In their master’s theses in craft science, Kerttu Tolmunen and Niina Niinimäki studied the environmental friendliness of the dyeing process using Finnish natural indigo obtained from woad (Isatis tinctoria) and the usability of the dyed materials. Niinimäki examined the dyeing results and colourfastness of cotton samples dyed with three different woad varieties and reduction methods. Tolmunen examined the washing resistance of various textile materials dyed with woad.
Color enhances the appearance and appealingness of the product, as well as increases its market value. The color naturally affects the aesthetics of the product, and a successful dyeing result is part of the high-quality finish of textiles. The safety of the dye is also an important criterion for the quality of the product.
When dyeing textiles, the color fastness must also be taken into account. Natural dyes tend to fade over time, but indigo retains its color well.
Indigo has been historically and is still today one of the most important sources of producing blue colour. However, dyeing with indigo involves environmentally harmful methods. Synthetic indigo is produced from oil refining by-products and uses chemicals that are harmful to the environment. Natural indigo would be more environmentally friendly, but it does not guarantee an environmentally friendly dyeing process. Indigo dyeing uses a vat method based oxidation-reduction reaction of the dye, and sodium dithionite is the most commonly used reducing agent, which is harmful to the environment. Therefore, research and development of dyeing methods in addition to the dye itself is also important.
The influence of woad varieties and extraction methods on color in Niinimäki’s thesis
In her master’s thesis, Niina Niinimäki examined the dyeing of cotton with indigo obtained from woad (Isatis tinctoria) grown in Finland. Three different strains of woad were compared, from which indigo precursors were isolated by extracting them into warm water by three slightly different methods. Indigo was reduced to its water-soluble form with fructose and glucose, and control samples with sodium dithionite. The colour yield as well as abrasion and washing resistance were examined in terms of woad strain, extraction method, and reducing agent. The goal was to find an environmentally friendly dyeing method suitable for home dyers and craft teaching.
Environmentally friendly methods for reducing indigo have been studied quite extensively, and also the use of different sugars in indigo reducing has been considered in the past. However, most studies related to reducing indigo have focused on the reduction of either synthetic indigo or natural indigo derived from plants of the genus Indigofera. Since the natural indigo always contains some impurities, it was interesting to do the examination specifically for the woad grown in Finland. The effect of different extraction methods on indigo yield has also been studied somewhat in the past, but differences in different strains of woad have been studied less.
In the comparison of different extraction methods the dyeing result and color fastness were examined in respect of the extraction time and the way leaves had been chopped. Whole unchopped leaves and a short extraction time (10 min) gave the weakest dyeing results. Chopping the leaves and increasing the extraction time to half an hour clearly improved the dyeing result. However, in addition to indigo, chopped leaves and longer extraction time also resulted in increased amount of other colorants in the dye. This could be observed from the CIELAB measuring results, where the samples dyed with this dye bath were greener and yellower than their controls. However, in laundering, these samples faded less and their colour turned bluer in color. Presumably chopping the leaves released among other things yellow flavonoids, which were then washed away from the samples in water.
Out of the different woad strains, the strain from the 2002 row spacing and seed quantity test conducted by the MTT Agrifood Research Finland (now known as the Natural Resources Institute Finland) created the largest total color change and the darkest dyeing samples. However the dyed samples of this strain were yellower and less blue than their controls, although the differences were small and not statistically significant. Thus, it was likely that this strain contained more, not only indigo precursors but also other dyes such as yellow flavonoids. It would therefore be interesting to study the amounts of these other dyes in different strains of woad as well. It would also be a good idea to carry out further research on the comparison of strains, as the leaf material of this study could only be collected at the very end of the growing season, when the number of indigo precursors in woad leaves has already clearly decreased. From this review, it is not possible to conclude whether the strain that gave the best dyeing result initially had the most indigo precursors, or whether they only decreased from the leaves a little slower than from the other two strains. There were no differences in the laundering and abrasion resistance tests between the woad strains.
Indigo was successfully reduced with both fructose and glucose, but color yields were lighter and less blue than with sodium dithionite. There were no significant differences in color yield between fructose and glucose. However fructose scored slightly better than other reducing agents in laundering tests in terms of both color fading and staining. Although the dyeing results of sugar-reduced baths were lighter than sodium dithionite, sugar reduction could be a viable option for example for craft education where the use of environmentally harmful chemicals is avoided.
Color fastness and ideal washing instructions of textiles dyed with woad
The aim of Kerttu Tolmunen’s research was to examine the colour fastness to rubbing and laundering of textile materials dyed with Finnish natural indigo derived from woad, as well as to find the best possible washing instructions for products dyed with woad.
Three of the samples examined were finished commercial products and three were textiles dyed as tests of the colourant. All samples were woven fabrics and their materials were wool (n = 1), silk (n = 3), cotton (n = 1), and linen (n = 1). The samples were tested according to the standards SFS-EN ISO 105-X12: 2003 for colour fastness to rubbing and SFS-EN ISO 105-C06: 1997 for domestic laundering. In addition to these, colour fastness of the textiles was determined by adjusting the standard and washing the samples at 20 ° C and 40 ° C with 3 different detergents. The used detergents were Erisan for sensitive skin (pH = 8.7), BioLuvil wool & silk (pH = 7.3) and Ole hyvä eco & vegan (pH = 8.2). According to the manufacturers, all detergents are suitable for delicate materials. Change in colour and staining of the samples were were assessed with standardised grey scales. The changes in colour after different washing conditions were also examined instrumentally by determining the colour before and after each wash with a CIELab-spectrometer. The color fastness scores of the samples were also compared with acid-dyed Finnish sheep wool yarn, which was subjected to the same tests.
As a rule, dark textiles detached more color when rubbed than lighter ones. The control sample dyed with synthetic dye received a rating of 4/5 (on a scale of 1-5, where 5 is the best rating) in the abrasion resistance test, both dry and wet. Some of the samples dyed with natural indigo got better values than this, so a synthetic dye is not a substantially better alternative than a natural dye, at least in terms of abrasion resistance of the colour.
Compared to synthetic dye, the color fastness properties of natural indigo (both abrasion and water wash resistance) were even better in some of the tests. Especially the linen and cotton fabrics dyed with natural indigo kept their colour well and hardly stained the test fabrics at all.
Based on this study and from the point of view of color fastness, products dyed with woad, especially woolen and silk materials should be washed with a detergent suitable for these materials, with a pH close to neutral. In cotton and linen, the dye remained as well in standard (pH 10, temperature 40 oC) as in gentler (pH 7-8, 20 oC) washing. The pH of the detergent affects the color fastness of delicate materials. None of the detergents selected for the study were above the others, and no direct relationship was found between color fastness and different pH values of the detergents.
The washing temperature affected the color fastness so that in colder washes the colors did not change at all. Only the cotton sample (sample 5) which slightly changed in color in all washes, also changed its color slightly in washings at 20 ° C (color change value 4/5). Wool yarn (sample 2) also changed color at 20 ° C when washed with BioLuvil detergent (color change value 4/5). Based on the results of this study, it can be said that the colors of the products dyed with woad stay better when washed in a cool (20 ° C) temperature.
The study suggests that Finnish natural indigo is a competitive alternative to synthetic dyes. However, research on the subject should be continued and deepened, and the laundering resistance of Finnish woad dyes should be examined more broadly. In a single wash, the colors seem to stay well, but it would be valuable to study how the textiles keep the colour when washed more frequently.
Niina Niinimäki, M.Ed. (craft science).
Niinimäki, N. (2020). Puuvillan indigovärjäys morsingolla (Isatis tinctoria) : Morsinkolajikkeen, uuttomenetelmän ja pelkistimen vaikutus värjäystulokseen ja värinkestoon. (Master’s thesis, University of Helsinki)
Kerttu Tolmunen, M.Ed. (craft science).
Tolmunen, K. (2020). Suomalaisella värimorsingolla (Isatis tinctoria) värjättyjen tekstiilimateriaalien värinkestot. (Master’s thesis, University of Helsinki)
Main picture: Test samples attached to test fabric (Tolmunen’s study) Picture: Kerttu Tolmunen