This question came in via the cosmetic science discussion forum and it was something that we hadn’t covered before. The question specifically was “what is % solids and what does it tell you?”
The notion of % solids is something that you probably first learned about in your high school or college chemistry courses. Simply put, it is the amount of solid material present in a liquid (or semi-liquid) sample. For cosmetic formulators it can be useful for things such as formula duplication and quality control.
Determining % Solids
Figuring out % solids of a material is easy enough.
Step 1 – Get a sample the raw material or finished product and figure out its starting mass. It’s helpful to weigh out a sample of 100 g or so.
Step 2 – Put the sample in an oven set at slightly above 100C. The idea here is to drive off as much water as you can via evaporation.
Step 3 – Record the sample mass at set time intervals. When there isn’t a significant change in the mass, you’ve approached the value of the % solids of the sample.
This is just a rough way of figuring out % solids and there are fancy machines you can use, but this gets the job done, and in the cosmetic industry, that’s usually good enough.
Formulating with % solids
The way that you use % solids to formulate is by getting a competitor’s product. Take a look at the ingredient list and figure out where most likely is the 1% line. Next, use the % solids to determine how much water is in the formulation. Then, make guesses as to the concentration of the ingredients above the 1% line. Make your formulation and compare it to the original control.
This is an iterative process but the more you do it, the better you will become at it.
% solids for quality control
The primary use of % solids in the cosmetic industry is as a quality control measure. When you get a raw material you should check the % solids on the Certificate of Analysis (COA) and determine whether the sample meets the % solid specification. If it doesn’t, you may have a quality problem. You can do the same with your finished products.
The % solid is a simple test and can be done rather quickly. Every cosmetic chemist should use it both for formulation efforts and quality control.