- published: 30 Jan 2016
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A lava lamp (or Astro lamp) is a decorative novelty item that contains blobs of coloured wax inside a glass vessel filled with clear liquid; the wax rises and falls as its density changes due to heating from an incandescent light bulb underneath the vessel. The appearance of the wax is suggestive of pāhoehoe lava, hence the name. The lamps are available in a variety of styles and colours of wax and liquid.
A classic 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 mineral oil, paraffin wax and carbon tetrachloride. The water and/or mineral oil can be coloured with dyes. The density of common wax is much lower than that of water and would float on top under any temperature. However, the carbon tetrachloride is heavier than water (also nonflammable and miscible with wax), and is added to the wax to make its density at room temperature slightly higher than that of the water. When heated, the wax mixture becomes less dense than the water because wax expands more than water when both are heated. It also becomes fluid, and blobs of wax ascend to the top of the device where they cool (which increases their density relative to that of the water) 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.