First of all, this is not a guide but notes for my self, that hopefully others might find useful. Stuff you find here might already have been described elsewhere on this site.
First the basics
What makes the lava lamp flow is basically the liquid solution being made correctly for the specific wax your lamp uses. In other words, you might not be able to take one liquid from one lamp, and pour it into another lamp with some other wax. This might work, BUT the wax has to have the same properties as the one you got the liquid from, to get the same flow. I've read that Lava Lite uses the same type of wax and liquid in all of its newer lamps, just dyed differently, but i cannot confirm this, but this means that if you want swap a liquid from such two lamps, it should be possible.
What happens is that as the wax heats up it melts, which lowers its density. Once the density is lower than the liquid, the wax will rise up from the bottom of the lamp. When the wax has risen, it will cool down again, due to it being farther away of the bulb, which will make the density rise, making it higher than the liquid again, which lets it fall back to the bottom again. Now there are other factors that play a role here, like surface tension, but this is more or less the essential part of how it works as far as i know.
So how to make your own perfect liquid
That's a good question! After messing a lot with redoing liquids, I've stumbled upon some different scenarios and behavior of the flow i obtain. Its not entirely regardless what solution you use to increase the density of the liquid.
To begin with you'd want some water to mix with. You have three options here:
Someone say tap water is fine, but i really haven't tried that. Deionized water should be the cleanest water of the three of them, but Distilled water is just as good for this job, and should be cheaper. I'm using Deionized water in my examples as this is simply the easiest to get here i Denmark. No mater what type of water you choose it should have, or at least be super close to, the density of 1,000 g/cm3.
Not necessary, however very useful, would be to get hold of a device/tool to measure the liquids density. I've bought my self an aquarium hydrometer. Something like this: eBay Link or another eBay link . I cannot however confirm how precise those specific ones are, but what you might notice is that they list specific gravities / densities between 1,010 - 1,030 g/cm3, which is a good range, since most liquids I've made at least, which includes Mathmos, Lava Lites and other China brands, usually end up between 1,013 - 1,025 g/cm3. This tool however is not required, but an easy way for you to create a new liquid, if you already have some of the original. The one i have looks like this one:
Its also a very neat thing to have, once you have a liquid with a flow you like, since you can easily measure it afterwards and note it down for later.
How to raise the density of your liquid
Next up is what solution you need to choose to raise the density of your upcoming liquid. There are some options here:
Just by looking at this, one might think that Glycerin is the way to go, since you would need way less of it to get the density up, compared to Propylene Glycol, which would require you to make an almost 50/50 mix between it and Water. However this is where the tricky part comes in. Because your choice does very much affect your flow and wax, even if you end up with the same density as the end result. What I've discovered is that this is dependent on your wax, but I've also heard that good results is mostly guaranteed if you use Propylene Glycol.
Take this Jet as an example:
This liquid solution works, as we see the wax going all the way from bottom to top, and making a nice column. BUT its all full of bubbles! A no go for me.. The properties for the solution is 1,017 g/cm3 which is obtained by using Epsom Salt, and 1ML of SLES Surfactant (more on this later).
Take a look at this one, which is the same lamp and wax, but with a different liquid solution:
Still a good column shaped flow, and now no bubbles in the wax ! The properties of the liquid used here, which is with the same lamp and wax, is also 1,017 g/cm3 with 1ML of SLES Surfactant (more on this later), but the difference here is that instead of using Epsom Salt to increase the liquids density, I've used Fog fluid. So as you can see, even though the basic properties of the two liquids are the same, there's a huge difference in the outcome.
What about the other stuff in the liquid
The only other thing i have ever used, other than a solution to raise my density, is a Surfactant. What it does is, it reduces the surface tension of your liquid and helps the wax to "flow" through the liquid. It will sorta make the liquid slippery for the wax. You have serveral options here too:
I'm using SLES which i got from eBay a long time ago which is the pure choice. I haven't needed to get anything since, because i ordered 1,3 gallons before i realized how little you actually need of this stuff. For the larger Grande lamps you will need about 40 - 60 ML pr solution and for the smaller ones you will need around 2 - 8 ML. So buying 1,3 gallons was probably .. too much.
The actual mixing
But lets get back to the task.
For the first time of creating a liquid, i would recommend you use Epsom Salt. It is important that you DO NOT use regular household salt or other normal salt. It has to be Epsom Salt. Why? Because this is the purest you can get. If you use other types of salt, you will properly end up with a liquid that is foggy and not clear, which would be a shame. Also i recommend you to read the Other Observations part before you begin. And if you happen to have a liquid in the lamp, save it in another container, just so that you can always go back to that if you mess up. I expect that you have already gotten the cap of the lamp. Since Mathmos is a screw cap type, its no problem (you might need some force however), but for Lava Lite check out Dr. Whats GooKit tutorial as he explains there how to take off a cap of a Grande.
The above is how i do it. And I've had some good results.
Here are some other notable things I've encountered when redoing liquids:
To much surfactant seems to draw out color from the wax into the liquid!
The below picture is of two globes holding the same wax (it wax taken from a Grande). The liquids in them are also of the same properties, BUT the one on the left has 8 - 10 ML of surfactant (SLES) where as the one on the right only has around 1 - 2 ML. Notice how the one on the left has a blueish color in the liquid? This was not dyed in, and this tint of blue happened with only a few runs of the lamp with that high amount of surfactant in it where the one on the right was running the same amount of time. The only logical explanation I can come to think of, is that if you add too much Surfactant, it can draw out the color from the wax and mix it with the liquid.
Glycerin seems to prevent column flow in smaller lamps (Viscousity?)
More on this later...
Theres ALWAYS a hair or other 'not-supposed-to-be-in-my-lamp'-thing in my lamp!
When redoing liquids becomes a common thing, you will most certainly notice now and then that some hair or other thing which is not supposed to be in the lamp is in there. I'm struggling with this my self, and i really don't have a solution for it other than this.
The hardest part for me has been to simply accept that its impossible to avoid this completely, unless you are able to keep your work space super clean.
What if the wax has a density LOWER than water? or
The wax rises all the way to the top, even with pure water as my liquid!
More on this later...