Besides the two essential additives we've already talked about, stearic acid and Vybar, there are many other products that can potentially be added to a paraffin wax base to achieve the results you want.
Here's a non-exhaustive list of some of them and the way you can use them in your candlemaking projects.
We have all looked into a shop window one day and noticed an advertising poster that probably featured beautiful and vivid colors when it was first put up, but is now reduced to some faint shades of blue after several months of exposition to daylight and sunlight...
The exact same thing could happen to your candles after some time: ultraviolet emissions carried by daylight start altering their colors (and never in a good way); even uncolored candles are affected: with time, a white candle will turn yellowish for the same reasons.
To give your candles the best chances to hold on to the colors you gave them for as long as possible, always add a tiny percentage (about 0,1% is enough) of UV-inhibitor to your wax blend. It protects against both UVB and UVA.
The universal additive is a proprietary (read ultra-secret) blend of different additives and generally has the following properties:
You probably already read the articles about stearic acid and Vybar, so you might wonder what the difference is since the properties of the universal additive are a combination of the properties of the two additives mentioned earlier... Easy: there are no differences, besides the fact you don't have to do extensive testing to come up with the perfect mixture, the one that will produce the best results.
Furthermore, depending on what spot on the planet you call home, universal additive can be extremely hard to come by. If you're in that case, don't panic, it is far from essential and can easily be replaced by more common additives.
Mineral oil is used as an additive to give candles a mottled (snowflake) appearance.
This effect is frequently occuring by accident when the amount of fragrance oil in the paraffin wax is too high or if you didn't add (enough) Vybar to your wax blend, but it can also be intentionally achieved by deliberatly raising the percentage of oil in the wax (fragrance oils are a bit too costly for that, so we substitute mineral oil) and by extending to a maximum the cooling process of the candle.
For more information about this relatively simple, easy to reproduce technique, read the article titled "3 surface techniques".
Back in the time when I was just starting to make candles, my biggest frustration, one widely shared with all my european candlemaking friends, was the complete impossibility to obtain a container candle wax blend, a soft wax with a low melting point that would not pull off from the glass container walls or create wet spots when cooling off. That frustration was made worse when I sat up to read the american candlemaking discussion boards where pretty much everyone was bragging about the container blends commonly available in the United States.
Fortunately, that was twenty years ago and things have changed. And even though the selection of wax blends available to us europeans is still substandard compared to what can be purchased in the US, it is now possible to get a decent container wax blend pretty much wherever you are.
But just in case you're not that lucky, or in case you're a die-hard who loves a challenge and doesn't mind the hard work, I've preserved on this new version of the site the "homemade" recipes I devised, improved and used a lot back in the time. And the vegetable shortening was an important ingredient in many of those.
Because of its chemical structure, paraffin wax expands when heated and contracts when it cools off.
As far as molded candles (pillar candles, votives) are concerned, that's a good thing as it makes unmolding the finished candle much easier. But in container candles, where the liquid wax is poured in glass containers, the contraction stage can have nefast consequences: wet spots can appear or the or the wax can completely pull off the sides of the container, turning your candle into a rattle. The solution to that is a wax blend that reacts as little as possible to temperature changes. By lowering the melting point of the wax, the contraction / expansion process is reduced.
Vegetable oils, also because of their chemical structure, do not expand and contract in function of their temperature. So we can conclude that by mixing equal parts of paraffin wax and vegetable oil, we should end up with a blend that is less sensitive to temperature changes than straight paraffin wax.
Being a liquid at room temperature, vegetable oil is not a good choice for making candles. On the other hand, its solid form, vegetable shortening, is perfect for the job.
The exact proportions to use depend on many things: grade of paraffin wax, type of vegetable shortening, additives used...
You will have to test and test again to find the formulation that works best for you.
As a starting point, get some inspiration from the recipes you'll find in the article dedicated to Container wax formulas.