Liquid motion lamps, of which the Lava lamp is one trademarked brand, consist of a clear glass lamp body filled with two different colored liquids, along with a small light source and a heating element, both at the base of a lamp body.
How Liquid Motion Lamps Work
The two liquids inside the lamp have similar, yet slightly different, densities, and their different chemical compositions prevent them from mixing with one another. At room temperature, the density of the more viscous liquid, the "lava," is greater than that of the other liquid, so when the lamp is off, the lava settles to the bottom of the lamp. When the lamp is turned on, the light source and the heating element warm the lava, causing it to expand and decrease in density.
Now less dense than the other liquid, the lava rises toward the top of the lamp. As it moves farther away from the heating element, it cools and contracts, and when its density is once again greater than that of the other liquid, it falls back toward the bottom of the lamp. There it is heated again, creating a constant cycle of motion.
The Lava Component
The lava fluid needs to be viscous so that it will hold together in blobs and flowing shapes, and it must not be water-soluble so that it won't dissolve into the other liquid. Paraffin wax was used as the basis for the lava fluid in early liquid motion lamps, and although the exact chemistry of current formulations is kept secret by lamp manufacturers, wax is likely still the primary component in the lava.
Other components, such as sodium sulfate, are added to the wax to help it flow and move in the expected manner, and wax-based paints and dyes are added to create different color variations between individual lamps.
The Other Liquid
The liquid through which the lava flows is usually made up primarily of water. The oil-based wax of the lava will not mix with water, so the two fluids remain separate. Substances such as sodium chloride and propylene glycol are added to the water to adjust its density and bring it close to that of the wax, and water-based dyes are added to the water to achieve unusual color combinations in individual lamps.
Glitter lamps look similar to Lava lamps, but in a glitter lamp, metal-coated flakes float through a liquid in the lamp body on convection currents created by the heat of the light source in the lamp's base. In these lamps, the fluid is a mixture of propylene glycol and polyethylene glycol.