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. |
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 yellow | Unaltered, becomes white on boiling. | Becomes paler on boiling, liquid yellow. | Blackened. | Melts. |
| Naples yellow | On boiling orange, then white. | Reddish yellow. | Turns brownish black. | Melts at a high temperature. |
| Massicot | Turns white. | Partially soluble on boiling. | Blackened | Melts with some difficulty. |
| Lead iodide | Turns 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 yellow | Yellow solution. | Yellow solution, white residue. | Unaltered. | Melts with difficulty. |
| Cobalt yellow | Red solution. | Colourless solution, greyish blue precipitate. | Unaltered. | Becomes blackish at high temperatures. |
| Orpiment | Unaltered. | Colourless solution. | Yellow solution. | Volatilises. |
| Turpeth mineral | Dissolves | Unaltered. | 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.