees – were an absolute rarity in the neighbourhood of the smelteries.
Fortunately, some 15 years later special furnaces had been developed, which prevented the sulphur dioxide from escaping into the atmosphere. The sulphur dioxide was then utilized to produce soda (sodium carbonate Na2CO3). At that time, soda was a key product used for soap making and for manufacturing glass.
Coming back to the local ores, another particularity has to be mentioned: Where the local ore deposits had formerly been mined, the soil is highly contaminated with toxid lead-, zinc. and cadmium-minerals. In these locations an extremely specialized group of herbs has managed to adapt and to survive in this environment (Evolution). The most famous species of the aforementioned population of plants is a small, yellow blossomed violet, called viola lutea calamaria (Galmeinveilchen). It is a worldwide unique botanical speciality, which indeed can only be found in the neighbourhood of Stolberg (endemic).
The method of zinc smelting
Quite similar to brass making, the general problem in producing zinc
resulted from the fact, that the zinc content of the ore was not set
free as a liquid but as zinc vapour. In addition, the zinc vapour
instantly reacted with the oxygen of the air, thus forming zinc-oxide
powder. The problem was solved by condensing the zinc vapour under
complete exclusion of air.
The retorts (also called muffles) were placed within a furnace, where they could be heated to more than 1000 Co. The muffles were filled with a mixture of pulverized zinc ore and pulverized coke. A condenser was firmly placed against the open end of the muffle and then the junction was covered with clay to avoid any leakages. The condenser thus protruded out of the hot furnace.
Sketch: F. Holtz.
When the content of the muffle gradually heated up, zinc vapour together with carbon monoxide was set free, as was the case in brass making. When reaching the condenser, the gases cooled down and the zinc vapour condensed to liquid zinc, whereas the carbon monoxide streamed out of the other and open end of the condenser, thus keeping away the oxygen of the air. The liquid zinc collected in the condenser and could then be taken out by a special spoon and cast bars for rolling coils subsequently.
The carbon monoxide leaving the condenser was immediately ignited to form less dangerous carbon dioxide.
Normally the outlet of the condenser was elongated by a so called allonge. Small amounts of zinc vapour or microscopic droplets of liquid zinc which escaped from the condenser, solidified within the allonge. The resulting dust (either zinc or zinc oxide) collected at the inner walls of the allonge and could be utilized to enrich the next charge of ore.
Discharging and refilling of the muffles (called maneuver) was very, very hard work indeed. It had to be done under working conditions, we could not even imagine today. The replacement of a muffle for instance had to be done by hand; no lifters or auxiliary tools were available. And a muffle was not only heavy, it was also preheated to above 800oC.
Typically, the so called maneuver was started at 4 or 6 o’clock in the morning, depending on the season of the year. In other words, the extremely hard work was done during the cooler hours of the day. Even though the maneuver took 4 to 6 hours, the workers were paid for a complete 12 hours shift.
In the 19th century zinc was a totally new material and, more importantly, it was virtually resistant against corrosion. You may well compare it to the plastics of today. So we can look back at a wide variety of final zinc products such as bathtubs, buckets, watering cans etc.
Also today, zinc is commonly used for producing roof's gutters, facade cover plates etc. Nowadays, architects increasingly make use of zinc as a design element. Actually, this has a long tradition and started in the second half of the 19th century. At that time the Prussian architect Karl Friedrich Schinkel promoted the utilisation of zinc ornaments and sculptures such as:
- neo-Gothic window frames and
Everywhere, and not only in Germany or Europe, zinc ornaments were implemented in architecture as fashionable decorations. Since a total of three zinc smelting plants were located in Stolberg, it was no wonder, that a company was founded, which specialized in producing zinc ornaments.
Most of the cast zinc sculptures were produced as follows: First of all, a moulding form, shaped according to the desired statuette, was filled with hot liquid zinc. At the surface of the cooler mould, the liquid zinc began to solidify. After a few seconds the mould was turned upside-down. The still liquid zinc could flow out, whereas the solidified zinc remained in the mould. In this way hollow, moulded sculptures were obtained, thus saving material and weight.
Numerous highly attractive examples show quite clearly why zinc decorations were so popular at those times: After having gathered some patina, the zinc statues almost looked like being made of stone. In many cases it was next to impossible to visually recognize the statues as zinc objects.
As so often in technology, the well established process of zinc casting became obsolete, because it could be done even better and cheaper. Instead of casting, the objects were manufactured by using more or less thin zinc plates in a deep drawing process. The different parts were subsequently soldered together as required to obtain the final sculpture.
More details will follow soon !!!