A size is a substance that acts as a protective glaze on cellulose fibers to reduce the absorption of liquid, so when you paint in water-based medium, the paint would stay on top of the substrate – paper or fabric – instead of getting soaked into it.
The wide acceptance of gelatin or glue* as size stems from its unique ability to deposit a tacky viscous film from a warm water solution, which provides a strong, resilient bond upon drying.
Sizing the paper is the first and foremost essential step in painting in the Eastern tradition, even though there are pre-sized papers available for purchase, knowing the process gives you much more flexibility for making artwork.
*Gelatin or glue is derived from the hydrolysis of collagen – the protein substance contained in the tissues of living organisms. In a technical sense, gelatin is the purified form of collagen, while glue largely contains gelatin, may contain other protein material or impurities associated with collagen. These terms may be used interchangeably.
A standard formula for size follows:
- 5g dry animal glue solid
- 2.5g (1 tsp) alum powder
- 250g water
To prepare the solution
1. Prepare the glue: soak the glue solid in water until it becomes jelly-like, and heat up to 70˚C to dissolve, filter the mixture
2. Stir in alum powder in the warm liquid until fully dissolved
- When the solution is cool, apply on the front side of the paper with a dedicated flat brush in slow, horizontal motion, allowing the size to soak into the paper and displace extra air. Do not leave puddles on the paper.
- After the paper is dry, repeat from the back side. (for heavier paper)
- Once the paper is dry, you can check whether the size is enough by brushing water over it. If the size is not enough, there would be a visible difference in places where water gets soaked into the paper from where it stays on top. Then apply once more on the front side.
It’s recommended to size the paper on a sunny day, when the air is dry. Otherwise, if it takes too long to dry, the paper will harden in a buckled state, making it difficult to mount on board.
Keep in mind the formula provided above is not a one-size-fit-all, but a starting reference for a heavy paper (say 300gsm). I’ve come across many formulas of glue/alum mix; sizing depends on the substrate material, for example, the formula for sizing silk would be more diluted. Meanwhile, atmospheric temperature also affects performance of alum and glue; according to classical text in the Chinese painting tradition, the alum-to-glue proportion should be 2:8 in winter, 3:7 in spring and autumn, and 4:6 in summer. While it may sound complicated, a simpler approach is to mix lighter size and apply multiple layers.
So what happens if you add to much alum?
Alum is an acidic material, so adding too much will make the paper brittle, and may shorten its life. On the other hand, if too much glue is added, the surface becomes slippery, making it difficult to apply paint.
Further reading: Why alum?
Questions about the process? Drop me a line!