A friend of mine has a premium lipstick they use for serious socialising, and a cheaper workaday one that is good enough for when they are seeing less important people like me. She hit on a whizzo wheeze of melting the small amount left in the bottom of the pricey one and pouring it onto the other one to get the full value out of it.
The results were disappointing. The hybrid stick was too soft to be usable and had to be thrown away. Well it was worth a try. The problem was probably due to the way it cooled down. Lipsticks are not at first sight particularly complicated things from a chemist’s point of view. They are simply a blend of waxes containing some pigments. What could go wrong?
As it turns out quite a bit. Wax isn’t quite as simple as it as first appears. Molecules have shapes and those shapes vary a fair bit at different temperatures and this affects how they interact with each other. The very act of melting a wax is a good example. Below the melting point the wax molecules are not moving very much relative to each other. From our perspective they form a solid – but at the molecular level it is simply that they are stuck. Heat them up and they start moving and the whole lump of wax becomes mobile.
There is no chance at all that all the molecules will return to the exact same positions when you cool them down again. If they arrange themselves in a different way they might well end up having different physical properties. The speed at which they are cooled down has a lot to do with it and the rate of cooling needed to get the desired result is often key. Lipstick manufacturers and cheapskate lipstick owners have to take this into account. Cosmetic factories are often equipped with what are called heat tunnels which are long ovens which allow the speed at which the product is brought down to room temperature after blending can be carefully controlled.
This problem is incidentally not remotely restricted to the lipstick industry. The makers of chocolates, candles and even drillers for oil all have problems with getting the right crystal structure.
This is difficult to reproduce at home, but you might be lucky. It all depends on the way the ingredients mesh together. You could have luck and hit on just the right conditions. But frustrating as it is, a lot of the time the lipstick left in the bottom is simply unobtainable.