The legendary Idrija mercury mines – UNESCO World Heritage in Slovenia

Whenever we travel, we usually have a look at the UNESCO World Heritage sites in the country. So, when we spent a week in Ljubljana we tracked down as many of the buildings as possible by the architect Jože Plečnik. The Idrija mercury mines were another UNESCO site, we figured, that we could visit in a day trip from the capital.

So, we made our way by public bus from Ljubljana to the small town of Idrija on a grey and rainy day. Just the right setting for a visit to the mercury mines of Idrija.

What is special about the Idrija mercury mines?

Along with the mines of Almadén in Spain, the Idrija mercury mines have the prestigious status of a UNESCO World Heritage since 2012. In the historic mines of Idrija, mercury has been mined as early as 1500. The mines were active until the 20^th century and belonged to the biggest mercury mines in the world for centuries.

Apart from the historical importance of the mining itself, the mining conditions also shaped everyday life. Around the mines, a prosperous mining town evolved, complete with miners’ living quarters, a theatre, and later some manufacturing industry.

Who needed all this mercury?

A shiny, liquid metall forming silvery droplets? Mercury is absolutely intriguing! In the 10^th century the Caliph of Cordoba had a small mercury pool to impress his visitors with light effects. During the 16^th century Spain needed mercury to extract gold and silver from ore in their new mines in South America. Mercury easily combines with precious metals turning it into an amalgam. This amalgam could be quite easily separated into pure gold and mercury again. The mercury from Idrija was exported to Germany and Holland and from there it went to South America. Especially the German Fugger family made a fortune from the mercury trade. Later the Idrija mercury was also used for scientific instruments and in the production of modern weapons.

A visit to the Anthony’s Tunnel in Idrija

From the bus station it is only 400 m on foot to the entrance of the Antonijev Rov or Anthony’s Tunnel. This simple building is one of the oldest preserved mine entrances in Europe and a visit into the Anthony’s Tunnel is only possible with a guided tour. So, we are lucky that one of the tours starts just when we arrive.

The tour begins with an introductory movie in the former assembly hall. This large hall was not for festivities but the place where the miners gathered before their shift to get their job assignments. When they went down into the mines, they wore an identification tag. That was a small, numbered medallion, which they put back on a board after work. The intention was to check if everybody had returned safely from underground. Imagine those days when miners were missing in the evening! It must have been scary and depressing. And in fact, in the 500 years the mine was active, around 300 miners died. Still, this is quite a low number in comparison to other historic mines, we learn.

 “Srečno” is written over the entrance to the mines – Slovenian for “Save return”. Today we wear safety helmets and a protective raincoat and follow our guide Matic into the narrow rock passages.

Mercury – a poisonous metal

Work in the mines was not only dangerous but also quite unhealthy. Although it is considered a metal, mercury is liquid at room temperature! In the Idrija mercury mine, around 30% of the mercury occur not as a compound (which is usually solid), but as pure mercury contained in the rocks. And indeed, even today, walking along the rocky walls, we see glittering silver droplets in fissures of the grey stone. These are tiny balls of pure liquid mercury, held together by surface tension. For the safety of the visitors there is a plexiglass screen in place in front of most of these droplets.

We remember the old-fashioned fever thermometers from our childhood, which always came with a warning. “Stay away from them,” the adults would caution us, because if the glass breaks and the mercury leaks out, it is highly poisonous. Inhaling a certain amount of mercury fumes can have a negative effect on the body and lead to cancer. 

„As children we played with the shiny liquid pearls,” our guide Matic recounts. “We had no idea that these droplets are toxic.”

Today it is well established that a quite high number of miners got a mercury poisoning. And even today the cancer rate in Idrija is higher than in the rest of Slovenia. The official advice is now that the inhabitants of Idrija should not grow vegetables in their gardens because of the heavily polluted soil.

In the Idrija mercury mines, safety measures have been lenient at best throughout history. Safety clothing such as masks and gloves were only introduced in 1960s.

History of the Idrija mercury mines

It was in 1490 that a barrel maker found mercury in the water, thus starting the mining operation in the small town. As demand grew, the mines became ever bigger. And with the invention of the steam engine in the 19^th century, the output of the mine increased further. Eventually the Idrija mercury mines were among the most technically developed mines in Europe.

In the early 20^th century, however, the presence of a lot of water in the rock became a problem for the electrification of the mines. Nevertheless, the mines reached their peak output in 1913, at 820 tons of mercury. After World War I, the mines belonged to Italy for some time before becoming part of Yugoslavia. But they suffered from economic crises and became ever less profitable. They finally closed for good in 1988.

Daily life in the Idrija mercury mines

The tour through the Idrija mercury mine takes around one hour and leads us deep into the rocks. Inside the mine, life-sized puppets mimic the life of the miners and give an impression of the work in the mines. Compared to other mines we have seen (like Blaenavon in England or the mines at Banska Stiavnica in Slovakia) the mine waggons seem small. That’s because mercury is exceedingly heavy: 14 times as heavy as water! Accordingly, the small waggon the size of a removal box could hold up to 300 kg of ore and mercury!

The regular miners worked only two 8-hour shifts. The third shift at night was reserved for the dynamiters. At the end of the tour, we see a virtually recreated blasting of a tunnel section. At that point we are about 80 m deep under the mountain, and we don’t want to imagine how a real explosion would feel. It does make sense that we pass a chapel inside the mine at the end of our tour.

Visiting the mercury smelter and the museum

The mercury mining business did not stop in the mine itself. In fact, the whole town was created around the mercury processing facilities. There was an ingenious, continuous ore movement system that moved recently mined ore up and out of the mine. From there it moved in a kind of ropeway across the river valley to the smelter on the other side of the small town.

So, after the mine, we also visit the remains of the smelter. In the smelter, the mined ore was first crushed and sorted and then transported to the smelting furnace by conveyer belts. The remains of the furnace are still quite impressive: Basically, it is a 36 m long, 2 m diameter, slightly inclined pipe. This pipe was heated throughout, while the crushed ore was gliding down inside. Mercury turns into gas at a temperature of 356 °C. At some point inside the pipe, the mercury would turn to gas and float up so that you could separate it. The gaseous mercury was then cooled down again and collected. The work in the smelter with its gaseous mercury was even more hazardous for health than the work in the mines.

In the adjacent museum we learn more about the different uses of mercury. For instance, the Swiss alchemist and physician Paracelsus in the 16^th century advocated mercury to treat Syphilis. He died of mercury poisoning himself, however. In 2011 mercury was banned in the EU.  And in 2017 the so-called Minamata Convention, designed to protect human health and the environment from emissions and releases of mercury, became effective.

Where else in Idrija can you see remains of the mining history?

The mining history of Idrija is present everywhere. But if you want to see more remains of the actual mining there is the area of the Joseph’s shaft (Jožefov Jašek) a bit up the mountain. You cannot enter this shaft, but there is an old waterwheel to see and some administrative buildings from the mining area. We went there at dusk when it was still lightly raining.

Should you visit the Idrija mercury mines?

We think that the Idrija mercury mines are fascinating evidence of a now-forgotten industrial history. In many countries, there are old mines. And often, they have insightful museums – in the German Ruhr area, for instance, you can explore the whole industrial heritage through numerous museums and old coal mines. We find that industrial heritage gives you entirely new insights into the fabric of a country: Be it coal mines in Belgium, silk spinning in Japan, or the silver mines of Banska Stiavnica or Hall in Tirol.

It was also quite interesting to learn more about mercury. Mercury was still present in daily life in our childhood but is very distant now. It is usually only in books and archaeological records that we find references to mercury. For instance, we often visit sites and museums relating to the silk road in Central Asia during our guidebook research. We have been to ancient cities like Paikent or Akyrtas, but also the historical museums in Samarkand or Tashkent. These museums often display “mercury flasks” of clay. Apparently, the scientists producing the famous glazed tiles of Uzbekistan and Persia also used mercury. For gold making, or to extract their ingredients for the shiny glazes. 

How to get to the Idrija mercury mine from Ljubljana

We took a bus from Ljubljana bus station (next to the train station). They did not go very often – less than every hour – so we had checked the timetable for both directions beforehand. The bus cost 6,30 € each way (in 2022) and took just over an hour.

NB: We were not sponsored or paid in any way to write this blogpost about the mercury mines of Idrija. We paid all expenses ourselves.

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