9. Should fail by gradual wear, not by disintegration.

10. Should leave a good surface for repainting.

11. Should not require an unreasonable amount of skill or muscle in

application.

12. Should be homogeneous.

13. Should dry fast enough.

14. Should not be readily ignited.

15. Should have power to absorb and remove moisture or dampness

from the metal.

16. Should have properties that will prevent corrosive action of traces of

water in contact with the metal.

Best Vehicle for Paint

Durability of paint depends just as much upon the vehicle as it does upon the pigment. Up to the present time no vehicle has proved to be anything like as good as pure, raw, linseed oil, notwithstanding the fact that many substitutes have been tried. Some of these substitutes are valuable as thinners of linseed oil, if used in moderate quantity, because they often improve somewhat both its drying and its working properties. The usual reason, however, for the adoption of such thinners or adulterants is to reduce the cost of the paint; and too often this is done at the expense of its quality. It is generally conceded that "the base of the best substitutes for linseed oil is linseed oil itself."

The author once had great hope of Leucol Oil as a vehicle; and, as a test of it, he used Leucol Red-Lead Paint on one of his Mexican bridges in competition with a number of other paints on several nearby structures; but it failed to give satisfactory results, the surface quickly assuming a whitish tinge, and the protection failing much sooner than it should have. It should be stated, though, that the climatic conditions in the tierra caliente where these structures were located were unusually severe for bridge paints. All but one of the paints then tested failed unequivocally—but of that exception, more anon.

Boiled linseed oil was much used for bridge paint in times long past; but experience has shown that the boiling is a detriment to the vehicle instead of an improvement thereto.

Use of Driers

Some engineers consider all driers merely as adulterants, employed solely for the purpose of cheapening the product; but the author is of the opinion that they occupy a legitimate place in paint manufacture, provided they be used in moderation—especially for priming and intermediate coats, which often need to dry fairly quickly in order not to delay the application of the succeeding coat. As far as he knows, the best drier to employ is