Many of us love the beautiful, retro, tacky look of a lava lamp. But long-term ownership means you'll need to address cloudy and receding water. Here's how to replace the water and make your lava lamp work like new.
Before replacing the water in your lava lamp, ensure that it is long cool and that the "lava" (wax) has fully hardened. Remove the outer plastic cap and use a bottle opener to pry off the inner metal bottle cap. Remember, you're going to be putting this cap back on when you're finished, so go slowly and avoid bending it as much as possible.
Pour out the water. If any of the lava has hardened in pieces that are small enough to fall out of the bottle, use a strainer to avoid losing them. Refill the lamp with distilled water. Leave some space at the top—the black cap will cover it.
Add a few drops of dish soap to the bottle. Cover the top with your hand and shake. The water will foam up, but don't worry—the bubbles won't last. The dish soap will act as an emulsifier.
Add a small amount of canning/pickling salt to the water solution. Be sure to use this type of salt, as it is the purest. Return your lava lamp to the base and plug it in. Wait for the wax to soften. See if you've added enough salt to make the wax move around. If it sits on the bottom, gradually add more.
Turn the lamp off and let it to cool again. Be sure that the mouth of the bottle is dry and reattach the cap using a fast-drying epoxy. Do not use heat-sensitive or water-soluble glue. If you have access to a bottle capping device (sold for hobby beer-brewing), you can also use it to recap your bottle. However, lava lamp bottles have no neck, so you'll need a special kind of capper.
Hot lava lamps can be a burn hazard. Do not handle the bottle while your lamp is hot. Epoxy is an extremely strong glue that should not be touched. Use it in a well-ventilated area.