1. The Preparation of the Fabric. The fabric must be clean. A spot of grease or other soil allows that part of the fabric less opportunity to extract color from the dye-bath, with the result that the dyeing is not level. Thorough washing with soap and warm water followed by a good rinsing will remove most spots. The methods of the dry cleaner will be necessary for others.
All sizing must be removed. Sizing consists of substances such as clay, flour, starch, gum, oil, tallow, soap, etc., put into the fiber, either before or after weaving, for the purpose of giving weight, body, strength, stiffness, softness, finish, or other desired quality. Some sizing prevents the extraction of color from the dye-bath. In other cases the sizing itself absorbs a large share of the color, and later when the sizing is removed by washing this color goes with it. Vigorous washing with soap and hot water, followed by rinsing, will remove most sizing. A fabric which suffers injury by the removal of its sizing is not suitable for dyeing.
Previously dyed fabrics are often uneven in color due to spotting and fading. Level dyeing cannot be done over such color. The practice of the professional dyer is to remove or level this color before attempting further dyeing. This often calls for skill beyond that of the inexperienced worker. Sometimes prolonged boiling in strong soap suds will accomplish this. Many colors will also yield to hot hydrosulphite solution followed by a good rinsing.
Of course where goods have been previously colored there are limitations as to the colors that may be obtained by additional dyeing. For example, blue goods cannot, without the removal of the blue color, be dyed yellow but may be dyed green or purple or a deeper blue. Yellow goods cannot be dyed pink, but may be dyed orange, green, brown or black. This will be discussed more fully later.
Some fibers are hard and harsh and resistant, like crash and most new cotton cloth. This is due to the presence of gums, resins, waxes and other impurities, substances which either attract dyes or else prevent their taking the fiber. It is not possible to dye such goods successfully without first rendering the fibers soft and absorbent. This may be accomplished by prolonged boiling in washing soda. Repeated washing and exposure to air will also do it, hence old cotton usually dyes well. Technically there are elaborate and carefully carried out chemical processes of preparing cotton cloth for dyeing, and entire mills are busy doing this one thing.
The Javanese and other Oriental peoples purchase cotton cloth from Europe and treat it for many days, alternately soaking in oil and boiling in lye from ashes, then exposing to the hot sun. This process is repeated until the cloth is soft and absorbent and suitable to receive color decoration.
The cloth should always be thoroughly wet before putting it into the dye-bath. This should be done in water of the same temperature as the dye-bath.
It may be briefly mentioned here that a fabric must sometimes be given a treatment with some chemical, called a mordant, before it will exhaust certain dyes from the bath and be properly dyed.
2. The Making of the Dye-Bath. For simple dyeing, as mentioned at the beginning of this chapter, we must observe the following things:
The dye-bath must be sufficient in quantity to cover the goods well and to permit of their being worked thoroughly during the process of dyeing.