Most American made cloudy lamps can be fixed by simply running the lamp. The lamp can be put on a timer to run the lamp for you. Set the timer to run the lamp for 8 hours on (or as long as it can be run without overheating), and 8 hours off. Run this cycle until the cloudiness goes away. If the cloudiness does not go away after a month, then there might be a more severe problem with the lamp. Some will clear up better if you let them sit in a cold place for a while.
If the lamp is a China made lamp it may never clear up. The same methods can be tried as if it were an American made lamp but your results may not be as good. Newer China made lamps have responded better to filtering after being ran enough times, but filtering seems to change the flow of the wax. As of November 2010, Lava Lite claims to have fixed their issues with clouding in their lamps, but this has yet to be seen. Even after several promises Lava Lite still sells cloudy lamps to much of our heartache.
2012 update Lava Lite is attempting to correct the formula to stop lamps from being sold in a cloudy condition.
2021 update. The Chinese made lamps are still cloudy...
How to filter a lava lamp
To properly filter a lava lamp, you will need a ceramic filter such as the MSR MiniWorks. Dont get the EX filter as it has a carbon center. Any non-ceramic filter will not work because it will not filter down to about .2 microns. This means you cannot use a carbon filter, or a coffee filter. Keep in mind that the MSR MiniWorks filter pump comes with an optional carbon filter attachment. DO NOT USE THIS! It will remove some chemicals from the fluid that should not be removed. Also do not use any other filter such as a coffee filter because it will not filter the small particles suspended in the liquid that cause the clouding and will likely introduce fabric fibers into the liquid. The reasoning for using a ceramic filter is explained below.
Filtering a lava lamp is simple, but there are some very important steps to follow to get crystal clear fluid. The reason lamps cloud up is because chemicals from the wax get released into the liquid due to shaking or age. Usually running a lamp many times will cause these chemicals to re-enter the wax, but if that does not work the only way to remove them is by using a ceramic filter.
Make sure you have ran the lamp for at least two weeks - 4 weeks or more is preferred. If there is white powder underneath the wax be careful not to remix this powder back into the fluid, that powder is what clouds lamps. Let the lamp cool until it reaches room temperature before filtering.
Have one container ready to hold all liquid from the lamp, glass is preferable because some plastic containers can be dissolved by the liquid formula. Remove the cap on the lamp and slowly pour the liquid into your container making sure not to pour any white powder underneath the wax into the container. You will lose some fluid by doing this, but it is required for a clear result.
Put the filters hose into the glass container. Put the filter on top of the globe you originally took the fluid out of. Pump all of the fluid back into the globe, you may have to pause for a while if too many bubbles form. The bubbles are the surfactants in the fluid and they are vital for proper flow, so make sure you get them all out of the filter!
After the bubbles settle down run the lamp for a full cycle. If the lamp is low on fluid you can refill up to 1/2" with distilled water. If you have liquid from another lamp of the same make (Ex: USA to USA, China to China) you can use much more to top the lamp. Filtering should not remove more then 1/2" of liquid so if your missing more fluid it is probably still in your filter. Once you are sure your lamp is flowing properly and has the correct fluid level you can reseal the lamp.