CHARCOAL. - Sticks of this material are very extensively used for polishing several of the metals, and the action seems to depend on the silex disseminated throughout the substance of the charcoal. Considerable discrimination is required in the selection of pieces, from the bulk of that which is prepared from small green wood for metallurgical and domestic purposes, as but few pieces possess the requisite cutting quality; the workmen generally try it either on the teeth or finger nail.
The stick of charcoal is applied at an angle of about 40 or 50 degrees to the work, the position best suited to every piece being found by trial. Some pieces will cut rapidly and coarsely with water, others more slowly and smoothly with oil; and pieces of good quality are very highly prized by workmen. Some artizans conceive that charcoal cuts more greedily when moistened with vinegar, but which fluid is objectionable as it stains the metals.
In the course of polishing, the charcoal picks up the abraded particles of metal, they sometimes enter its pores, and would scratch the work if allowed to remain on the charcoal, consequently two pieces are mostly used; the one merely to clean the other by rubbing them together at their ends in the same manner that the painter rubs two lumps of pumice-stone together to clean their surfaces. In finishing delicate works, and laying the grain, abundance of oil or water should be used, so as to float off the minute particles of metal removed in the process.
The charcoal prepared from the wood of elder appears to have the decided preference especially for polishing the steel and copper plates used by engravers, both in their first preparation, and in the removal of the burrs thrown up by the graver. To ensure the possession of the true sort, it is recommended to obtain the waste pieces of elder from the rule maker, to cut them into short pieces, and then to burn them in a crucible filled with sand, in order to exclude the air, otherwise the entire substance of the wood may be burned to ashes; the kitchen or forge fire may be used, and the crucible should be allowed to cool in the embers.
The charcoal made from willow truncheons, is described as being much in esteem by the manufacturers of copper plates for engravers; and elm wood is also stated by Mr. Thomas Gill as being suitable for making the charcoal for polishing. See Tech. Repos. vol. ii. p. 264.