Bitumen is a naturally-occurring, non-drying, tarry substance used in paint mixtures, especially to enrich the appearance of dark tones

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Bitumen became very popular as a paint additive in the late eighteenth century and early nineteenth. However, because it does not dry it eventually causes often severe darkening and cracking of the paint. This can be seen in the work of Sir Joshua Reynolds, Henry Fuseli, and Sir Thomas Lawrence or example.