This commodity is sometimes contaminated with lead, because the fruit which yields the oil is submitted to the action of the press between leaden plates; and it is, moreover, a practice (particularly in Spain) to suffer the oil to become clear in leaden cisterns, before it is brought to market for sale. The French and Italian olive oil is usually free from this impregnation.

Olive oil is sometimes mixed with oil of poppy seeds: but, by exposing the mixture to the freezing temperature, the olive oil freezes, while that of the poppy seeds remains fluid; and as oils which freeze with most difficulty are most apt to become rancid, olive oil is deteriorated by the mixture of poppy oil.

Good olive oil should have a pale yellow colour, somewhat inclining to green; a bland taste, without smell; and should congeal at 38° Fahrenheit. In this country, it is frequently met with rancid.

The presence of lead is detected by shaking, in a stopped vial, one part of the suspected oil, with two or three parts of water impregnated with sulphuretted hydrogen. This agent will render the oil of a dark brown or black colour, if any metal, deleterious to health, be present. The practice of keeping this oil in pewter or leaden cisterns, as is often the case, is objectionable; because the oil acts upon the metal. The dealers in this commodity assert, that it prevents the oil from becoming rancid: and hence some retailers often suffer a pewter measure to remain immersed in the oil.