Chlorine bleaches fruit-stains, but turns the colour of blood-stains to an olive-green.
Red dyes fixed by a mordant are not influenced by ammonia.
Iron stains are usually blackened by ammonium sulphide.
Red paint may contain red oxide of iron; digest with hydrochloric acid and test for iron, by adding ferrocyanide of potassium to obtain the Prussian blue. Iron stains may be of a reddish-brown or orange colour, and insoluble in water, so that HCl is used to dissolve them.
Citrate and malate of iron stains are soluble in water; the addition of ammonia to an aqueous solution produces no change; guaiacum will give a blue reaction if a persalt of iron be present. The addition of hydrochloric acid and ferrocyanide of potassium will give the Prussian blue reaction. A drop of nitric acid added to the solution will oxidise the iron to the ferric state, and on the addition of a few drops of fresh-made aqueous solution of sulphocyanide of potassium the port-wine colour of sulphocyanide of iron will be produced.
A control test must be made with distilled water to prove the purity of the reagents, and the two results compared with each other.
Aniline stains resembling blood are changed to greenish-yellow or yellow on the addition of dilute nitric acid. Eosin stains produce a fluorescent solution when dissolved in water. Grease, tar, pitch, snuff, and paint may be mistaken for blood, especially on dark fabrics. They may be detected by two methods:
(a) The Wet Method.—Having failed to obtain a solution by the aid of the ordinary solvents for blood, other solvents must be used; ether or benzene for grease, paint, or tar. The solution obtained must be examined with the spectroscope.
(b) The Dry Method.—Place the cloth or other fabric stain down upon a clean white filter paper; then on pressing the cloth with a hot laundry iron, grease, tar, or pitch will stain the paper, paint or snuff will not.