“Bolus”
Pronunciation: /ˈboʊləs/ (BOH-ləs)
Part of Speech: Noun
Quick Definition: A fine, smooth, natural clay (usually a hydrated aluminum silicate colored by iron oxides) used as a ground layer for water gilding.
General Use: “The restorer carefully applied several thin coats of red Bolus to the altarpiece frame. Consequently, this layer would provide the monumental flexibility and color depth required before the gold leaf could be applied.”
The Red Hue – The most common and monumental use of red Bolus gives gold leaf its characteristic warm, deep tone. When the thin gold layer is applied over the red clay and polished, the red color subtly reflects through the gold, adding visual richness and depth.
The Sliding Secret – The hidden function that makes Bolus irreplaceable in water gilding is its composition. It remains soft and slightly greasy even when dry, acting as a lubricant. This monumental quality allows the burnishing tool to rub the gold leaf into a mirrored shine without tearing it.
Armenian Origin – Historically, the finest clay for Bolus was sourced from Armenia (hence “Armenian Bole”). The original clay was so prized for its purity and smoothness that it was considered a monumental trade commodity and was even used for medicinal purposes.
Did you know?
In conservation, the color of the Bolus used is often a clue to the original artisan’s school or time period. While red is traditional, yellow Bolus provides a lighter, brighter tone, and black Bolus (sometimes seen in Baroque art) creates a stark, cold contrast beneath the gold, manipulating the monumental luster.
Bolus Definition (Primary Context)
The Bolus is primarily used in the technique of water gilding (or water-size gilding), which achieves the highest possible monumental shine on a gilded surface. The preparation process involves mixing the finely ground clay powder (hydrated aluminum silicate, often containing iron oxide for color) with a binder, typically animal-hide glue (gelatin size). This paste is thinned with water and applied in several thin coats over a pre-prepared ground layer (such as gesso or chalk). The resulting Bolus layer is crucial because it provides both a smooth, absorbent surface to which the water-size and gold leaf adhere and a soft cushion that enables the highly polished, monumental finish achieved through burnishing.
Latin: bolus or bole, referring to a mass or lump of earth or clay.
Armenian Bole, Gilder’s Clay, Bole Armenia, Clay Ground.
Gesso (the rougher ground layer beneath it), Varnish (a protective coating on top), Oil Gilding (an alternative technique).
Pigment, Substrate, Ground, Medium.
Historical Context of Bolus
The use of Bolus dates back to ancient Egyptian gilding practices, but it was perfected and systematized in Byzantine and Medieval European traditions, becoming indispensable for decorating monumental icons, altarpieces, and illuminated manuscripts. The material was a staple in the workshops of master gilders, and trade routes specifically transported the prized, high-quality clay from the Near East across Europe. The continuity of its use, from antiquity through the Renaissance and Baroque periods, confirms its monumental value as the best preparation for the most brilliant gilding.
Social Context of Bolus
In the context of sacred art, the use of Bolus and gold leaf conveyed monumental status, sanctity, and wealth. The shimmering, permanent nature of the gold symbolized the divine and eternal, making it suitable for religious images. The expense of pure gold leaf and the specialized skill required to apply the delicate Bolus and then the gold meant that these objects were commissioned only by the Church, royalty, or the wealthiest patrons, elevating the resulting artworks to a monumental level of prestige.
| Area of Influence | Significance and Impact |
| Gilding Quality | It provides the necessary monumental smoothness and cushion. This allows the gold leaf to be burnished to a high, reflective luster. |
| Optical Effect | The color of the Bolus (e.g., red or yellow) subtly influences the final warm or cool tone of the applied gold. |
| Conservation | The Bolus layer is crucial for identifying historical gilding techniques. It often helps conservators determine how to safely re-adhere flaking gold. |
| Material Trade | The historical sourcing of fine clay, especially Armenian Bole, highlights the monumental long-distance trade networks for art materials. |
Terms Related to Bolus
The study of Bolus involves terms related to traditional surface preparation and gilding techniques. This provides essential context for art conservation. This provides a clearer view of the technical framework surrounding the monumental craft.
| Term/Concept | Description and Relevance |
| Water Gilding | The technique of applying gold leaf using water-size (diluted glue). Bolus is essential for this monumental method. |
| Gold Leaf | Extremely thin sheets of gold. This is the material applied over the Bolus layer to create the final gilded surface. |
| Gesso | A mixture of animal glue and gypsum or chalk. This is the rougher ground layer applied to the wood before the Bolus. |
| Burnishing | The act of rubbing the gilded surface with a smooth, hard object (e.g., agate). This compresses the gold onto the Bolus and creates a mirror shine. |
| Size | The weak adhesive (often diluted rabbit skin glue) used to mix with the Bolus and, in a more diluted form, to activate the Bolus surface before gilding. |
| Tempera | A painting medium (pigment mixed with egg yolk) often applied directly onto the gesso or Bolus surface adjacent to the gilding. |
Sources & Credits
Sources
- Eastaugh, N., et al. “Pigment Compendium: A Dictionary and Optical Microscopy Guide.” Butterworth-Heinemann, 2004. [Historical source]
- Mayer, R. “The Artist’s Handbook of Materials and Techniques.” Viking Press, 1991. [Historical source]
- “Bolus.” Getty Conservation Institute (GCI) Art and Architecture Thesaurus. [Museum site]
- The Victoria and Albert Museum. “Techniques of Gilding.” Conservation Notes. [Museum site]
- Cennini, C. “Il Libro dell’Arte (The Craftsman’s Handbook).” (c. 1400). [Historical source]
