The Odd Couple: Watercolor and Gouache

In true Odd Couple form, watercolor and gouache can live together, work together, but they behave very differently.

When I posted on making paint, I made up a bit of both watercolor (L) and gouache (R).  They are both the same recipe (water, gum arabic, glycerine, honey,and  a bit of thymol), except gouache has a bit of Calcium carbonate.  This addition is what makes gouache opaque , so you can cover and correct things.

This is watercolor medium and pigment:

This is gouache medium and pigment:

At first blush, why wouldn't you want to always have opacity?  Why would you ever use watercolor if you could use gouache?  There must be some horrible, horrible catch...

AND THERE IS...sort of.

The catch for this chalky addition to your paint is the dulling it causes.  All gouache will dry duller (more chalky) than the brilliant, wet paint.  This means that light colors will darken and dark colors will lighten... in a dirty looking sort of way.  You can imagine the nightmare of trying to remix/color match a wet paint to a dried color.  In the picture above, this is the mostly scraped glass that I made my paints on.  The watercolor was made on the left and the gouache is on the right.  You can already see that the watercolor is darker and richer.

But this paint is Prussian Blue.  It is a bit transparent, as paints go, but not really.  If you look at a paint like Alizarin Crimson, which is really transparent as a watercolor, it is very different as a gouache.

Starting from the top, Alizarin Crimson watercolor, then its gouache, then Prussian Blue watercolor, and then its gouache.

Close up!

But don't hate on gouache just yet.  It has so much more body than watercolor and gives you a much more substantial painting.  I will often put down a base painting in gouache and then 'glaze' with watercolor on top of it.  Also, if you spray your gouache with a varnish (see footnotes), the colors will go back to their wet appearance.

On an interesting side note, Calcium carbonate is an extender/filler that is put in cheap paints by some manufacturers.  This is generally why expensive paint is more brilliant and the transparent pigments more 'clear' than their cheap counterparts.

So check out the ingredients on your paints before you buy them.

There are many different types of varnishes, but I recommend these from Golden:
http://www.dickblick.com/products/golden-archival-spray-varnish/?clickTracking=true
There is a matte, satin, and a gloss.  I have all of them but I generally use the gloss because I am really shallow and like shiny things.