Liquid motion lamps, also called lava lamps, work by combining two different fluids inside a glass lamp body. One of the fluids is usually wax-based and the other is typically water-based, and the densities of the two fluids are almost, but not quite, equal.
When you turn on the lava lamp, a heating element heats the wax, thereby decreasing its density until the wax is less dense than the water. The wax then rises toward the top of the lamp, where it cools. Its density decreases until it's once again more dense than the water, and at that point the wax sinks toward the bottom of the lamp, where it's heated again.
Over time, or if the lamp is dropped or jostled, the water in the lamp can turn cloudy. If you do it carefully and correctly, replacing the water can make the lamp look almost like new.
Removing the cap on top of the lamp body or otherwise disassembling the lamp will void the lamp's warranty, so only do so if the warranty has already expired.
Removing the Cap
The first step in replacing the lamp's water is to remove the cap at the top of the lamp body. Be sure the lamp is unplugged and completely cold before you remove the cap.
On some lamp models, the cap is a simple screw cap that you can unscrew either by hand or by carefully gripping it with locking pliers. Other models, such as the Lava Lamp Grande, have crimped-on caps that can be removed by carefully prying around the perimeter of the cap with a small flat-head screwdriver inserted under the lip of the cap.
Some models are sealed with a plastic plug under the cap. Remove the plug, and dump out the water, being careful not to dump out the cold wax -- which should be solid and at the bottom of the lamp.
Adding New Water
Refill the lamp with distilled water, leaving between 1 and 2 inches of space at the top. Add a teaspoon of canning salt, pickling salt or Epsom salt to the water, and agitate it gently until the salt dissolves.
Alternatively, you can make an Epsom salt solution outside the lamp, adding Epsom salt to cool distilled water until the salt will no longer dissolve in the water. Fill the lamp gently with the solution, taking care not to disturb the wax at the bottom.
Adjusting Salt Content
After the initial addition of salt, you'll have to add more salt to adjust the density of the water, and you'll have to do it very gradually so you don't overdo it and have to start over. Begin by letting the lamp heat for about two hours, and then add more salt a small amount at a time. One method is to dissolve salt in a separate solution and then add small amounts of that solution to the lamp, using a drinking straw as a pipette.
Wait about an hour after each adjustment, with the lamp on. When the water's salt content is almost correct, the wax will begin to form a dome in the bottom of the lamp, and when you reach the correct density, the wax will begin to float upward.
The wax is likely to rise in one large blob until you add a surfactant -- a substance that will break the surface tension of the wax and cause it to form multiple, undulating blobs. Add a very small drop of a clear dish-washing detergent to the water, and the wax should begin to divide into smaller blobs. You may need to add more detergent to get the blobs to the desired consistency, but adding too much will make the wax runny, so go slowly.
Replacing the Cap
Once the lamp is operating satisfactorily, replace the plastic plug and cap carefully. If you've had to pry off a crimped cap, you can reseat it by carefully tightening a hose clamp around it.