(The black represents carbon, the red oxygen and the white hydrogen)
Glycerine is a colourless, odourless, quite viscous liquid that tastes sweet – its name derives from Greek word glykerós (sweet). It is usually referred to as VG – vegetable glycerine, although this name is a bit misleading. Most glycerine available on the market comes from fats and oils. Those two classes of compounds are – chemically speaking – esters of glycerine. In order to obtain pure glycerine, oil has to be hydrolysed.
As we can see from the chemical reaction above, there are two products: glycerine (right) and so-called salts of carboxylic acids, commonly known as soaps (left).
Some glycerine is produced during the production of biodiesel.
Of course this glycerine is a crude product and needs to be purified, usually by distillation.
Finally we obtain the high purity product (>99.5%) which is usually tested for impurities to meet pharmaceutical standards (EU Pharm, USP, BP). Only such glycerine should be used for DIY liquids.
Some glycerine is synthesized, but this product is mainly used by the chemical industry.
For VG the boiling point is 2900C, but we have to remember that heating this compound over 2800C results in thermal decomposition – the glycerine molecule turns into acrolein, the simplest unsaturated aldehyde, having quite low boiling point of 530C. Acrolein is a very reactive compound, toxic and irritating, especially for the eyes and skin. Every vaper experiencing dry puff should be aware of the fact that they are inhaling an amount of acrolein. This should be avoided as it is not good for our health.
Glycerine in e-liquid plays two important roles: first it is, of course, solvent, second it helps making thicker clouds. If a vaper wants to have more visible aerosol, they should increase the percentage of glycerine in their liquid. But we have to remember that glycerine is viscous. Increasing the amount of solvent makes liquid thicker and this may result in much worse transport to the wick. Vaping using liquid made entirely from glycerine would be almost impossible, as this would result in frequent dry puffs. So we usually use a mixture of glycerine and propylene glycol. One piece of practical advice: if your liquid is still too thick and you don't want to dilute it too much with PG, use some drops of 95% alcohol instead. But remember, just a few drops, because even a small amount of alcohol decreases significantly the viscosity of e-liquid.
When you use glycerine for DIY liquids, consider warming it (40-500C) before mixing with other ingredients, as this will decrease its viscosity. Do not heat it directly over the flame, it's better to use a so-called water bath. Of course you don't need to buy those expensive laboratory water baths. Just use a small pot, pour in some water and put on the stove. Remember an important rule: the container with the glycerine in, which you put into the water bath, should be open and therefore it should be secured in such a way that it would not topple. A simple rack would do.
VG itself has a sweet taste, so it also makes liquids taste sweeter. Usually this is good for fruit or candy liquids, but if you don't like sweets use less VG and more PG in the recipe. Liquids with VG usually require more flavouring, on the other hand the more VG the bigger the clouds. Some vapers prefer VG liquids because the throat hit is noticeably diminished as compared with PG liquids.
Happy experimenting, friends, but remember to be cautious!