Sometimes, for specially brilliant fabrics, the skeins are “lustred” by machinery; this is the so-called “metallic lustring” when the silk, generally enveloped in steam so as to be both hot and damp, is pulled out between two steel arms until it has been stretched a considerable percentage of its original length. This undoubtedly lessens the strength of the fibre considerably and diminishes its elasticity, but under this strain each fibre is stretched out perfectly smooth and thus becomes much more brilliant and lustrous. =Dyeing Wild Silks.=—It has been found difficult to handle satisfactorily the different sorts of wild silks in the factory. The bleaching of them has been very troublesome, although of late years the problem has been solved pretty well. And the ordinary process for dyeing silk with Acid dyes in a broken soap, or boiled-off liquor, bath is, for full deep shades at any rate, not always satisfactory. In consequence most of the genuine pongee or Shantung cloth is sold in the natural unbleached color, a pleasant shade of tan, or else in light shades. Perhaps the best results in dyeing pongee silk full, deep, even shades are obtained by mordanting the material with tannin and tartar emetic, just as cotton is mordanted before dyeing it with Basic colors, and then using in the dye-bath one or the other of the so-called “Janus” colors,—a group of colors on the border line between Basic and Acid, of which the best are Janus Yellow G, Janus Yellow R, Janus Red B, and Janus Black 1 (_Metz_). This process, however, is too complicated for the unprofessional dyer to use with much success. For all but the very full shades the craftsman is advised to use the Acid colors, as, for instance, some of the selected colors of the different houses, listed in Chapter VII, in a bath acidified with acetic acid, and without the use of soap. For dark dull shades the Sulphur colors can be used, especially if some care is taken to reduce the alkalinity of the bath by neutralizing or nearly neutralizing the sodium sulphide with a little acid sodium sulphite. If the desired shade is so dark as to necessitate heating and dye-bath, it is also advisable to add a little gelatin. For full shades of rather brighter quality the Vat dyes may be employed, also with precautions against the tendering action of the caustic alkali upon the fibre. Before, however, starting in to dye a piece of pongee on the assumption that it is made from tussah silk, it is very advisable to examine it carefully, picking out the individual threads and untwisting them, and to make a few dyeing tests upon small samples. For a large proportion of so-called pongee, which in color, lustre, feel, and general appearance resembles the genuine Shantung very closely, is simply made from spun or waste silk, and can be dyed like ordinary silk.