Today we're going to talk about mordants. It's a huge topic, in many ways it's the very foundation of natural dyeing, so this will be barely the tip of the iceberg, but if you are new to dyeing it is a crucial thing to understand as a mordant should be used for all adjective dyes. What the heck is an adjective dye? Well... it's a dye that requires the use of a mordant. Perhaps it's easier to talk first about substantive dyes, since this is a much smaller category.
A substantive dye is the opposite - a dye that does not require a mordant. There are VERY few of these dyes, the most notable being Indigo. Others are safflower (which has a unique dye method you can read about here), and some tannin-rich dyes such as walnut (and for these you can still mordant to enhance and extend the dye).
So essentially, almost all natural dyes are adjective, or non-substantive dyes. So you need a mordant! And all it is, is a metal salt that is used to fix a dye in the fibre. Without it, your dyed fibres will not be lightfast, or washfast. Natural dyes don't attach directly to the fibres, they bond to the mordants themselves, which is why we apply the mordant first. Simply put, the mordant bonds to the fibre, and the dye bonds to the mordant.
What type of mordant you use, and how much, will have an affect of the final colour of your fibre. In general, less mordant will give a lighter shade, and more will give a stronger shade. But if your aim is a lighter shade, you are better off using less dye, rather than less mordant, since the mordant is generally the less expensive or less precious of the two.
The most common mordant is aluminum, in various forms, which gives a clear, bright shade for most dyes, and are relatively inexpensive and safe to use. There are several different types of aluminum salts, but the most used one is potassium aluminum sulfate, (often just referred to as Alum, but experienced dyers will also use the short form PAS, to distinguish it from aluminum acetate, for example). It is the type that we sell in our shop, and generally one of only two mordants that I use often. It is also an active ingredient in some household items, such as deodorant and baking powder.
The other mordant I use is iron, or ferrous sulfate, which is the same type of iron that is found in an iron pill for folks who are iron deficient. Iron has the effect of darkening or "saddening" the shades, and in some cases shifting the colour all together. For example, the baby onesie above was dyed in a weak cochineal bath and the entire garment was pre-mordanted in alum. I then post-mordanted (as in, mordanted after the dyeing) the bottom half of it in iron, which shifted the shade to purple, and darkened it substantially. In the sample squares in the very first image, the top left square is osage dyed wool mordanted with alum. The bottom left square is osage dyed wool mordanted with iron, which gives a gorgeous olive green, rather than the bright yellow more commonly associated with osage. You only need a teeny tiny amount of iron, as little as 1% WOF, and you want to be careful not to use too much as it can harshen the feel of your fibres, and nobody likes crunchy fibre.
There are a few other types of metal salts that you may see folks mention, such as copper (which has the effect of 'greening" shades) and tin (which has some very desirable properties, such as yielding a bright red with cochineal). Both were used widely in classical dyeing, before synthetic dyes, but are now generally not recommended for home dyers, because of their relative toxicity. They require much greater care in handling and disposal, and with so many colours available with only the use of aluminum salts, I would just not bother.
Finally, and crucially, beware the dye recipe that calls for vinegar or salt as a mordant. These are plain and simply not mordants, and will not act as one. Vinegar is a pH modifier, and can play a role in natural dyeing, but mordanting is not that role.
For me, good old PAS, (Alum, Potassium Aluminum Sulfate, all the same) is really the only mordant I need.