Page:Popular Science Monthly Volume 29.djvu/365

Rh pot, which is kept warm by a moderate flame and serves the purpose of a cooling-vessel.

The button, or whatever is the article manufactured, is still only in the rude state, with the edges yet rough and the surfaces uneven, but already provided with holes for the after-insertion of metallic eyes. The rough edges are smoothed away by grinding on the grooved periphery of a wet sandstone, being held to it by a wooden clamp which is managed by the right hand while it is turned with the left. The surfaces are ground with wet sand on horizontal, fast-turning iron plates, and afterward polished on the face of soft wooden wheels roughened with Tripoli dust. To speed the operation, the workman presses upon the piece with both hands and gives it a peculiar rotary motion that equalizes the stronger friction to which the parts nearest in contact are exposed. The proper application pf this movement is a matter of knack, and is founded on mathematical principles, which also appear when the object is rubbed on a solid base, in the epicycloid lines which it is made to describe. On account of the relatively long time required for the operation of polishing, the smaller articles are subjected to what is called a fire-polishing, in which the smoothly ground pieces, imbedded on a plate of clay in fine sand, are heated in a muffle till their surface runs. If more strongly curved plates are wanted, to form a rose, for instance, the disks, previously prepared by notching and perforation in the middle, are placed in funnel-shaped crucibles in the hot muffle. The central part of the disk sinks on being heated. The hollowed leaves are then set one in another, in the order of their diminishing size, and fastened together by a glass-headed pin.

The foundation of the design is formed of a brass plate which has been previously shaped and perforated. Additional decorations are given by means of little beads, which are melted off in the glass-blower's lamp from thin threads of glass, and find their places in minute holes in the plate. Black sealing-wax is added to heighten the gloss and the blackness, as well as to cement the parts together. In other cases lighter figures are made by partly polishing or by etching them out on the smooth background. Iridization of beads, buttons, etc., has been much in vogue for a few years past; by this process those articles are given a metallic appearance. The luster of gold or silver is imparted by covering the black glass with a silver-or gold-leaf varnish and afterward heating moderately in a muffle. Peculiar tarnish effects are given by the application of what are called luster-colors; and, lastly, these are shaded by a brief treatment with chloride-of-tin vapor. The glass articles, hung upon a wire, having been previously warmed in the muffle-furnace, are drawn through the thick white vapors which are formed when a spoonful of the tin-salt is dropped upon red-hot iron. A long experience and considerable manual dexterity are required to make sure of getting the particular iris-color