Ijen is a quietly active volcano on the Indonesian island of East Java, and it is also a place of business. Local workers hike up the side of the mountain and down into the crater at the top to harvest its sulfur—a byproduct of the gas that escapes from the volcano’s vents and collects near the shores of an acidic lake at the crater’s center. The chemical is used in industry worldwide, from making matchsticks to vulcanizing rubber, but Ijen’s sulfur goes mostly to local factories, which use it to bleach sugar.

Ijen is one of the few volcanic sulfur mines remaining in the world: Mining an active volcano is dangerous work, and there are easier ways to get the chemical. I first became curious about the volcano after seeing it featured in the 2001 documentary War Photographer. The film showed the protagonist James Nachtwey coughing furiously as he clicked his camera amid clouds of sulfur. Being a photographer myself, I wondered how the place would look in person and whether the conditions could be as bad as they seemed. It struck me as impossible that the slight-statured miners shown in the film could really carry heavy loads of sulfur up and down a mountain.

I left from Hamburg, Germany, and it took me three flights, one train ride, two motorcycle trips, and a long trek up the volcano before I saw the mine for the first time. A photographer I knew in Jakarta introduced me to a former miner named Imam, who served as my guide. I spent two days photographing the workers there.

Ijen spits out striking blue flames that are only visible at night, which is when tourists hike up the mountain to see them. The sulfur miners get started shortly afterward, around sunrise, to do a few hours of work before the heat of the day sets in. They use simple tools such as stones, steel bars, and shovels to break the sulfur into chunks small enough to load into baskets and bags, which they then haul two hours back down the mountain. They can sell their harvest for about 7 cents a kilogram, so some miners carry up to 90 kilograms on their shoulders in large baskets connected by bamboo poles. The few miners who are capable of making the full four-hour trip up and back down the mountain twice a day can earn about $11.

These workers are exposed to danger even before they enter the mine’s toxic atmosphere, beginning with the slippery, rocky paths up the mountain. The crater’s air stings the lungs and eyes. Sometimes a miner will get trapped in a cloud of the sulfurous smoke that pours out of the volcano, which can lead to coughing fits or even loss of consciousness. The gas mask I wore wasn’t leak-proof, so I found myself coughing for weeks after my time in the crater.

To get some of these photos, I had to lie down on my side inside the crater, or sit on a stone or chunk of sulfur, and wait for the smoke to clear. I had initially planned to make a series of portraits of the miners, along with personal stories, but I soon found this wouldn’t work. Imam’s English wasn’t strong enough for him to do interviews for me, and in any case the miners didn’t want to talk that much while we were in the crater; they just wanted to load their baskets and get out. Instead, I took photos that told a story about working conditions in the mine. Since the colors of my surroundings changed frequently—sometimes the volcanic smoke would clear and the sun on the sulfur would make the whole crater seem a vivid yellow, while other times the smoke would render the scenery drab and colorless—I used color corrections to give each picture roughly the same proportions of yellow and orange so that they could be viewed as a series.

Tourists who visit the mountain will often photograph the miners trekking down it with fully loaded baskets. The miners will pose for them. But not many people go down into the crater itself as I did. In there, the miners are focused on doing their jobs as quickly as they can so they can leave and escape the smoke. Miners do not pose in such situations.

My guide Imam, 38, quit mining six years ago; the physical toll of his job was still visible on his shoulders. After our first day on the volcano, we were washing ourselves in the river of a nearby village and I noticed his wounds and asked if I could photograph them. I took this picture standing on a chair in a hut, where a small bulb was the only light source.

The miners slowly walked up the steep crater walls, stopping frequently while clouds of sulfurous gas blanketed the crater in the background. I don’t think this miner realized I was photographing him. Here, near the crater’s rim, the miners often just seemed relieved to be breathing fresh air again.

I liked the image of this miner wielding his steel bar against the rocky ground, almost as if it were a blind man’s walking stick. As he emerged from a cloud, he looked to his left, where the smoke was less intense.

I noticed Suwono Licin, 38, blinking and coughing a great deal. In this close-up, one can see that his eyes were red and his whole face, including his eyelashes, was covered in sulfur dust.

The miners used parts of old ceramic pipes to prop up their baskets, which they connected with bamboo sticks. Then they had to creep under the bamboo sticks to position the baskets on their shoulders. A load like this could weigh up to 90 kilograms altogether.

Mistari Ubret Giri, 52, calmly smashed a large chunk of sulfur into a manageable piece. I framed this picture to show the subdued sunlight in the background.

I had to kneel on the ground to take this shot of Madikan Blimbingsar Lincins, 45, running out of the cloud. The smoke burned the miners’ eyes, so they would often work until they were nearly blind, bringing heavy yellow chunks of sulfur with them when they reemerged.

Panggung Licin, 28, started mining at Ijen a few months before I traveled there. He was not well-equipped for the hostile environment—he wore a cap and scarf instead of a helmet and gas mask.

Sometimes the sunlight could just barely break through the smoke covering the crater. When this happened, the sun looked like a small, glowing spot in the sky.

A pipe network connected to the volcano’s fumaroles—the holes that spit out the mountain’s toxic gasses—collects smoke, directing condensed sulfur into barrels where it hardens from a red liquid into a vibrant yellow solid. Miners used water from a nearby reservoir pumped through hoses to cool down the pipes. I saw this miner running toward me with a hose in his hand while I rested on a stone.

A worker rested, surrounded by warm smoke. There was another miner a few meters away who was invisible in the cloud.

Despite having worked in a smoke-filled crater all day, Akso Panggung smoked a clove cigarette to get rid of the sour taste of sulfur.