Iron (Ferrous Sulfate) Mordant

I have always had mixed feelings about using iron as a mordant when dyeing.  Partly because it must be handled in such a way that the yarn/fabric doesn’t contaminate other yarn/fabric with loose iron before it is thoroughly rinsed and also because there is literature that indicates that yarn/fabric is damaged by iron.   On the other side of the equation, is the idea from Charlotte Kwon that even small amounts of iron will help with lightfastness.  I used 2% wog iron in the  logwood experiments so as to mitigate the lightfastness issues with logwood but felt that I needed to have a better understanding of iron used as a mordant.  Damage in historical textiles with gallic/iron pigment and/or tannin/iron dyes has been documented.  Go to your local museum, look at historical textiles dyed with natural dyes, and see how well the blacks are holding up in the textile.

I did some research and came across the following information specific to historical textiles.   Tannin based dyes + iron will damage textiles due to oxidation and/or the formation of sulphuric acid. The sulphuric acid is formed with unattached iron.  If the textiles are thoroughly rinsed, this is less of an issue as the unattached iron is washed away.  It is also possible that some of the damage is due to high temperatures used in dyeing.  Analysis cannot differentiate between heat versus dye damage.

The Colourful Past: origins, chemistry and identification of natural dyestuffs  discusses degradation of silk due primarily to the lack of regulation of silk dyeing. More iron was used in dyeing silk to get dark colors and the silk may not have been well rinsed.  Wool dyeing was regulated; particularly with regard to the amount of iron that could be used and the dye process quality.

Other sources conclude that tannin/iron damage is more of an issue with cotton, linen, and silk because the dye penetrates the whole fiber.  The dye and iron are found through out the matrix.  With wool, the dyes don’t penetrate the interior of the fiber.  If one rinses thoroughly to get rid of any loose iron, then it is possible it will not be an issue.

I came across another article in Dyes in History and Archaeology – Volume 16/17.  The bottom line in this case is that iron and tannins cause the degradation.  Iron used with dyes that do not contain tannins (madder for example) should be stable. The same paper suggests that the post treatment of an historic textile by an extract from the hinau tree which is a gallotannin will cause the binding of the loose ferric ions to the tannin and stop the degradation.

Cardon says something similar – she suggests that the small amount of iron used with reds could also be a factor in the lack of degradation whereas more is used to get black.

Conclusions?  There hasn’t been a lot of research and tannin/iron dye degradation.   The research does suggest that if the yarn/fabric is thoroughly rinsed and the iron % is not high, it is not an issue.  It also suggests that iron can be used safely with non tannin based dyes.  I did not find any studies on lightfastness with iron mordants; I imagine that is the next area of exploration.  This is all book theory.  If one is not creating art work for posterity, it is non-issue.  If one is, more tests are in order.



3 Responses to “Iron (Ferrous Sulfate) Mordant”

  1. William Bailey Says:

    In French, saddening colors with iron and tannin is called “bruniture”. I have gotten some great colors with this technique.

  2. William Bailey Says:

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