Epsom Salts are also known as magnesium sulphate. Epsom crystals have many health uses but they make the lava lamp come alive!
You can get them at any pharmacy or drug store. Walmart has a nice supply.
You just need the smallest container they have, less than 200 grams is needed.
You'll need a bowl of warm water and a glass of tap water (lukewarm). Start adding the sea salt into the glass of water and stir it around to dissolve the salt crystals.
At some point they will no longer dissolve and this is due to the temperature limitations. To correct this fill a small bowl with HOT water and center the glass in it. Continue to stir it vigorously In a few minutes the rest of the crystals will dissolve into the water. You want about 1/4 of the glass worth of salt in the end dissolved in the water.
Only add 1 teaspoon at a time and repeat after each one has dissolved in the glass. you need to stir vigorously.
From sitting there on heating up (cap off) the wax will either sit there and become a pool of wax at the bottom of the water, or it will ride up the sides making cool forests but it will not likely do much else. The soap drops will help this but the salt seems to activate the lamps best properties. If you already added the soap drops don't repeat this!!!
The next step will be to grab your straw and pretend you're a kid again with soda. Put the straw in block the mouth end with your finger... The lamp will actually seem to work after a while but this isn't the end yet. You have to get the right blend of salt and soap in there to make it bubble just right. A few drops of soap and some straw fulls (about a quarter full actually) will do it.
You need to put a little salt into the bottle and let it keep going see if anything changes. Give it sufficient time (about 30 minutes if the wax is melted and the lamp hot. You should see something happening pretty quick actually since it's hot and mixing is easy (do not stir! The lamp is self stirring if you stir it you will create a brand new cloudy lamp!)
Put the straw in on the edge and release the solution (salt and water) slowly. If you throw it in too fast the wax could break up into thousands of tiny pieces... this takes finesse. Another way to do it is to dip the straw about an inch into the water and slowly release the salt (still on the edge you want it to go in on the edges not where the wax is blobbing).
What's happening is the salt raises the waters density making the wax less dense (so it can rise up) than the water/solution. This also has a side effect of keeping the blobs together as if water were traveling through a balloon. The blob shape is the result of this network of concepts.
When you initially add the salt the lamps contents will sort of swirl about and start glooping. This is normal and passes. When it settles down again you can add more salt. Treat it like adding gasoline to a fire! What you want is for the wax to bubble up and float to the top then fall like you're used to seeing.
Half way up and falling means something is missing or too much of something was introduced. Usually you will see a column or bubbling that only goes half way before descending again like it's too heavy. You may need to mess with it for an hour slowly monitoring how it reacts to each dose of salt.
The lava lamp pictured below has blobs that start to rise but stop midway this is an example of looking ready but not yet being. This still needs more epson salt solution (may be only a few crystals from ready!) ...
This lamp is fully restored with this process and working great!
This is the lamp at the top of the page that was cloudy and useless!
I'm sure the lava company made theirs to last years so this may not be that stable, but it's safe and effective.
If it lasts a year I'd be happy, but capped off and glued (so the water doesn't evaporate out), I think it will last a long time.