A simple test is to make a freezing mixture of ice and salt, placing a small portion of the suspected oil in it, with the bulb of a thermome ter in the oil. If the oil congeals to a solid or butter-like consistency at a temperature higher than eighteen degrees below zero, it is not pure linseed oil, but may contain an excess of water, or may be adulterated with rosin oil (freezing point 6° F.), or with rape seed oil, which is apt to come from imperfectly cleaned seed and which freezes at 25° F. Fish oil has a still higher freezing point, 32° F., or that of water.
Testing White Lead. White lead purchased in unbroken packages bearing the name of any reputable corroder is reasonably certain to be pure. Occasionally it may contain a trifle of silver, or of uucorroded blue lead, either of which renders the white lead off color. But when white lead is offered under fancy names, either branded with a jobber's or grinder's name, or with no firm name, and especially when it is offered at less than the current market quotations for white lead, one may feel certain that it is a so-called "combination" lead, or a mixture of white lead, zinc white, and barytes.
To test white lead in oil for purity, scoop a small hole in a piece of charcoal, and place in it a portion of the lead about the size of a small pea. A gas flame, or the flame of a spirit lamp, is then directed upon it by means of a blow pipe. The lead must be held in the point of the flame, and a steady blast must be main taMed. A minute or two should reduce the
white lead to a button of metallic lead, provided it is free from adulteration. If any zinc, barytes, whiting, clay, or silica be present—even if only five or ten per cent—no metallic button can be formed, but the substance left will be a whitish, yellow, or gray cinder-like mass.
To test white lead for the presence of barytes (the most usual adulterant), place about twenty grains in a test tube, which should be filled about half full of dilute nitric acid. Pure white lead will completely dissolve, with some effer vescence, while barytes will remain as an insolu ble precipitate in the bottom of the tube. The oil may be removed from the white lead ground in oil, by placing it on blotting paper and satu rating it with benzine or gasoline, which is then allowed to evaporate.
lead, however, may be pure, yet of inferior quality, owing to imperfect corrosion or lack of care in its manufacture. A good white lead should be reasonably white, with a very slight yellow tone as distinguished from the dead white of zinc white. The quick-process leads, as a rule, are whiter than those made by the old Dutch process. A dark, somewhat gray color indicates imperfect corrosion. The lead should be of good density and not too oily, but should be ground so as to "string out" when taken up from the keg with a paddle. Lead of this character does not break up so readily as a "short" lead, and is much more durable for outside work.