What does percent solids tell you?

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?"

% solids

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.