Ochre Browns
:
ON THE SEMI-NEUTRAL, BROWN.
The slight affinity of sulphur for yellow ochre, with its merely
temporary effect thereon, was observed in the eighth chapter, where
allusion was made to the action of sulphuretted hydrogen and sulphide of
ammonium on the earth. Sulphur alone, and in the dry state, ignited with
yellow or other native ochres converts them into browns, varying in hue,
and of greater or less durability. Those browns, however, which we have
made by this process, although standing well in a book, have not
withstood exposure to light and air. They have all become pale, whitish,
or of a drab cast, evidently through the oxidation of the sulphur, or
rather the sulphide of iron formed during the calcination. Practically,
therefore, ochres have an antipathy to sulphur, moist or dry, by itself
or in combination; and are, so to speak, the disinfectants of the
palette. Ever waging war against sulphurous vapours, the native earths
serve to protect a picture from the damaging influence of impure air,
whether they be used alone, or employed in admixture with such pigments
as are injured thereby.