Gouache paint is similar to watercolor, but is modified to make it opaque. Just as in watercolor, the binding agent has traditionally been gum arabic but since the late nineteenth century cheaper varieties use yellow dextrin. When the paint is sold as a paste, e.g. in tubes, the dextrin has usually been mixed with an equal volume of water. To improve the adhesive and hygroscopic qualities of the paint, as well as the flexibility of the rather brittle paint layer after drying, propylene glycol is often added. Gouache differs from watercolor in that the particles are typically larger, the ratio of pigment to binder is much higher, and an additional white filler such as chalk - a "body" - may be part of the paint. This makes gouache heavier and more opaque than watercolor, and endows it with greater reflective qualities.