In our March 2023 post, we explored the outcomes between ferrous sulfate and ferrous acetate. We discovered that ferrous acetate produced substantially darker shades, especially when the dyestuffs contained tannins. That result led us to a new curiosity—what happens when cellulose fibers, mordanted with and without tannins, are immersed in iron?

Since cellulose fibers can be mordanted in a number of ways, we focused on two common methods. The first is our standard mordant method, which involves an initial immersion in oak gall tannin extract followed by aluminum acetate. The second is an immersion in aluminum acetate followed by “dunging” the fibers in calcium carbonate to fix the mordant in the fibers.

We intentionally chose dyes for this experiment that, to our knowledge, do not contain significant amounts of tannin: madder and osage orange. We used both dyes at 30% weight-of-fiber (WOF) and the following fabrics: (i) cotton sateen, (ii) wool/hemp/silk twill blend, and (iii) cotton double cloth.

The below swatches show the results of the dyes without any iron immersion. A couple of interesting things to note here:

• The wool/hemp/silk twill developed a deeper shade when mordanted with aluminum acetate + calcium carbonate.

• The two cotton swatches developed their deepest shades when mordanted with oak gall tannin extract + aluminum acetate.

We generally expect using tannin + aluminum acetate will result in the most saturated colors. And this occurred with the cotton swatches. When zooming in on the wool blend swatches, the silk yarns took the dye the least. Our guess is that the acidity of the tannin reduced the silk fibers’ ability to absorb the mordant. The wool yarns also seem a bit less saturated; however, wool is usually aided by acidity (i.e., cream of tartar) during the mordant process, so we’re not sure why this is the case.

Below are the results of immersing the same swatches above in iron, which answers our initial question—do mordant methods affect iron's efficacy? The answer is unequivocally "yes."

For the swatches dyed with osage orange, the iron affects not only the depth of shade but also the hue. The swatches mordanted with tannin are darker and olive green whereas those mordanted without tannin are lighter and a pale, dusty yellow.

This is a similar result for the madder dyed swatches, although the difference in hue is subtler.

Here are our conclusions from this experiment, outside of answering our initial question:

(i) tannins are necessary to develop darker shades with iron, particularly army and olive greens that begin with yellow dyestuffs;

(ii) tannins create brighter, more saturated colors, which we anticipated for cellulose fibers;

(iii) mordanting with calcium carbonate, though cheaper and faster than mordanting with tannins, produces softer results that are ultimately less light fast due to the absence of tannins;

(iv) silk, which can generally be mordanted the same as cellulose, seems to be affected by the presence of tannins, likely due to increased acidity; and

(v) wool is also impacted by the presence of tannins, though we currently lack a hypothesis as to why.