1. In printed fabrics is the pattern clearly discernible on the back of the cloth?
2. If the fabric is printed on both sides, how may this fact be proved?
3. What is the difference between printed and dyed fabrics?
Experiment 26—Bleaching by Sulphur Dioxide
Apparatus: A quart bottle.
Material: Sulphur, worsted or silk fabric.
Bleaching powder cannot be used in bleaching animal fibers such as woolen and silk fabrics. It injures the fibers and at the same time leaves them yellow.
Animal fibers are best bleached by immersing in an aqueous solution of sulphurous acid or exposing them to fumes of burning sulphur.
Wet a piece of dyed worsted or silk fabric and hang it in a quart bottle containing fumes of burning sulphur.[26] The fumes of burning sulphur have an affinity for coloring matter—dyestuff. The fumes (called sulphur dioxide) do not in most cases destroy the coloring matter as chlorine does, but simply combine with it to form colorless compounds which can be destroyed. The color can be restored by exposing the bleached fabric to dilute sulphuric acid.
Questions
1. Why is it necessary that the fabric be moist in order to be bleached by sulphur dioxide fumes?