on alum

Natalie Stopka, April 2018

I’ve recently undertaken a series of tests, as scientific as I can make them, to answer some questions I commonly receive from students and satisfy my own curiosity.

Firstly: What is the difference between the various types of alum?

If you have done any natural dyeing or marbling, you have most likely used alum as a mordant. Alum is a metallic salt which bonds to both the fiber substrate and the coloring agent, creating an insoluble bridge linking them together. Whether you are using dyes or paints on fabric or paper, an alum mordant will increase the amount of color that can latch on, and improve the washfastness and lightfastness of the finished piece. Alum is the most commonly used mordant because it is inexpensive, colorless, permanent, and toxic only in large doses.

I, like many marblers and dyers, am in the habit of using the term ‘alum’ to refer to two different compounds: potassium aluminum sulfate and aluminum sulfate. These two metallic salts function essentially the same way for mordanting, but it's worthwhile to consider their differences. I will not include aluminum acetate in this discussion, as it is always specified by name and is not in common use for marbling paper.

Potassium Aluminum Sulfate
Historically called potash alum, potassium aluminum sulfate is a translucent white crystalline powder. It is naturally occurring, and has been extracted from alunite in volcanic areas since at least 1500 BC for purifying water, as a styptic, and a mordant. Today it can be refined from bauxite or alunite, or made in a laboratory by adding potassium sulfate to aluminum sulfate. Potash alum is the historical mordant called for in traditional dye and marbling instructions, and in making lake pigments from natural dyes.

Aluminum Sulfate
Aluminum sulfate can be made in a laboratory or refined from various types of stone. In appearance its crystals are jagged, with an opaque white dustiness. Since its introduction as an industrial product in the 19th century, it has replaced potash alum in many applications such as water purification and paper sizing, and is sometimes called papermaker's alum.

What's the difference?

Potash alum and aluminum sulfate may be used interchangeably as a mordant on both cellulose and protein fibers.
Potash alum is slightly more expensive.
Some dyers claim that potash alum gives clearer colors, but I have not noticed any difference between the two. If you have trouble with muddy colors, make sure your alum is from a reputable vendor. Any impurities, such as iron, will sadden your colors.
If you are forging historical artwork, using aluminum sulfate to mordant is a dead giveaway.
Both potash alum and aluminum sulfate are acidic. Whichever you use, follow mordanting instructions to avoid making your artwork overly acidic and brittle.
Most important for practical purposes: Aluminum sulfate is more water-soluble at room temperature. When mordanting paper, most historic manuals call for heating the water before adding potash alum (they don’t specify which alum, but potash alum is implied). Heating the water will allow the potash alum to dissolve quickly.

I made this chart from what scant data I could find, showing the solubility of both sorts of alum in one pint of water. Paper marblers generally use between 1-4 tablespoons of alum per pint, or about .5-2 ounces. Though both potash alum and aluminum sulfate are soluble at room temperature in that amount, in my experience warm tap water helps to speed up the dissolution of the crystals.

Please be in touch if you have any information to contribute, or reading to recommend! The alums and their history are a big topic.

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