A practical review of water gilding gesso and bole in Travelling Beneath the Gold Surface – Part I: Study and Characterization of Laboratory Reconstructions of Portuguese Seventeenth and Eighteenth Centuries Ground and Bole Layers by Sandu, Paba, Murta, Pereira, and Ribeiro (2014).
Why This Paper Matters to Gilders and Painters
Most modern gilders learn water gilding gesso and bole from books that compress three centuries of practice into one page of instructions. Buy gesso. Mix it with rabbit-skin glue. Apply seven layers. Burnish with agate. The recipe works, more or less, and a beginner can make a respectable gilt panel before lunch. But it is a flat recipe, stripped of the choices and quality checks that older artists made every day. When the panel cracks at year five, or the bole refuses to burnish, or the gesso lifts in humid weather, the modern instructions offer no clue.
This paper goes the other direction. The researchers took two real Portuguese treatises — one by Philippe Nunes from 1615, one by José Lopes de Baptista Almada from 1749 — and tried to make their water-gilding grounds in a lab, step by step, with period-appropriate raw materials. They then used X-ray diffraction, optical microscopy, and scanning electron microscopy to check whether what they made matched what real altarpieces contain. Notably, the work is part of the GILT-Teller project, funded by the Portuguese Foundation for Science and Technology, which focuses on gilding materials and techniques in Portugal between 1500 and 1800 (Sandu et al., 2014, p. 96).
For a working artist, this is not an academic exercise. It is a guided tour through the decisions that the printed recipes hide: which gypsum to buy, why your rabbit-skin glue may not behave like the historical glue, how to test whether your “anhydrite” is actually anhydrite, and why the artist’s hand still controls the outcome even when the chemistry is correct.
The Core Finding of Water Gilding Gesso in One Sentence
Reproducing a historical water gilding gesso and bole recipe correctly requires that you control three variables the recipe itself does not name — the mineralogical purity of the gesso, the source and processing of the animal glue, and the experience of the hand mixing them — and analytical confirmation is the only way to know whether your reconstruction is faithful (Sandu et al., 2014, pp. 111–112).
What the Researchers Actually Did
The team selected two Portuguese treatises representative of the seventeenth and eighteenth centuries: Nunes (1615) and Almada (1749). Both describe water gilding, the technique that allows the gold leaf to be burnished to a high gloss against a bole layer. They then attempted full laboratory reconstructions on flat pine and oak panels, documenting every step with photographs and video (Sandu et al., 2014, pp. 96, 98).
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