Day 6 — Gum Arabic and Watercolour: Polysaccharides, Flow, Rewetting, and Light

Gum Arabic and Watercolour: Polysaccharides, Flow, Rewetting, and Light

Binding Agents Masterclass, Day 6

Gum Arabic: The invisible force binding pigment into luminous watercolour washes. A glassy polysaccharide resin, it mediates pigment flow and transparency. (Source: Tate, 2024)

It is late 18th-century London— The botanist Joseph Banks returns with exotic acacias. By candlelight, chemists extract a sap so versatile it will write, paint, and even sweeten medicines: gum arabic. To this day, nearly every watercolour—Turner’s vaporous sunrises, Cézanne’s crystalline washes, and your own palette—owes its shifting, rewettable luminosity to this unassuming exudate from wild trees.

The Chemistry: Polysaccharides at Work

Major polysaccharide units in gum arabic resin. The blend enables solubility, flexibility, and gloss (Tate Conservation, 2020).

Gum arabic is a glassy, complex blend of polysaccharides—mainly arabinose, galactose, rhamnose, and glucuronic acid (Bai, 2023). These proteins and sugars dissolve in water to form a sticky, stable solution with pH near neutral. Once mixed with pigment, the solution:

• Forms microscopically thin films as water evaporates, trapping pigment particles in a flexible, light-diffusing matrix.
• Allows repeated rewetting and layering, as the dried binder is readily soluble.
• Increases gloss, transparency, and flow—distinct from matte, hydrophobic binders like animal glues or egg.

Gum arabic is thus prized for its "open time," beautiful transparency, and unbeatable reactivation on the page—all direct consequences of its unique chemistry (Tate, Horie 2010).

History in Practice: From Manuscript to Modernity

First recorded in Egyptian hieroglyphs for ink and adhesives, gum arabic’s use has been continuous (British Library, 2022):

• Medieval miniatures (e.g. Livre de Kells) – used as both binder and gilding aid.
• Albrecht Dürer (1471–1528) – earliest surviving European watercolours ("Hare", 1502) employ gum arabic washes; source: Albertina Museum.
• J.M.W. Turner (1775–1851) – technical analyses show gum arabic in his atmospheric watercolour skies and glazes (Tate Technical Reports).
• John Singer Sargent – used both gum arabic and ox gall in lush late-19th-century watercolours (Boston MFA pigment analysis).

Today, all major watercolour brands (Winsor & Newton, Schmincke, Daniel Smith) rely on high-purity gum arabic, sourced mainly from Acacia senegal trees.

Pros and Cons of Gum Arabic as a Binder

Summary comparison: gum arabic’s practical advantages in watercolour vs. key limitations. (Tate, 2022; Gettens & Stout, 1942)

Advantages	Limitations
Brilliant transparency and gloss	Susceptible to rewetting, water damage
Permits glazing, layering, granulation effects	May crack if applied too thickly or dried rapidly
Readily available, non-toxic	Not suitable as a permanent binder for all supports (e.g. canvas)

Practical Workflow: Making and Using Gum Arabic Watercolour

1. Prepare gum solution: Dissolve high-purity gum arabic (granular or lump) in distilled water, ratio 1:2 to 1:3 by weight. For 100g gum, use 200–300g water.
2. Add preservative: Add a drop of clove oil or a pinch of sodium benzoate to inhibit mold (source: Winsor & Newton technical info).
3. Mix with pigment: Gradually blend high-quality pigment into the gum solution, grinding with a muller on a smooth glass slab. Aim for a smooth but not runny consistency.
4. Modulate flow: Add small amounts of ox gall or honey for increased wetting/flow, if desired (source: Handprint, 2022).
5. Storage: Pour into pans or tubes, seal tightly. Let harden (if for pans) for several days before use.

Process Control: Timing, Acceleration, and Retardation

• Drying time: Typical thin watercolour washes dry within minutes; thicker applications or humid conditions can extend this to 30–60 min.
• Acceleration: Gentle airflow speeds up drying; do not use heat, which can craze the gum film (source: Conservation Wiki, 2024).
• Retardation: Additives like honey or glycerin retard drying, useful for extended blending or in arid studios.
• Curing: Unlike oils, there is no oxidative cure—paint is simply dry once water is gone. Full evaporation is complete within hours.

Gum arabic watercolour’s famous rewetting ability: still responsive to water even after years (Tate, 2020).

Best Uses and Caveats

• Best for: Paper-based works; glazed luminous washes; effects exploiting transparency, diffusion.
• Not recommended for: Permanently outdoor display, high-humidity situations, or on unsized supports.
• For lasting display, use archival-quality paper, frame under glass, and avoid direct contact with moisture (Tate, 2024).

Conservation and Scientific Evidence

• Tate, Metropolitan Museum, and Getty Conservation studies confirm gum arabic’s stability on rag papers—films darken only if exposed to continual elevated humidity or pollutants.
• Loss of flexibility and increased embrittlement has been measured in 19th-century watercolours, usually linked to environmental exposure, not inherent flaws (Horie, 2010; Tate Conservation).

Key Takeaways

• Gum arabic uniquely enables the rewettable, glossy, flowing qualities of watercolour.
• Its chemistry creates luminous, easily layered paintings like those of Turner or Sargent—but requires archival paper and display care for longevity.
• Modern brands use acacia gum of remarkably consistent quality, offering artists the same toolset prized across centuries.

Sources

• Tate Technical Bulletin: Modern Watercolour Materials (2020, 2022)
• British Museum: Guides to Gum Arabic (2022)
• Winsor & Newton: What's in Watercolour – Gum Arabic (2023)
• Horie, V. (2010). Materials for Conservation, Routledge.
• American Institute for Conservation – Gum Arabic in Watercolour Conservation (1997)