Silk weaving has reached a very high standard of industrial efficiency. In fact, today a number of varieties of silk fabrics are produced on handlooms and sophisticated power looms. This requires different qualities of raw silk. In order to assist the weaving industry in the selection of the required raw silk, it must be first tested and classified. Further, the raw silk reeling industry requires well-defined standards, which can only be achieved by silk testing. As the demand for silk is global and a number of countries compete in the trade of raw silk, it is necessary that there should be industry standards for raw silk quality so as to enable buyers to purchase raw silk at internationally accepted grades. This is the reason why all raw silk produced should be classified following testing.
The testing of raw silk is based on the procedure laid down by the International Silk Association (I.S.A.). This procedure is quite conservative and the equipment used is consistent with traditional patterns when compared to procedures followed for general textile products. The traditional method has been widely preferred in silk producing and consuming countries of the world. The mechanical testing procedure is similar everywhere, but the compilation of test results and standards for various grades differs slightly from country to country.
In 1914, an essay competition was held in New York for quality silk goods. During 1921-1927, silk classification committees were set up. In 1928 and 1929, International Technical Conferences were held in Yokohama and New York, respectively. All silk producing countries such as Japan, China, Russia, France, Italy, etc. and silk consuming countries such as the United Kingdom and the United States, were represented. The organization of the International Silk Association and the organization of the third International Technical Raw Silk Conference in Zurich in 1949 followed by a fourth one in New York in 1950, contributed to placing silk technology and silk research on a global basis. In 1949-1950, a Bulletin was issued by the International Silk Association (I.S.A.) on International Standard Methods for Raw silk Testing and Classification. In 1961 a revision was made and it was decided to divide them into three categories. A second amendment was made in raw silk testing and classification in 1974. Recently, an electronic testing system was introduced into I.S.A.s methods.
Two main tests are usually applied, namely the conditioned weight test and the test for quality. The quantity tested from a consignment depends on the total weight. Usually, 5-10 bales of 60 kg each or 20 bales of 30 kg each is the unit of testing used for the export of raw silk.
11.2 Quality test
11.2.1 Visual Inspection
This test is conducted on the whole lot of raw silk, which has to be classified.
This is carried out in an inspection room, which is well illuminated. There are three main factors that have to be tested. These are:
In this test, the entire lot is inspected to assess the uniformity of colour, lustre and feel. The result is not recorded in the I.S.A. method, but is classified as good, fair and inferior under the Japanese method.
11.2.3 General finish
Here, considering the presence and degree of a number of defects assesses the general finish of the lot. These defects are:
- Re-reeling. Gummed skeins; gummed spots on skeins; double ends; irregular traverse and partial flack of traverse.
- Finish. Tangled filament, defective lacing, filament out of place in skein (pulled filaments loose).
- Arrangement. Lacing of booking card through skeins; non-uniform skeins; wrong twisting; raised filament; streaky filament; cut ends; discoloured skeins; foreign matter on skeins; irregular skeins on book; knots on skeins; skeins or books of different types.
- Damage. Books of irregular shape; gummed books, soiled filaments; frayed skeins; insect attached skeins; musty skeins, etc.
The results are expressed in terms such as good, fair, poor or inferior. Outstanding defects are mentioned in the Test Certificate.
The degrees of Colour and Lustre and the Smoothness or Hand of the lot are inspected and indicated in the following manner:
- Colour: light, medium or deep
- Lustre. bright, medium or dull
- Hand. smooth, medium or rough
11.3 Sample test
After completion of the test on the whole lot, the following tests have to be conducted. To conduct these tests, it is necessary to take out 50 skeins off the lot if the skeins weigh below 120 g. Alternatively, 25 skeins are taken if the skeins weight over 120 g. Having extracted the required number of skeins, the following test is carried out:
11.3.1 Winding test
- Winding frame
- Wheels and bobbins
Number of sample skeins drawn out for this test should be 40 skeins out of 50 skeins for a lot which weighs approximately 70 g each and 20 skeins out of 25 skeins drawn out for each skein weighing approximately 140 g.
When winding is started only the top half of the sample skeins should be wound. The winding should be carried out at a predetermined speed for a specific duration. The number of breaks that occur should be counted and noted. When breaks occur, it is necessary to note the cause of each break and this should be recorded.
Table 34. Average speed and winding period for winding test