Holbein Gum Arabic Medium is an artist-grade medium that enhances gloss and transparency when mixed with watercolors. Mixing Gum Arabic Medium with opaque colors such as gouache and poster colors also increases transparency. - Holbein Gum Arabic - 60 ml bottleHolbein Watercolor Mediums, GUM ARABIC 60ML - This is an artist-grade medium that enhances gloss and transparency when mixed with watercolors. Mixing Gum Arabic Medium with opaque colors such as gouache and poster colors also increases transparency.