Definition of "natron"

Natron and salt are soluble by liquid, but not all liquid but only such as is cold. Hence water and any of its varieties melt them, but oil does not.

1931, Aristotle, translated by E.W. Webster, Meteorologica, Bk. IV, ch. 6

You know the mysterious idols they were supposed to set up to worship in their chapters – were they really human heads treated with natron after the Ancient Egyptian pattern – idols of Persian or Syrian provenance?

1974, Lawrence Durrell, Monsieur, Faber & Faber, published 1992, page 242