Tech Talk: Pigments

Watercolor test strip

I'm going to do a number of posts on the technical aspects of painting with watercolors. To start things off, I want to talk about paints. I was inspired to begin this series after receiving a couple of sample sets of a new line of watercolors, Mission Gold, a few months ago. I'll talk more later about the color test strip shown above, but first let's talk about how you can check the pigments used in your paints.

The first thing I do with an unfamiliar tube of watercolor is to look on each tube for the pigment information.  These days, all reputable brands of watercolor list the pigment number or numbers on each tube.  I've circled the numbers to make them easy to find on this assortment of watercolors from various manufacturers.

Pigment numbers (circled)

Pigment numbers are in the form PY## for yellows, PO## for oranges, PR## for reds, etc.  Once you have the pigment number (or numbers in the case of a mixed color), you can check several sources to learn more about them.  For years, I have relied on The Wilcox Guide to the Best Watercolor Paints and Hilary Page's Guide to Watercolor Paints.

You can check the raw pigments as well as the ratings for individual manufacturer's paints.  Hilary Page has tested all the paints herself and gives personal ratings, pointing out her favorites in each color, but Wilcox gives the most comprehensive information on the raw pigments -- pointing out pigments that may be suitable for oil or acrylic paints, but not for watercolors. 

There is also an online resource by Bruce MacEvoy.  He has created an exhaustive catalog of information related to watercolor, with a very thorough section on pigment information. By clicking on the color links at the top of that page, you can see every pigment by number along with detailed information about each one. There is an overwhelming amount of material, but for the purposes of this post, the critical thing is the lightfastness rating (Lf), the column at the rightmost edge of the rating tables. Lightfastness is rated from 1-8, worst to best.

 Now back to my color tests.  After looking up the pigment numbers on the paints in my sample sets, I was very concerned because some of them were ranked as fugitive or unreliable by both Wilcox and Page.  However, since this is a new line, neither Wilcox nor Page has tested the actual paints, so I decided to do my own lightfastness tests.  I had 14 unique colors in the sample sets, so I took an 11x14-inch piece of watercolor paper and divided it into 14 equal spaces.  I printed out the color name and pigment numbers in the center of each space and then painted a full-strength stripe across the middle of the paper, with identical lighter stripes on each side of the center. You'll notice I blotted out the paint at the center of the darker colors in order to see the names and pigment information. (I also lifted a stripe down the right side of each color, but that is unrelated to my test for lightfastness. I merely wanted to see how staining each color was.)

I cut the paper down the middle, labelled the backs, and taped one to a piece of acid-free matboard that I set where it will get as much full sun as possible. The other half is sandwiched between two pieces of acid-free matboard and tucked into a flat file drawer.  At present, the daily dose of sun is not great -- a few hours at best -- but my plan is to compare the two halves of the test sheet every two weeks for at least 6 months.

Stay tuned.