There are three only which we need consider:—
- 1. Soda and water.
- 2. French chalk.
- 3. Pure redistilled glycerine.
The first is perfectly satisfactory when freshly applied, but soon dries up and evaporates.
The second falls off; and unless the chalk be of the softest kind, free from all grit and hard particles, it will soon do more harm than good.
The third, glycerine, is for ordinary purposes by far the best, and has a beneficial rather than a deleterious effect on the rubber; but it must be pure. The redistilled kind, free from all traces of arsenic, grease, etc., is the only kind permissible. It does not evaporate, and a few drops, comparatively speaking, will lubricate fifty or sixty yards of rubber.
Being of a sticky or tacky nature it naturally gathers up dust and particles of dirt in course of time. To prevent these grinding into the rubber, wash it from time to time in warm soda, and warm and apply fresh glycerine when required.
Glycerine, unlike vaseline (a product of petroleum), is not a grease; it is formed from fats by a process known as saponification, or treatment of the oil with caustic alkali, which decomposes the compound, forming an alkaline stearate (soap), and liberating the glycerine which remains in solution when the soap is separated by throwing in common salt. In order to obtain pure glycerine, the fat can be decomposed by lead oxide, the glycerine remaining in solution, and the lead soap or plaster being precipitated.
By using glycerine as a lubricant the number of turns that can be given a rubber motor is greatly increased, and the coils slip over one another freely and easily, and prevent the throwing of undue strain on some particular portion, and absolutely prevent the strands from sticking together.
§ 12. The Action of Copper upon Rubber.—Copper, whether in the form of the metal, the oxides, or the soluble salts, has a marked injurious action upon rubber.
In the case of metallic copper this action has been attributed to oxidation induced by the dissolved oxygen in the copper. In working drawings for model aeroplanes I have noticed designs in which the hooks on which the rubber strands were to be stretched were made of copper. In no case should the strands be placed upon bare metal. I always cover mine with a piece of valve tubing, which can easily be renewed from time to time.