Iron is incredibly useful for the natural dye process and palette. It darkens/saddens colors and efficiently creates shades of black. Iron can be used in one of two ways: (i) as a mordant or (ii) as an afterbath (i.e., immersing pre-dyed goods into an iron solution). We generally immerse goods in an iron afterbath rather than mordant the goods with iron before dyeing. This way, we can watch the color shift in real time, which happens in a matter of minutes.

Natural dyers can create iron solutions in a few ways:

• Ferrous Sulfate - Readily available from natural dye suppliers, ferrous sulfate can be used for wool and cellulose fibers but not silk.

• Ferrous Acetate - Created by combining equal parts ferrous sulfate and sodium acetate, ferrous acetate is less damaging to fibers and generally recommended for all textiles. This can also be created by combining rusty iron scraps with vinegar, but the amount of ferrous acetate in the solution will be unknown and make results unpredictable.

• Iron Pots & Scraps - If ferrous sulfate is not readily available, dyers may use an iron pot or place iron scraps into a stainless-steel pot in which some of the iron will dissolve and attach to the textiles. Generally alum is added to the bath to increase acidity, which helps to increase the amount of iron in the solution.  

We use only ferrous sulfate and ferrous acetate as we can control the exact percentage of iron available in the afterbath. Usually iron is used between 1% to 4% weight-of-fabric (WOF). For this experiment, we decided to observe the differences, if any, in using ferrous sulfate and ferrous acetate. In each case, we obtained three results: (i) dyed but not modified with iron, (ii) modified with ferrous sulfate (8%) and immersed for approx. 6 minutes, and (iii) modified with ferrous acetate (8%) and immersed for approx. 6 minutes. Note, we usually would use no more than iron at 4% WOF, but given the weight of the swatches and sensitivity of our scale, we had to use an 8% solution. The fabric we used for these swatches is a 100% cotton double cloth that we used for our Convertible-Collar Shirts. Below are the results and details of each dye process.

Oak galls are not often used as a dye by themselves. Generally they're used as part of the mordant process for cellulose fibers or with iron to obtain purplish grays and blacks. For the unmodified swatch, we soaked the fabric in oak gall tannin extract and gave it only a light rinsing to give a sense of color oak galls impart. For the others, we took the swatches steeped in oak gall tannin extract and immersed them in iron solutions. The ferrous sulfate solution turned the swatch a light gray while the ferrous acetate solution turned the swatch near charcoal. In both cases, the gray leans cooler, almost purplish.

Myrobalan is a yellow dye that also contains plenty of tannins. Each swatch was mordanted with oak gall tannin extract and aluminum acetate and then dyed with myrobalan at 20% WOF. The unmodified swatch shows the typical calming yellow, buff color of myrobalan. When placed in ferrous sulfate, the buff color shifted to a lichen or sage green. The ferrous acetate solution, however, shifted the color to a much darker, pewter green.

Madder gives turkey reds at high WOF percentages but can also be used to obtain sun-washed reds, pinks, and oranges, depending on the variety of madder used. The swatches were mordanted with oak gall tannin extract and aluminum acetate and then dyed with madder at 150% WOF. The deep, turkey reds of madder are shown by the unmodified swatch. The swatch immersed in ferrous sulfate turned a pleasing shade of maroon, and the one placed in ferrous acetate turned out a couple shades darker. Nowhere near as dramatic as the difference between the oak gall and myrobalan swatches. Our guess is that the lack of tannins in madder is the culprit.

The unexpected result from this experiment is the difference in shades obtained from ferrous sulfate and ferrous acetate, particularly when tannin content in a dyestuff is high. When researching this difference, we came across one possible explanation. The acidity of ferrous sulfate is higher than ferrous acetate, and that increased acidity may hinder the reaction between iron and tannin in the fabric.

While we typically default to using ferrous acetate on our goods, ferrous sulfate can still be utilized on cellulose textiles. This ultimately means that we can obtain a greater range of shades.