(or Astro lamp) is a novelty item used more for decoration rather than illumination — the slow, mesmeric rise and fall of variously shaped blobs of wax in it is suggestive of lava; hence, the name. The lamps are available in a variety of styles and colors of wax and liquid.
The lamp contains a standard incandescent bulb or halogen lamp which heats a tall (often tapered) glass bottle containing water and a transparent, translucent or opaque mix of wax and carbon tetrachloride. The wax is slightly more dense than water at room temperature but is less dense under warmer conditions. This occurs because wax expands more than water when both are heated. When heated, the wax becomes fluid, its relative density decreases, and blobs of wax ascend to the top of the device where they cool (which increases their density relative to that of the water's) and then descend. A metallic wire coil in the base of the bottle acts as a surface tension breaker to recombine the cooled blobs of wax after they descend. The underlying mechanism is a form of Rayleigh–Taylor instability.
The bulb is normally about 25 to 40 watts. It may take 45 to 60 minutes for the wax to warm up enough to freely form rising blobs (depending on the original temperature). It may take 2 to 3 hours if it has been in a cold room for a long time.