In general, salt should be avoided because it can oxidize the coil and it also reacts with perc (try dropping a grain of undissolved salt on warm lava and see what happens....). When dissolved, it's less aggressive but yeah, it's bound to cause problems in the long run.
It has the advantage that it's easy to adjust the density directly in the globe if you're going by trial and error, since it doesn't add any volume to the water, and you don't need to use any "extra-globe" test rigs. I literally used just a teaspoon and some patience to adjust the density on my lamps (I only bothered testing samples of wax and fluid with the very first one, and I still had to add salt manually to the filled globe, so I opted to skip the procedure and mix directly in the globe).
On the converse, adding a liquid with known density such as a precise mixture of water/glycol would be safer, but you need to think ahead to account for volume changes.
E.g. if you start with pure water and in the end you realize that you need to add 1/3 pure glycol to obtain the proper mixture for your goo, you must somehow have started with a half-full globe in order not to overfill your globe. And if you discover that it's under-filled and you need to add more liquid with the same density as the one inside the globe...it's a headache ;-)
It really depends on the method used: I did all adjustments for my lamps by trial and error without any specialized lab equipment, and with no "test dipping" of goo into sample fluids. A bit ghetto, but it worked.
Although, if I always had the proper materials and equipment to make salt-free fluids with known densities without "by eye" trial and error, I would prefer it.
Yeah, the original formula and most commercial lava lamps contain propylene or ethylene glycol.
The problem is that you don't know how much you'll need to add beforehand, and that compared to salt, it adds a LOT of volume unless you can find it pure, which is not always the case.
Prop glycol (safe) or Ethylene Glycol (which is POISONOUS) can be commonly found in antifreeze products, but they are already mixed with water.
Also, prop glycol has a lower pure density of 1.03 compared to eth. glycol (1.11) so e.g. 10% of pure prop glycol to distilled water, won't change it very quickly. Even using pure prop glycol would only give 1.03 density, which wouldn't have been enough for certain of my goos (there's also the uncertainty of the actual goo's density). Ethylene glycol can give a max of 1.11 is used pure In order to accomodate for such slow density changes when adding glycol, you'd have to start with a near-empty globe, and once you figure out the precise ratio of glycol to water, you'd have to replicate it OUTSIDE the globe as well, if you wish to add more fluid.
With solid salts, you can start with the correct amount of fluid and just add them as you go, without worrying about e.g. filling the globe way before having reached the correct density.
As I said, the proper procedure would be to determine the exact density of YOUR ooze beforehand by measuring samples of it, and then engineer the fluid specifically for your needs, all outside the globe. But it's longer, tedious, and not really worth it unless you make A LOT of lava lamps and/or you follow exactly the same formula for each of them.
To make a comparison, a fully saturated saltwater solution can reach density 1.20, which is quite a leeway to play with.
Yeah I do. Basically it's a tradeoff between durability (still to be seen) and ease/range of adjustement.
Salt allows for easier adjustments at almost constant volume and more density ranges to play with, at the expense of (potential) long-term problems.
Using other means of adjusting density means that your perc will also have to be within certain density restraints (e.g. too dense perc can't be handled by non-corrosive, salt-free solutions) and that getting a precise amount of fluid of the type you want will be harder.
All lamps I made so far use salt (and one even has iodized salt, with no ill effects so far).
As always, there's a big "but" to everything: it's possible to create an entirely salt-free and additive-free lamp as I did today: my little "secret" was in the way I mixed the wax with the perc, allowing me to add more perc to the wax as needed instead of adding additives to the water.
As promised here's a pic of my latest creation: Poison Ivy, a fluo green wax + purple liquid beauty :-)
Despite the name, it uses very high quality materials and most importantly - no salt.
The fluid is just distilled water coloured with some blue pen ink, and I adjusted the perc in this way: I initially mixed just enough perc so the wax wouldn't just go up in flakes, let it float up in one bubble, and then added perc little by little at the top of the bottle (directly ONTO the wax) until I achieved the desired density. There, no salt, so it should be in theory corrosion free. No antifreeze either - except for a tiny amount I used more for color correction.
Hi Marcel. Just wanted to chime in here and let you know something in case you don't. Animals adore antifreeze. It kills them in pretty short order. Be careful not to break that lamp if you have pets. They will be attracted to the liquid spill like flies to .....well you know what.