Lava lamps were invented in the 1960s and many used lamps are still in circulation. Some in the salvage market need restoration, since the wax "lava" inside is fragile. Just shaking a lamp can permanently cloud it. Even leaving it in the sun can eventually turn the coloured water clear. Fortunately, you can solve both of these problems with a straightforward restoration job.
Unplug the lamp and let the wax cool for several of hours.
Unscrew the cap at the top of the lamp globe and pour out the liquid contents. The wax will be solid at room temperature and only flakes of it that cloud the water will pour out.
Fill the bottle with distilled water. Do not pour it directly on to the wax at the bottom in the globe. Do not shake or stir the water. Pour out the water. Repeat this step if there are still particulates left in the lamp.
Fill the bottle up with distilled water again, leaving a 5 cm (2 inch) air gap at the top. This gap gives the heated wax at the bottom room to expand, and therefore gain buoyancy.
Turn on the lamp again and let the wax warm up for an hour. You can leave the cap off.
Microwave a drinking glass of distilled water for 10 or 20 seconds. Dissolve as much Epsom salts or pickling salt in it as you can. This will increase the water's density to be less than that of the heated wax, without clouding the water with the iodisation of table salt.
Dip a drinking straw 2.5 cm (1 inch) into the salt solution. Cover the open end of the straw. Drop the solution into the lamp. Wait 10 minutes for it to distribute. Repeat this every 10 minutes until some of the heated wax first floats to the top of the bottle. That's when the water and wax densities will be correctly balanced. The water is thus less dense than the cooled, contracted wax and denser than the heated wax.
Add a small drop of washing-up liquid and two drops of food colouring. Again, let it diffuse on its own instead of shaking it. Now screw the cap back on and you're done.