In the previous post I talked about the step-by-step process of sizing paper; we will examine the use of alum as an essential component in this post, as well as its application during the painting process.
Image above: silver metallic paper sized with alum and glue for painting fine lines.
If you know about fabric dyeing, then the use of alum as a mordant would be quite straightforward. If you are wondering about what a mordant is – basically, a substance that binds the dye by forming a coordination complex and attaches to the cellulose substrate. In this case, Alum, precipitated as aluminum hydroxide, acts as a bonding agent by attracting the hydroxyl groups of the cellulose and glue into the hydration shell of the complex aluminum ion.
If glue alone is used as size, it is very easily removed from paper by wash. So it is conceivable that in traditional Chinese gonbi paintings (or more generally art of East Asian cultures influenced by China), in which multiple washes are involved in the painting process, alum has become an established component of glue size.
In The Whatmans and Wove Paper, the author John Baston mentions that other than the mordant-like action in bonding the glue to the paper, the use alum also helps in increasing the jelly strength of the glue film; and it possessed some properties in retarding, if not preventing, a degree of mold growth. Furthermore, if alum has not been added in excess, there should not be any residual acidity in the paper to attack the cellulose, so the benefit of using alum should outweigh the potential danger of harming the paper.
Other than the initial preparation of paper or fabric for painting, there are a few more times when you might want to apply size. When colors are applied in thin layers and multiple washes are involved during the painting process, alum and glue size is used to “set” the colors, so the layers of pigments would not be lifted easily in subsequent washes. This is also done with small amount of alum, however, so that the paper won’t become too brittle.
In addition, some artists may choose to paint on waterleaf (i.e. unsized) paper to create certain effects, then once the paint is dry, size can be applied on the paper for painting detailed work on top.
When metallic leafs are used, alum and glue size can be applied to make the surface suitable for painting on top; the pigments would grab onto the surface rather than slipping off the metal. Size also forms a protective barrier that prevents tarnishing for silver and copper leafs, or gold leafs which is not pure 24ct gold.
These are some applications of size that I encountered in my own practice, added to it are compiled information from research paper and other artists. I hope this article gives you a practical overview on paper sizing. Please feel free to drop any comment or questions.