Rosin is a resin derived mainly from pine. It is a bit like one of those odd job men you run into who can do all sorts of odd things. It is used to coat violin bows so they run smoothly over the strings. It is used as a base for some kinds of glue. It also gets used for some nail paints and in some topical pharmaceuticals where it is known as colophony. It is even used as a food additive under the number E915.
The most surprising use in some ways is the one which calls for the highest rosin level is in depilatory waxes. Those are the ones that are melted just prior to use, then painted onto the skin and allowed to set under a strip. The strip is then rapidly removed taking the hair with it. It is a bit ouchy but very very effective. It is the most troublesome and least comfortable hair removal method, but the effort is rewarded by the good results.
This is another one of those cases where a naturally occurring polymer outperforms anything the chemist can come up with. To be fair, science has only been at it for a few centuries. Nature has had millions of years to experiment and come up with materials with unique properties.
Unfortunately it suffers from a problem that is not uncommon with natural products, which is that it is rather prone to causing allergic reactions. The words ‘natural’ and ‘harmless’ are often used as synonyms but it s far from the case. Rosin is also used in pharmaceutical applications so there is a medical grade. Again, ‘medical grade’ is often put forward as a badge of safety. And again the idea that something that is medical is automatically benign is contradicted by the example of rosin. There are plenty of examples that can be put forward to bust both these myths but I think rosin is the only one that does both simultaneously.
Although it gives excellent results it is a nightmare to work with so it tends to only get used when there really is no suitable alternative. Its tendency to cause allergies doesn’t help either. So I can foresee the day when it is no longer part of the cosmetic industry’s palette of raw materials. But for now, it is an interesting if difficult ingredient.