Have you ever bought yourself a face scrub and wondered how long those lovely scrubby bits and pieces will stay suspended in their gel or cream home?   Maybe you are a wannabe or actual scrub maker wanting to know how long your particular brand of loveliness will remain nice and tidy on the shelf?  Maybe you just don’t give a damn but have nothing better to do for the next 5 minutes of your life.


Well you guys are in luck as I’m going to let you into my secret world of stability testing starting with method validation.

Facial scrubs can come in all different shapes and sizes but are most commonly either in a cream or a gel base.  Cream scrubs start off life as a normalish moisturiser to which something bubbly and then something scrubby are added.

Gels start off life-like a typical hair gel and also get pimped with a little something bubbly then scrubby.

Both can be just as rough or mild with creams winning out for ultra-dry skin due to their inbuilt oil phase.

As many a cosmetic chemist will tell you the problem with facial scrubs is keeping the scrubby bits suspended – fighting gravity.  This can be achieved in a number of ways including matching the density of the scrub particle with the base (pretend that the two are weight lifters, they need to both be the same grade),  making the continuous phase (usually water) thicker and harder to ‘move through’ (try to visualise your scrub particles running through water, then custard, then thick toffee….).  Finally and most importantly are suspending agents which act like spiders webs by structuring the continuous phase (water) like a net, preventing your scrubby stuff from falling straight through it.

Get these things right and you can have the most amazing scrub that stays shelf pretty for over 3 years, get it wrong and you can have an epic fail in less than 24 hours.  Of course,  the general public never get to see the epic fails as they show themselves right away leaving the chemist to cry alone but that gap of time between a few hours and three years is where all of the drama lies and that’s why we speed things up in our stability lab.

The machinery of choice when evaluating scrubs is the centrifuge.  This bit of kit spins the product around super quick exposing it to G Forces (Gravity) far in excess of what you would normally experience here on earth.  These excessive forces strain and age the product in minutes rather than days, weeks or months giving me (the tester) the ability to give the product a pass or fail with minimal down time.


However, there is one complication and that’s method validation.

Before chucking your goods into the centrifuge you have to know if the test is fair and the only way to gauge that is to run a validity test. This ensures that your failures as well as your passes are interpreted accurately.

The cosmetics industry does give guidelines on how to carry out stability testing of this kind but as there is no absolute method for facial scrubs the detail is down to the individual tester so what I do is keep an eye on how other products on the market perform.

Given that I can’t access the private files of other cosmetic brands I have to make certain assumptions about the other products that I choose to test.  One key assumption is that the ‘best before’ data displayed on the pack is accurate and has also been validated.  With that in mind I hunt out between 3-5 products that are already on the shelf, that display the appropriate best before or stability markings and that are of a similar type to the product that I’m interested in.  I then run them through several cycles in my centrifuge starting with low speeds and working up until I either get widespread failures or I reach 20 minutes at 3000 RPM intact – the industry recommended guide level.   If I am in doubt of the results I run a second round of tests and another if needed.  Once that has all been done it’s time to run the test product through its paces.

The test product is put into three vials and spun at the validated level for the required time.  If it passes the testing is finished, if it fails I reduce the speed of the spin, decant new samples and try again.  At a reduced spin level we can no longer give the product a pass but we can use this to work out how far away from passing we are.  And so it goes on.

If a product fails then it may end up back on the bench as we  try one of any number of tweaks to fix the problem which may or may not be fixable – it’s tricky, especially when you want to keep the product looking and feeling like it did before.  I do this kind of project as part of my bread and butter lab session work for a brand, including the products that I develop (as with all the best will in the world it can be difficult to produce robust, amazing products exactly to a customers specifications first time – especially when under time or budget constraints) and I must say it is my favourite part of brand development.  The stuff that you learn from carrying out this kind of investigative work always thrills me and while I can’t always give people a perfectly happy ending, I never get tired of trying.

There are other stability tests that we carry out to ensure that your product remains both functional and good-looking but I’ll save that bore fest for another day. In the meantime thank your lucky stars for the centrifuge and other gravity defying laboratory paraphernalia.  Horaaaay for stable products!