A thick, dark resinous substance similar to tar or bitumen; a sticky material produced by alchemical or chemical processes.
From Arabic al-qitrān (tar or pitch), with al- being the Arabic definite article. The word entered European languages through alchemical texts.
This is one of those words that shows how medieval alchemy was actually about real materials—tar and pitch were hugely valuable for waterproofing ships and building, so alchemists studied them intensely even if they called the work 'sacred.'
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