An example is the process of weighting which is primarily suit able for silk. Weighting of other fibres, notably rayon, has been attempted, but silk for centuries has been treated with sugar, salts of tin and lead, and iron to increase its weight and give the flimsy, soft pure silk the body and close texture that women for the past three centuries have always associated with "good" silk. The silk dress that would "stand alone" was probably heavily weighted. During the 1920s, however, the question arose as to why the silk dresses of 5o years before lasted a lifetime and the same silk dresses of the current day went to pieces in a few weeks. The answer was the degree of skill and care that went into the weaving of the fabric and its dyeing and finishing. The number of threads across the width of the material, the binding of those threads in the material must be sufficient to stand the weight of the finished fabric. Weighting as a process was originally intended only as a means of producing the stiff fabrics demanded in the styles of the middle ages and the 18th and 19th centuries. All weaving and finishing was done carefully with close and precise attention given by each workman. As silk fabrics became a mass production problem in the loth century, this same care and thought was, of necessity, abbreviated. The demand for silk fabrics changed from that of one or two silk dresses in a lifetime to a new silk dress every month or so. To meet this demand, many started business as silk weavers and dyers who were rela tively unskilled in the more exacting of the operations. Com plaints were made by consumers that the present-day silks did not wear and that the reason was "over-weighting." To determine by laboratory research the effect of weighting on the silk fibre, groups both in the United States and in Europe compared weighted and unweighted samples and measured the tensile strength of silk fabrics periodically to learn to what degree the fabrics had lost strength. Contrary to the general impression, the amount of weighting did not of itself determine the tensile strength of silks. The manner in which the threads were woven, the ingredients used in weighting and the method followed in the weighting process were equally if not more responsible.
These tests emphasized once more the mystery that surrounds the physical reaction of silk as it absorbs the minerals without outward change of appearance. Silk can absorb the liquids in its weighting bath to the extent of 300% of its original weight with out any noticeable change in its outward appearance. The solu tion does not coat the fibre; it is absorbed by it and is not re movable except by laboratory treatment. For hundreds of years, the objective of the dyer was to develop his process so as to give the desired appearance to any fabric he handled. But weighting, or rather poor weighting, brought about a demand for satisfactory wear in fabrics as well as aesthetic appeal. Some fabrics were not usable except with heavy weighting ; for example, the fabrics used in facing men's dress clothes. Contrarily, unweighted fabrics were found to be more popular for underwear. The unweighted fabric is termed pure dye although the words are sometimes misunder stood by the layman. They have no reference to the quality of the dyestuffs but are probably a distortion of an expression in tended to describe a finish as "pure dyestuff," i.e., nothing but dyestuff, although the origin is not clearly established. The United States Government edict, in 1938, demanded that where any weighting was present in the fabric, the percentage of weight ing in proportion to the silk fibre content in the finished fabric must be so stated. Similar regulations were put into effect in France.
The process of weighting begins with the degummed fabric. After being placed for several hours in the bath of weighting solu tion, the fabric is removed, washed and dried. The time it is per mitted to remain in the bath and the formula for the contents of the bath are, after many hundreds of years, still dependent largely on the skill and knowledge of the individual workman. Silk may
be given several baths of this kind and each time the threads gain weight. After a further treatment of special solutions, the silk is dyed. During this process, one of the valuable adjuncts is the seri cin or boil-off liquor which was saved after the degumming process. The gum solution gives added weight of a natural kind. The gum content of the silk is a cause of dissension as to the basis of ex pressing the amount of weighting in a fabric. The European method is based on par weight or the weight of the silk before de gumming. The European silk manufacturer considers that he may add weighting to the degummed silk up to the original weight before degumming, without technically "weighting" the silk. The percentage of weighting would be figured from the par weight not the degummed weight. The actual amount of weighting or foreign matter in the fabric would therefore be more than stated. In the United States, Government regulations now require that the per centage of weighting be expressed as the proportion of metallic weighting to the amount of actual silk fibre in the finished fabric, with no credit for any loss of weight in the raw silk due to de gumming.
Printing of silk fabrics is done by the same general methods fol lowed in other textiles. Both the roller type of printing and that by stencils, termed "screen prints," are followed. The most popular is the roller printing done by engraved copper rollers, similar to book or magazine printing. Each colour in the design is a separate roller and in referring to silk designs, reference is made to the number of rollers as synonymous with the number of colours. Designs for roller printing run from one to four colours and can be application or discharge. In the first, the design is ap plied to the cloth ; in the second, the design is made by removing colour from the background through the use of the proper chem icals.
Screen printing is used for the prints of more than four colours but also for achieving a hand printed effect that is in demand in more expensive printed fabrics. Instead of a roller for each colour, it is a screen for each colour. The process is slower than roller printing and is frequently used where only a small number of yards of a given design is required.
Finishing.—To give the dyed and/or printed silk smoothness, dull lustre and suppleness, as well as to provide protection against certain characteristics of wear, such as creasing, perspiration, water-spotting, etc., the finishing of silk is an additional art in preparing the fabric for the markets. The materials used in clude starches, dextrin, glue, gelatin, sulphonated oils, mineral oils, glycerine, various organic chemical compounds used as soften ers and penetrants, and resin. Not the least of these are per fumes or essential oils used at the final stage for overcoming any unpleasant odours remaining from the use of the less pleasant of any other materials.
In addition to the oils and gums, the finishing treatment in cludes stretching, gassing, moiring and framing. Fabrics must be made uniformly wide, wrinkles removed, fuzz burned off the sur face. The natural strength and elasticity of silk provides ample opportunity for manipulation to produce desired effects and the infinite variety of types that are one of the attractions of silk fabrics.