L-ascorbic acid is the most studied brightening active there is and the hardest to keep alive in a bottle. It needs a pH below about 3.5 to penetrate, it oxidises on contact with light, air and metal ions, and it degrades faster the more of it you dissolve. Which is why most vitamin C serums on sale are either a stable derivative with weaker evidence, or L-ascorbic acid in a formula that will be inert before you finish it.
Aster & Fold have got the first half of the problem right and the second half wrong.
The formula
Twelve per cent L-ascorbic acid, 0.5 per cent ferulic acid, 1 per cent tocopherol, in a propylene glycol and water base at a stated pH of 3.2.
Those proportions are not arbitrary. The combination of ascorbic acid with ferulic acid and vitamin E is the one with published stability and photoprotection data behind it, and the ratios here are close to the ones in that work. Most products name ferulic acid and include a fraction of what would be needed to do anything.
Stating the pH on the carton is worth noting too. It is the single most useful number for this ingredient and virtually nobody prints it.
Eight weeks, two faces
On normal skin: a clear, photographable evening of post-inflammatory marks from week six, and a general brightness by week eight that was obvious side by side and would have been easy to talk yourself out of without the photographs.
On reactive skin: a fortnight of stinging that eased, then tolerance, then a similar result about two weeks behind. Low pH plus 12 per cent is a lot. Anyone who has struggled with acids should start at a lower percentage in this ingredient rather than assuming the sting is the point.
Neither tester saw anything that looked like the collagen claims on the box, which is what we would expect at eight weeks. Those come from longer studies.
The bottle
Clear glass. A dropper. An ingredient that degrades in light and air, in a container that admits both, opened once a day for two months.
Our first bottle was faintly straw-coloured at four weeks and unambiguously amber at six. Amber means oxidised: dehydroascorbic acid, which does nothing useful and can be mildly irritating. The second bottle, kept in a bathroom cabinet rather than on a shelf, made it to eight weeks still pale — which tells you the storage advice matters and that most people will not follow it.
An opaque airless pump would have cost pennies and made this a five-star product. Instead the formulation team’s work is being undone by the packaging team’s, and the consumer pays for both.
Would we buy it again
Yes, and we would decant it. Which is a ridiculous thing to have to say about a serum at this price, and the honest recommendation all the same.
