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About Mordants
by Michele Wipplinger
Earthues, A Natural Color
Company
5129 Ballard Avenue NW
Seattle, WA 98107
Telephone - 206-789-1065
Fax: 206-783-9676
www.earthues.com
Look in any book about natural dyeing and you will see many approaches to
mordanting.  Earthues uses alum, cream of tartar and plant-derived tannins to fix our
cloth and fiber for dyeing.

Most natural dyes need a mordant to fix the color to the fiber and increase lightfastness.  Some common examples of
mordant dyes include cochineal, madder, fustic, osage, and logwood.

Mordant literally means "to bite". The mordant is the chemical link that fixes the dye to a substrate by combining with the
dye pigment to form an insoluble compound. This chemical can be a salt or a hydroxide of aluminum or chromium. And, of
course, a variety of substrates can be mordanted and dyed: textiles, leather, flowers, and wood.

Whether you are an experienced dyer or a beginner, the easiest, least expensive, most available and safest mordant that
you can use is alum. It seems like a simple task to just go buy it and use it. However, you might ask yourself: should I buy
grocery store alum or aluminum sulfate? How are potassium aluminum sulfate and aluminum sulfate different from each
other? And, what is the difference between aluminum sulfate and aluminum acetate? Which is best? How much do I use?
Where do I get them? And which mordant is the best for cotton, wool, silk, hemp or abaca?

Potassium Aluminum Sulfate
True "alums" by historical definition are double salts of aluminum such as potassium aluminum sulfate. However, aluminum
sulfate has come to be known as "alum" also. The difference is subtle but may affect your dye results. Aluminum sulfate is
the result of the refining process of bauxite which is the raw state of aluminum ore. During this refining process sulfuric
acid is used to remove most of the iron and silica present in bauxite. Further purification with potassium yields an
aluminum sulfate with fewer impurities, especially iron which could dull the color on the fiber. Therefore, potassium
aluminum sulfate is the best alum to use for craft dyers and artisans. It is readily available and inexpensive. Iron-free grade
aluminum sulfate is also a good mordant for natural dyeing, although it may be more expensive than potassium aluminum
sulfate. If possible obtain a sample amount and test the mordant. The differences between these two alums are sometimes
unclear because they are not tightly regulated and are used so interchangeably. I once mordanted wool with aluminum
sulfate and the resultant cochineal dyed yarn was quite dull. In the process of solving this problem I studied the Material
Safety Data Sheet and discovered that the aluminum sulfate I was using had iron in it and the potassium aluminum sulfate I
had previously used did not.

Use alum to mordant all protein fibers and cellulose fibers that are classified as leaf (abaca, pineapple, agave, hennequin,
etc.), as well as tagua (corozo) nut.

Other uses for Alum
Alum has some other interesting uses beyond its importance as a mordant for textiles. Both forms of alum are used
extensively in the United States in water treatment plants to remove metals from city and industrial wastewater. It is used in
the paper making industry for sizing, as an agent in leather tanning, as an additive in glue, and as a mild
astringent/antiseptic for skin. It imparts flame resistance and water proofing characteristics to mordanted cloth. Alum is one
of the chemicals used to remove excess nutrients (such as phosphates which encourage the growth of algae) from fresh
water lakes. In the food industry, alum is used in baking powder, and as a catalyst in clarifying sugars and fats.

What Quality of Alum?
Another consideration is the grade or quality of alum to use. The purest and most refined is food grade that you find in the
spice section of the grocery store. It is of approximately the same purity as reagent grade (research quality) alum. Both of
these are expensive choices. However, the Schilling variety is available at midnight when sleep eludes you and the color
carmine does not. Otherwise, use technical grade potassium aluminum sulfate which is the best buy around. There is an
industrial grade available, however, it is the crudest form of aluminum sulfate and contains iron contamination. This cruder
form is what is often found in garden supplies and nurseries. Alum is the most successful mordant for textiles and is
available at any dye or chemical supply house.
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