A friend of mine has just asked about the sulphur that is mined at Ijen and this reminded me that I meant to include that information with the photos.
Here is some information from Wikipedia and also a fantastic link to some photos taken by someone who took a boat out onto the acid lake:
From an active vent at the edge of the lake, volcanic gasses are channeled through ceramic pipes, resulting in molten sulfur. The sulfur, which is deep red in color when molten, pours slowly from the ends of these pipes and pools on the ground, turning bright yellow as it cools. The miners break the cooled material into large pieces and carry it away in baskets. Miners must carry loads, which range from 75 kilograms (170 lb) to 90 kilograms, up 300 meters (980 ft) to the crater rim, with a gradient of 45 to 60 degrees and then 3 kilometers (1.86 miles) down the mountain for weighing. Most miners make this journey twice a day. A nearby sugar refinery pays the miners by the weight of sulfur transported; as of September 2010, the typical daily earnings were equivalent to approximately $13 US. The miners often use insufficient protection while working around the volcano and complain of numerous respiratory afflictions. There are 200 miners, who extract 14 tons per day or only 20 percent of the continuous daily deposit.
For more information and fantastic photos, follow this great link.