One of the best tests for linseed oil is its weight or specific gravity. The latter term is not so well understood among painters as it might be. A few words of explanation may therefore be given. One hundred gallons of pure water when weighed when the temperature is at 60 deg. F. weigh exactly 1,000lbs. This is taken as the standard of specific gravity for all liquids, and a scale or hydrometer is based upon it. One hundred gallons of boiled linseed oil at 60 degs. F. weigh only 940lbs., and the specific gravity of linseed oil is therefore said to be ·940. As it would be practically impossible to weigh as much as 100 galls. of the liquid which it was desired to test for specific gravity, an instrument called the hydrometer is used for the purpose. This floats in the liquid and bears a scale so that a portion of it stands out of such liquid. When put in pure water, the indicator would show, of course, at 1,000, and in boiled linseed oil ·940, and in raw linseed oil it would show about ·922, which is the specific gravity of raw oil. Cotton seed oil raw is the same as raw linseed oil, namely ·922; colza oil is ·915. In actual practice the specific gravity of oil is taken by means of a bottle or flask which is weighed when full of oil and then compared with the weight of the same bottle full of water.
CHAPTER XII.
Recipes, Tables, Hints and Notes
The author trusts that he has made it clear to the reader that the subject of paint and colour mixing is far more comprehensive than might at first sight appear. Yet it is of such great importance that every house painter worthy of the name should make himself acquainted with it, and, unless he be colour blind, he can do so without difficulty if he will only take the trouble to make a number of tests and experiments.
In “putting on,” i.e., engaging the services of journeyman, the master painter will find, as a rule, that only about one in twenty has any knowledge of colour mixing, yet these men could, if they would only do so, easily make themselves at least fairly proficient in the subject by devoting their spare time to making various mixtures and using a box of ordinary artists’ oil colours for the purpose. A very good box can be purchased for about ten shillings.
Having given some practical tests for colours, we may now add one or two for turpentine.
To Test the Purity of Turpentine.
—It is of considerable importance that turpentine used for painting should be quite pure. To test the purity in a practical way pour a few drops on a sheet of white writing paper; if it is pure the mark will evaporate in a few minutes, leaving the paper quite clean. If, however, paraffin oil has been added to the turpentine it will leave a greasy mark on the paper, which will not disappear for several hours or even days. Turpentine is sometimes adulterated with benzine. The test above will not detect this, as the benzine will not leave a greasy mark. The evaporation, however, will be more rapid than when the turpentine is pure. When turpentine is very old, it becomes “gummy” or thick, and is unsuitable for mixing with paint. This condition is indicated by a greasy mark left on writing paper when a few drops are poured upon it.
Another very simple test for the purity of turpentine is to place a sample in a small white bottle and shake vigorously, carefully observing the time that it takes the bubbles that arise from the agitation to disappear. If the turpentine is adulterated with paraffin oil the bubbles will hold longer than when it is pure. The best plan is to have a bottle containing pure turpentine and another containing the suspected sample, and to shake up both together, comparing the rapidity with which the bubbles disappear.