Pure ingredients

Until recently I never even thought about DIY skincare, because I read some articles about why you should´t do your own l-ascorbic acid serum, because it oxidizes real quick and that it would´t word as well and so on. But then I stumbled across some websites where you can buy the pure ingriedents like antioxidants, humectants and  it one recent discussion here in the forum it was said you could theoretically mix pure licorice extract with your moisturizer. Now I thought, could you mix vitamin e “ 95% active blend of natural mixed tocopherols”  with your moisturizer or even apply it pure? When I think of that usually there isn´t more than 2% in a product and how cheap it is to buy pure ingriedients it all sounds very attractive. It also made me think about that whole l-ascorbic acid serum thing again, because would´t it acually be better to do our own like once a week instead of buying one and having it exposed with air every time you open it for month?