Reactions of the White Pigments.

On Heating on Charcoal:—Lead pigments give a lead bead in the reducing flame, which is converted in the oxidising flame to lead oxide, forming a deposit on the charcoal surrounding the hole.

Antimony white gives a brittle metallic bead in the reducing flame. This burns in the oxidising flame with the evolution of white vapours, and is at the same time covered by small shining crystals.

Bismuth white in the oxidising flame gives a rainbow-coloured incrustation spreading far over the charcoal.

Tin white gives a malleable metallic bead, converted by nitric acid into a white powder.

Zinc white is converted into a green mass when moistened with cobalt solution and heated in the oxidising flame.

The more expensive white pigments are frequently mixed with cheaper substances, e.g., white lead with finely powdered barytes, chalk or gypsum. Such admixture is distinctly to be regarded as adulteration, since the added substances have not the covering power of white lead. In the case of coloured pigments an addition of a white material is not to be regarded as adulteration, since the addition is made in order to impart a paler shade to the colour.

Pigment.	Hydrochloric Acid.	Caustic Soda.	Ammonium Sulphide.	On Heating.
Antimony white	Dissolves,solution turbid on adding water.	Dissolves,	Turns reddish yellow.	Turns yellow and melts.
White lead	Dissolves with effervescence, solution gives crystalline lead chloride.	Dissolves.	Turns black.	Becomes permanently yellow.
Lead oxychloride	Dissolves without effervescence.	Dissolves on boiling.	Turns black.	Turns yellow.
Lead sulphate	Insoluble.	Dissolves on boiling.	Turns black.	Unaltered.
Permanent white	Insoluble.	Insoluble.	Unaltered.	Unaltered.
(barium sulphate)
Bismuth white	Insoluble.	Insoluble.	Turns black.	Evolves reddish brown fumes, which redden litmus paper.
Zinc white	Dissolves without effervescence.	Dissolves.	Unaltered.	Turns yellow,becomes white again on cooling.
Tin white	Dissolves.	Dissolves.	Turns yellow.	Unaltered.