Tuesday, August 18, 2009

Mineral make-up: Chemistry of pigments

I thought I'd take a look at the chemistry of the pigments we use in mineral make-up with more detail on how to use the various pigments tomorrow.

Inorganic pigments are formed from compounds of transition elements like iron, chromium, and so on. The colour is produced "as a result of the ease with which the outer "d" electrons can absorb visible light and be promoted to the next energy level" (in other words, the colour is determined by the outer electrons).

If you're interested in how colour works, check out this entry in Wikipedia (scroll down a bit).

They tend to be more opaque, more light fast, and more solvent resistant than the organic pigments, but they are more subdued than colours you'd find from ingredients like carmine (although micas definitely are shiny things!). They may be affected by alkali or acid - which explains why iron oxides or ultramarines do not play well with bath bombs - but tend to be less chemically reactive than the organic pigments.

The main inorganic pigments we use in mineral make-up are...
  • iron oxides
  • chromium oxide or hydroxides
  • ultramarines
  • manganese violet
  • titanium dioxide (I've already written about this one)
  • micas
There are generally 3 iron oxide colours - red, black, and yellow - which are combined to produce the other colours. They offer good stability and opacity in a mineral make up applications, but are discoloured by very low pH. When they are found naturally, they are called ochres (so they have been used for thousands of years in art and cosmetics!) They are non-bleeding, and moisture resistant.

Yellow iron oxide: This is iron (II) oxide-hydroxide or FeOOH.

Red iron oxide: This is iron (II) oxide or ferric oxide - Fe2O3. The rust colour comes the fact that it is oxidized iron. It can be obtained by the heating of yellow iron oxide.

Black iron oxide: Sometimes called magnetite, iron (II, III) oxide or Fe3O4, it comes from the controlled oxidation of ferrous sulphate in alkaline conditions.

Chromium oxide (Cr2O3) and chromium hydroxide (Cr2O3 x H20) are both shaded green and provide good colour stability in mineral make-up products. The chromium oxide offers better tinting than the chromium hydroxide. Chromium oxide - formerly known as viridian - is more of a grass green (left); chromium hydroxide is more teal or aqua (right).

Ultramarines can be natural or synthetic. The naturally found ultramarine has been used for centuries in painting. It is usually a blue colour because it contains lazurite (found in lapis lazuli gems), and it is the most compound used in mineral make-up (from a chemical point of view). The blue colour comes from the S3- anion on the molecule.

The synthetic form - the one we use - is more vivid that natural ultramarine blue, and less permanent. It is moisture resistsasnt and has great light stability. It is very unstable in acid, and will release hydrogen sulphide in an acid-base reaction (this explains the smell of rotten eggs if you use them in bath bombs). You can find ultramarine blue, purple, and pink. They tend to be more of a pastel colour than the iron oxides, although they can be very vivid.

This is another complicated molecule with good light stability for mineral make-up products. It is unstable in water, so it is not suitable for bath bombs or other products you might want to colour and take into the shower or bath. It makes a great base for purple and burgundy shades (when mixed with titanium dioxide).

Micas are defined as a hydrous silicate in a monoclinic crystalline form. But this really doesn't do them justice. Micas come to use as powders with various levels of shine and glitter. To me, they are the highlight of the mineral make-up, offering the colours we need to blend to get even lovelier colours. The size of the mica particle determines how shiny the mica will be; the larger the particle, the shinier!

Mica on its own isn't very exciting, so the mica is coated with iron oxide or other inorganic pigment, titanium dioxide or bismuth oxychloride to increase the shine or colour. If you find interference micas, they are created by layering various iron oxides and other pigments in relative thick layers so show off every colour.

Now you know the chemistry of colours! Join me tomorrow to learn a little about colour blending.


Mich said...

Hey Susan,
Great info!

It's probably smart to point out from the start that the ultramarines and the chromiums are usually considered NOT "lip-safe". SO DON'T EAT THEM. Or put them in lipstick.

Thanks for your great blog!

Susan Barclay-Nichols said...

Great reminder, Mich! Thanks!

Kelly Tanner said...

Hey Susan somewhere on the blog I saw a link that led to a page full of color recipes using ultramarine and oxides. I thought I book marked it but I can't find it anywhere. There were tons of colors on this site/page. If you know what I'm referring to please help! Thank you. I love your blog I can look for hours!

Susan Barclay-Nichols said...

Hi Kelly. Check out the links in the mineral make-up section of the blog as I'm not sure to what you refer. I have a bunch of green ones, but there are others, too.

Clark Danger said...

Hey Susan! Looking for a light matte cobalt blue mineral pigment. I've had that rotten egg smell problem with the ultramarine. Any suggestions? Thanks!