Student Projects, Pigments&Binders

Introduction Dyes have been utilized in both Eastern and Western societies since pre-Christian times, and have a rich and vibrant history not only in the Old World, but in the New World as well. Indigo, a blue vat dye so-called because it is applied in a special dyebath called a vat and does not require a mordant- was America's most important dyestuff during the 18th and 19th centuries. It produces a myriad of blue shades that are very fast to light and washing in both the materials of cotton and wool.
A Brief Background Chemistry of the Dyeing Process All natural dyestuffs must be dissolved in a liquid before textile materials can absorb them. Since water will not dissolve indigo, it must be dissolved in another liquid. When indigo is combined with a reducing agent, a compound is formed that will dissolve in an alkaline liquid. This process turns the indigo dye liquid yellow. When wool or cotton are dipped into this dyebath (known as an indigo bath) they readily absorb the dye in this yellow, reduced form. The dye is fixed on the fibers permanently by oxidation when the dyed material is exposed to air. During this final step the blue color returns and the dye reverts to its original insoluble form on the fibers. Repeated dipping and airing of the material results in a gradual build-up of blue color on the yarn or fabric.
Experimental Variable Historically, indigo dyers would try to improve the quality of their product by varying different aspects of the dyeing process. The alkaline bath was a popular mode of variance and is the variable of this experiment. I used the suggested potash as the alkaline bath, and then replaced the potash with NaOH and CaCO3 (lime and sodium hydroxide). I measured the difference in results by how fast they held to the fabric and by measuring the color with the spectrometer.
Materials 1 Large 1000mL Beaker 1 Glass Stirring Rod 1 Centigrade Mercury Thermometer 1 Electric Hot Plate 1 Small 250mL Beaker 18 Small plastic dishes 3 100% cotton sheets, apprx. 7.08 grams of NaOH, CaCO3, and K2CO3 3.54 grams of Indigo * 3 14.17 grams of Na2S2O4 * 3 Unlimited supply of H2O 1 Plastic Wash Basin
Procedure After gathering my chemicals in the plastic wash basin that will later serve as the dye vat, I used a Labtronix Mettler BB240 to measure out the mathematically deduced amount of each chemical required for the different dye vats. I recorded my observations of each chemical in table I.1. After having measured each chemical and placed each chemical in its' specific make-shift carrying capsule, I moved onto the lab and set up my operational area. After plugging the heating plate under the flame hood (since the smell of the indigo dying process can be quite pungent), I filled the large beaker half way with tap water. I then added the pre-measured amount of indigo (3.54 g) to the water, and stirred with the stirring rod until the indigo and water formed a watery, purple paste. The indigo particles stuck to the stirring rod, however, making an even distribution of the powder difficult. In a separate beaker I added the alkaline chemical with 250mL of tap water, and stirred until I formed the basic solution. After using pH indicator paper to determine the pH of each alkaline solution, I poured the alkaline solution into the indigo paste mixture. I then heated the solution at a setting of 7 on the hot plate, stirring vigorously for 30 seconds every two minutes for six minutes, carefully monitoring that the temperature stayed between 30-50 degrees Centigrade. I then added the sodium hydrosulfite powder, sprinkling it over the heated mixture, and observed the changes in each separate alkaline-indigo mixture, recording them in table I.2. After another two minutes of stirring, the dye vat was complete and ready to be used. I poured the dyevat into the plastic washbasin and for 2 minutes immersed the cotton cloth in the dyevat, covering every inch of the fabric with the chemical solution. I recorded my observations in table I.3. I then poured the remaining dyevat solution into yogurt jars, covering each with saran wrap for future use. I cleaned my supplies thoroughly, and repeated the dyeing process using a different alkaline agent.
How Indigo Works: Chemical Overview Indigo is insoluble in water before dyeing, but it is made soluble in the blue vat. The blue vat is a "vat" of chemicals in which the indigo dyeing takes place. Several combinations of ingredients will help to put indigo into solution. These account for the variety of instructions for making up blue vats that one encounters in dye recipes. The main purpose of these ingredients was to combine them with indigo to reduce it and make it soluble in alkali solutions; in this dissolved state indigo could be absorbed by textile material. In my experiment, sodium hydrosulfite was the agent which acted as a reducing agent which induced the fermentation of the indigo. Historically, chemical compounds such as bran, madder, or copperas were frequently used. Indigo is then dissolved in an alkali solution, which is usually made up of lime, potash, or soda in water. Dipping textile material in the dye solution deposits dye in the fibers. When the textile is removed from the dye vat, the air oxidizes the indigo, returning it to its original insoluble form with the noticeable color change from yellow to blue.
Links to Oxidation/Reduction Websites http://www.chem.uidaho.edu/~honors/redox.html http://edie.cprost.sfu.ca/~rhlogan/redox.html http://www.dist214.k12.il.us/users/asanders/ox.html http://www.chemistry.nmsu.edu/studntres/chem112.spring.1997/Redox.html
References Adrosko, Rita. Natural Dyes and Home Dyeing. New York: Dover Publications, 1971. Bemiss, Elijah. The Dyer's Companion. New York: Dover Publications, 1973. Cook, Mayer. The Chemistroy of Natural Coloring Matters. New York: Reinhold Publishing, 1943. McLaren, K. The Colour Science of Dyes and Pigments. Bristol: Adam Hilger Ltd., 1983. Seagroatt, Margaret. A basic textile book. New York: Van Nostrand Reinhold Company, 1975. Weigle, Palmy. Ancient Dyes For Modern Weavers. New York: Watson-Guptill Publications, 1974.

Sarah Leopold, 2000.

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