On the various eco-printing and natural dye fora there is a lot of confusion and mis-use of terminology going on when it comes to the processes of mordanting fabrics.
In short: Mordanting and tanning improve light-, wash-, and rub fastness. Mordanting is done with inorganic materials, and tanning is done with plant-based materials with a high percentage of tannic acids from different tannin groups.
Why do we need to be modern?
Natural dyes are divided into substantive (direct) and adjective (indirect) dyes. When a dye is not substantive, it means it needs a link (a glue, if you will) to connect it permanently to the fiber.
Substantive dyes will connect to the fiber without a mordant, but not all these dyes are lightfast (turmeric would be a great example), and a mordant is needed to improve light fastness and wash fastness.
All dyes (both substantive and adjective) get improved light fastness, rub fastness, and wash fastness from using a mordant before dyeing.
Examples of substantial dyes that do not need a mordant to dye fabrics;
Turmeric
Indigo (a separate category, needs to be used in a vat form)
Black Walnut Hulls
Often these dyes in itself possess a high amount of tannin.
All other dyes will need a mordant to connect permanently to the fabric.
What is a mordant?
Thessaurus says; a substance used in dyeing to fix the coloring matter, especially a metallic compound, as an oxide or hydroxide, that combines with the organic dye and forms an insoluble colored compound or lake in the fiber.
Or in simple words; a mordant is a metallic salt that functions to link between a dye and a fibre. The word mordant comes from the Latin word 'mordere' which means 'to bite'.
Inorganic mordants are;
Ferrous sulfate (often called 'iron') FeSO4
Aluminium sulfate Al2(SO4)3
Aluminium acetate (for cellulose fibers) Al(CH3CO2)3
Potassium aluminium sulfate (called Alum) KAl(SO₄)₂
Copper sulfate (often called 'copper') CuSO4.5H2O
Titanium Oxalate C4O8Ti
Stannous Chloride (Tin) SnCl₂
These are the inorganic mordants I use myself, in my home studio. There are more mordants available, and used in old dye books I can not recommend using as a hobby dyer because of toxicity, such as chrome and lead.
Tannins
The application of tannic acid/tannin to textile material is also called tanning. Tanning is used in the dyeing of adjective dyes with cellulosic fibers as part of the mordanting process, and also in the mordanting of silk fibers. Tannins are always organic material from plants, and their structure is very complex.
Sometimes you will find a tannin sold as 'tannic acid', this may be plant-derived but could also be synthetic. In both cases you should stay clear away, it can be very toxic and there are plenty of tannins with a good pedigree you can use. (Thank you Caroline Nixon for pointing this out.)
Gallic tannins. Clear tannins that do not add much color to the fabric;
Sumac (leaves, galls)
Ellagic tannins. Tannins with a lot of flavonoids will add a yellow color to the fabric;
Catechic tannins. Condensed tannins that will add brown and reddish hues to the fabric;
Black tea
Quebracho
Chestnut bark
See all the mordants and tannins available in the shop here.
Proper mordanting and tanning
Proper mordanting and tanning takes time, it can't be done doing a 'dip' as some people call it when eco printing. Example: If you are using a ferrous sulfate solution and merely dipping your fabric in it, what you are doing is using the ferrous sulfate as a modifier that combines with other mordents and tannins, but the fabric will not be properly mordanted, resulting in disappointing results over time. This is why your iron dip blankets that looked so stunning when you unbundled, washed out like nothing good ever happened!
In the above picture, Rita Trafford sent me, you see the difference. Left was done with a properly mordanted blanket, right was merely a quick dip.
A good mordanting/tanning process starts by measuring the proper percentage according to the Weight Of Fibre (WOF). I understand it sounds romantic to rust your own metal scraps in vinegar and water, but besides the health hazards of rust (tetanus!) you will never know how much ferrous sulfate is present in your rusty water. Knowing that you only need 1% WOF ferrous sulfate to make an effective iron blanket or mordant, purchasing 100 grams of this will make your work so much more precise and you will prevent textile rot.
Iron causes acid deterioration and catalyzes the photo-oxidation of the organic materials in textiles, resulting in significant or complete loss of strength over time.
In this piece of textile from the Israel Museum in Jerusalem you can see exactly what happens when a large WOF of ferrous sulfate is used in dyeing (in the 'old' days this could be up to 50%).
The dark pieces with ferrous sulfate (to make black), look like they have been eaten away by a mysterious bug, but it's sheer deterioration.
A complete step-by-step guide to properly scour and mordant your textile before dyeing and eco-printing with precise recipes can be found here.
In these marigold dye samples, you can see the effect the different mordants have on the outcome. Mordants each have a very typical effect on dyes. These samples were mordanted with (FLTR) nothing, alum, copper and iron.
