Reactions of the Yellow Pigments.

Pigment.Hydrochloric
Acid.		Caustic
Soda.		Ammonium
Sulphide.		On Heating.
Chrome yellow
and chrome red		Green solution
above a white
residue, which
is soluble on
largely diluting.		Becomes orange
on boiling and
dissolves.		Blackened.Fuses to a yellow
mass.
Cassel yellowUnaltered, becomes
white on boiling.		Becomes paler
on boiling,
liquid yellow.		Blackened.Melts.
Naples yellowOn boiling orange,
then white.		Reddish yellow.Turns brownish
black.		Melts at a high
temperature.
MassicotTurns white.Partially soluble
on boiling.		BlackenedMelts with some
difficulty.
Lead iodideTurns white.Dissolves.Blackened.Melts.
Barium yellow
(chromate)		Yellow solution which
gives white precipitate
with sulphuric acid.		Unaltered.Unaltered.Unaltered.
Cadmium yellow Dissolves with
evolution of
sulphuretted hydrogen. 		Unaltered.Unaltered.Melts with
difficulty.
Zinc yellowYellow solution. Yellow solution,
white residue.		Unaltered.Melts with
difficulty.
Cobalt yellowRed solution.Colourless solution,
greyish blue
precipitate.		Unaltered.Becomes blackish at
high temperatures.
OrpimentUnaltered.Colourless solution. Yellow solution. Volatilises.
Turpeth mineralDissolvesUnaltered.Blackened.Turns red.

On Heating on Charcoal:—Chrome yellow, chrome red, Cassel yellow, massicot and lead iodide give lead beads in the reducing flame. Chrome yellow and red when fused with soda give red masses soluble in water. Naples yellow gives a lead bead and white fumes without smell.

Orpiment gives an odour of garlic.

Cobalt yellow, when heated with alumina, is turned blue.

Cadmium yellow produces a brown incrustation on the charcoal.

The examination before the blowpipe serves especially for the recognition of lead, chromium, antimony and arsenic. If a pigment blackens on heating, the presence of an organic colouring matter is indicated, such as Dutch pink, weld lake, etc.

The adulteration of pigments free from lead by colours containing that metal is recognised by the blackening produced by ammonium sulphide.