The artists worked on wooden boards, which they sanded down and then coated with a paste of chalk and gelatinous glue. This priming process added considerably to the brilliance of the colors when they were applied. Charcoal, a sharpened goose quill or the ash of burnt bones were used to make preliminary sketches.
The next stage in the process was to apply the paints, or pigments. Because pigments were expensive and difficult to obtain, they were applied layer by layer. Pigments were made from ground-up minerals and organic substances. The blue used for backgrounds, for example, was made from azurite, while pigments used for the glaze were made from lapis lazuli; because the latter did not cover well, it had to be applied in very thick layers. Copper resinate mixed with malachite yielded green tones, while yellow and white came from salts of lead or tin.
The painters crushed the pigments to powder and mixed them with certain binging agents, according to their qualities. The Van Eycks experimented for many years until they had developed a durable binder which would dry evenly. To achieve this, they mixed oils of flax, hemp and nuts, to which they added resins. But we do not know the exact recipe. We also don’t know how they made the clear varnish used to coat completed paintings. All we know is that it contained natural resins.
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