What is gouache?
After a long search, I've discovered many opinions and no firm answers.
To start with, I'm not talking about acrylic gouache, that's not gouache. It's an acrylic paint designed to be not shiny and a few other characteristics to mimic gouache. It's out of the equation here.
Gouache is water-soluble, both before and after drying. It's highly pigmented, and dries matt (not shiny). It can be used thick, like oils, or watered down to mimic watercolours. It's more than a close relative to watercolours, but more on that in a moment.
That's about as much as people can agree on.
Gouache has been used by artists in Europe for a few thousand years, and probably longer elsewhere. But it wasn't always called Gouache. Actually, that's a very recet name, and for most of history, gouache was lumped in watercolours. It uses the same pigments and binder (thing that makes paint paint). Sometimes the pigment might be ground differently or binder ratio adjusted to get the desired results, but for the most part, we see it used as watercolours. In the late 1700s, early 1800s, in English, we start to see "body colour" and "opaque watercolours" to describe colours or methods that are less transparent than others. Often colours that include white or chalk.
Note, watercolour binders change over time in Europe.
In Constables stone hedge (when viewed in person, not so much with online copies) and his other watercolours, we can see his use of opaque, white, or body colour to help some objects stand out or fall to the back (general rule, transparent come forward)
Turner was another watercolourist who used this to great effect, but to be frank, most mid 1800 watercolour painters understood how body colour and white worked in watercolours.
Late 1800s, we start to see colour printing and photography. Posters, books, packaging, etc. With this, we see a need for illustrators to find an easy to work with, opaque, fast drying paint they can design the image with. This will become what we know as gouache, but to start with, we call it poster paint and variation thereupon. (Note, at the start, other paints were also used, but eventually gouache seems to become the industry standard).
Enter the 20th century.
We have two main trends going on.
1. Watercolour snobery.
We see a growing trend in England and North America to toss out historical watercolour traditions and give it a narrow definition. Old binders are out, and only gum Arabic are permitted. By the 1940s, most opaque pigments are shunned. By the mid century, white is added to the shunned list and the modern view of watercolours we have today is established and enforced.
2. Illustrations become more popular.
Books, magazines, everything for sale, etc, all of this needed art. The way to get that art is to paint and photograph it. Gouache fills this niche perfectly (and we start using the french word, gouache, for it as watercolours has banished it from their ranks). The pigments don't even need to be lightfast as the art isn't what's being consumed.
As people expierence the art second hand, perhaps as the wrapper on their handsoap, or magazine cover, gouache becomes the trademans paint. No longer shown in museums or galleries, it is degraded even as it comes into its own.
At least that is what I can cobble together of the history so far.
The problem is, we haven't solved "what is gouache"?
It seems the artists of the world are conflicted on what makes gouach gouache. Some suggest it's watercolours that don't fit our current idea of watercolours. So like adding chalk to the watercolour recipe, making it opaque, and therefore, now gouache. Examples
https://stonegroundpaint.com/pages/gouache
https://www.beampaints.com/collections/paintstones/products/gouache-6-set
Others say it is all in the pigment, grind or quantity. Or binder recipe, or...
Others have yet more options. It is the age of the internet, afterall. I'm curious to learn more about this.
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