I have been experimenting with some new materials lately, one of them is casein paints. They are a milk protein based paint and can be used like watercolors. There are several differences between casein and watercolors, most importantly they have a lot more body and weight and are best used on a hard surface like clayboard . To me they feel like a heavier, smoother version of gouache. I love the way they feel on the smooth clayboard and there is some room to actually build up some pigment, which is one of the things I miss about oil painting. The drawbacks are they are actually even more fragile than watercolors, the colors lift when it comes in contact with something wet and if you have butterfingers like me and frequently drop pieces parts of the paintinsg chip off. They also claim to be non toxic, which I personally don't believe as when I use them I am constantly wheezing and a red rash appears on my arms.
Here are a few of my sketches
These ones are very small, 6" x 6". I haven't worked in that scale in very long time.