bard graduate center residency

Through July 1st I'll be working in the beautiful TAC Makerspace above the Bard Graduate Center Gallery, grinding homemade lake pigments and creating experimental monoprints from them. Stop by to visit during open studio hours 11:30-4:30 Tuesdays and Wednesdays:

TAC Makerspace @ Bard Graduate Center Gallery
18 West 86th Street
New York, NY 10024

This week I'm testing botanically derived pigments on paper, and looking forward to pushing my craft into unknown territory!

stone patterns from lake pigments

indigo as pigment

Straying a bit further afield from the recent posts on alum and the quantity of alum used in paper marbling, let's address the question obliquely raised in the last post on lake pigments:

Indigo is a pigment?

Many of us natural dyers are well familiar with the process of concocting an indigo vat and the working properties of indigo as a dye. It is truly remarkable that so many historical cultures throughout the world developed a practical understanding of the complex chemistry governing the indigo vat, leaving us with a wealth of cultural and artistic heritage centered on this beautiful dye. But indigotin, the indigo blue which colors fibers dyed in the indigo vat, is an insoluble organic compound and can be used as a pigment. It may be mixed with a binder for use as paint without going through the trouble of making an indigo vat. Let's review the process by which indigo is made:

  • Indigo precursors naturally occur in the leaves of an indigo-bearing plant. After harvesting, these precursors are converted by enzymatic hydrolysis to indoxyl.
  • When oxidized, two indoxyl molecules join with oxygen to form indigotin.
  • Indigotin is indigo blue! Indigotin is insoluble, and can be used as a rich navy blue pigment.
  • Natural dyers now mix up their indigo vats to reverse the process. To work as a dye, an antioxidant reduces indigotin to leuco-indigo suspended in water. Fibers lowered into the vat become saturated with leuco-indigo, which spontaneously oxidizes to indigotin when lifted from the vat and exposed to air.

It's quite the feat of organic chemistry, but the peculiar dual nature of indigo provides its distinction as an artists' material. Used as a dye, indigo's working qualities have inspired resist techniques in cultures around the world. After dyeing, the indigo is not chemically bound to fabric and can be abraded from it with much washing and wearing, giving us the particular worn-in look of denim. And because indigo is inert when oxidized, it has better lightfastness that other natural dyes.

grinding indigo pigment
mulling indigo pigment

Sources of indigo pigment

In medieval Europe, indigo was prepared as a pigment by skimming and drying the flower from the surface of a woad vat, called blue florie, or grinding white lead with imported indigo. Indigo was widely available, fairly inexpensive, and in common use as a workaday blue pigment. It is a strongly tinting, dark, and slightly greenish shade of blue. Indigo is certainly a regal dye, one of the few lightfast historical grand teints. But as a pigment, its lightfastness did not match the more permanent and expensive mineral blues.

For use as a pigment today, pure indigo can be finely ground and mixed as watercolor, tempera, or oil paint. If you're a dyer, you can use the botanical indigo powder you already have on hand. You can also purchase the genuine botanical pigment from Kremer,* from Cornellisen in the UK, or you can buy Genuine Indigo paint pre-made by a reputable paint manufacturer.

Synthetic indigo was formulated in the late nineteenth century and is chemically identical to indigotin. Slight color variations between synthetic and botanical indigo are due to additional organic compounds extracted from different species of indigo bearing plants, including the colorant indirubin, which influence the shade of blue. Either botanical or synthetic indigo can be used as a pigment, but not indigo dye sold 'pre-reduced.'

*or get sidelined by Tyrian purple, the other famous vat dye with similar chemistry

 homemade indigo and maya blue watercolors

homemade indigo and maya blue watercolors

What is Maya blue?

Maya blue is an indigo derived pigment used by ancient Mayan and Aztec artisans to color murals and ceramics. This pre-Columbian azure colored pigment is so stable and lightfast it was long thought to have a mineral origin. It is made by heating a finely ground dry mixture of indigotin and clay, either palygorskite, attapulgite, or sepiolite, to 356 degrees Fahrenheit. At this temperature the indigo is sublimated as a gas and replaces water in the clay, becoming a stable compound. Once cooled, Maya blue pigment can be mixed with a binder and used in all sorts of applications. Given that Maya blue requires a small amount of indigo, is lightfast, and is a lovely turquoise shade, it is superior to pure botanical indigo as a pigment.

I have only found Maya blue pigment available from Rublev, but it is not a difficult pigment to make at home.

How do indigo and Maya blue fare in paper marbling?

Below are the results of some preliminary marbling experiments with homemade indigo watercolor and Maya blue watercolor.

  • Top: indigo watercolor
  • Middle: Maya blue watercolor
  • Bottom: both shades of blue
  • Column A: no mordant
  • Column B: pre-mordanted with 1.5 teaspoons aluminum sulfate/pint water
marbling tests with indigo pigments

I did struggle with getting the Maya blue to behave, though it functioned well in previous trials alone and mixed with lake pigments. The indigo blue has a beautiful rich hue, and will be useful for mixing dark shades.

As you can see in the right column, the pre-mordanted paper takes a crisper print with less streaking. The question of mordanting for indigo pigments was raised in the wonderful book Marbled Paper: Its History, Techniques, and Patterns:

Why, it may be asked, can some colors (such as indigo) derived from non-metallic materials, seemingly be employed without the use of a mordant? The answer can only be advanced in stages. To begin with, indigo, like the lake colors, which it resembles in several ways, has a low specific gravity. Secondly, it is likely that in its earlier, unsynthesized state, indigo contained minerals and metallic traces. There is good reason to suspect that somewhere during its manufacturing process or final preparation, alum was added, because earths, presumably containing alum and mineral salts, such as chalk, were often added as extenders to cut costs. But, most of all, indigo (like a few other dye-bearing plants) was insoluble in water, and had the ability to fix itself to an organic fiber without the aid of a mordant.
— Richard J. Wolfe

We know that historical paint manufacturers (and maybe some today) stretched or lightened indigo with the addition of various earths, inadvertently making a paint useful for marbling without a mordant. My homemade indigo paint is concocted of pure botanical indigo powder, which apparently does not have enough trace minerals to work quite so well without the help of a mordant.

However, Maya blue is useful for marbling without a mordant due to the chalk in its formulation. It is a lovely (though faint in this test), stable color which European marblers never had access to until the recipe was reverse-engineered in recent years.

Update: After 1 month of exposure to direct sunlight, no fading has been observed in either indigo or Maya blue watercolor samples on unmordanted paper. Lightfastness concerns about indigo pigment can be reserved for very long term light exposure.

alum & lake pigments

In the last post, we discussed the quantity of alum necessary to marble with watercolor and acrylic pigments on paper. There is another class of pigments that can be used for marbling, and was historically an important source of particular hues. These are lake pigments, made from natural dyes. As both a marbler and a natural dyer myself, these pigments are of great interest.

What are lake pigments?

Natural dyes (with the exception of vat dyes including indigo) are water soluble. This means they cannot be used for marbling, as they simply dissolve into the size. In order to make these beautiful organic colors, many of which have no counterpart among mineral colors, useful for  marbling they must be converted into pigments. A lake pigment is made when dissolved dye is precipitated onto an inert substrate - often potash alum. The precipitate is then filtered, washed, and ground. For marbling it can be mixed or mulled with a binder, usually watercolor medium, and used like any other watercolor paint.

If you've ever used the all-in-one dyeing method of mixing mordant, dye, and fiber in one pot, you may have noticed some of the dye spontaneously settling to the bottom. This means the dye and alum have bonded together, making a lake pigment.

mulling maya blue watercolor

What role does alum play in lake pigments?

As we learned about alum, it readily bonds to dye molecules, making it a substrate well suited to the formation of lake pigments. Other mineral mordants can function the same way, such as tin and copper salts. Each of these mordant substrates acts to stabilize the dye compound, improving its lightfastness. And just as with natural dyes applied to fabric, the choice of mordant and pH can strongly affect the shade of a lake pigment.

The example below is a lightfastness test of homemade lake pigments (yes, I need to practice patience in grinding pigments). From the top, madder lake with copper, weld lake with a little indigo, and maya blue pigment, all mixed as watercolors. The left side was exposed to direct southerly sunlight for one month, and the results I think are quite promising. Unfortunately lake pigments suffer from a bad reputation because they are not as permanent as their modern synthetic replacements. Some lake pigments are certainly fugitive, and care should be taken to keep artwork made with them out of direct sunlight. The most lightfast pigments are made from the most lightfast dyes, such as madder, buckthorn, and weld.

 Lightfastness test of madder lake, weld lake, and maya blue pigment. Left side exposed to sunlight for one month.

Lightfastness test of madder lake, weld lake, and maya blue pigment. Left side exposed to sunlight for one month.

Marbling with lake pigments

Because lake pigments contain a mordant within them, paper does not need to be pre-mordanted before marbling! I have read this in multiple sources, and the test below confirms it.

  • A: Unmordanted paper
  • B: Paper pre-mordanted with 1.5 teaspoons alum/pint water

The mordanted paper absorbs the marbled print a bit more quickly, but no difference in the quality of the pattern was observed.

Lakes are prepared for marbling just like any other watercolor pigment: each color is thinned to a good consistency and mixed with the appropriate amount of surfactant. You don't need to go through the trouble of grinding your own pigments to enjoy working with lakes, any 'genuine' watercolor lake such as rose madder or carmine will work. It can be difficult to know just what manufacturers put in their paints, and whether they are synthetic pigments simply bearing the names of historical shades, so it's worth seeking high quality paints if you want the real thing.

testing alum mordant with lake pigments

In addition to lake pigments, I've read that any genuine earth pigment may be used for marbling without a mordant, as earth pigments are composed of minerals. It is clear from natural dyeing that ferrous salts are a very effective mordant, so the iron present in earth pigments should work the same way. I hope to test this in future, and make a comparison of the lightfastness of these samples.

Update: After 1 month of exposure to direct sunlight, no fading has been observed in samples printed on mordanted and unmordanted paper. The alum and chalk present within the lake pigments appears to stabilize the dye colorants admirably.

Please feel free to contact me with any insights or sources for historical pigments!

alum quantity

When preparing paper for marbling, it is pre-mordanted with an alum solution. We discussed the role and varieties of alum in the last post, and let's continue with the question:

How much alum to use?

Marbling instructions vary greatly in the strength of the alum solution recommended to mordant paper. I've seen measurements ranging from 1.5 teaspoons to 5 tablespoons per pint of water. Let's investigate just how much is enough to make a successful print.

What can go wrong?

If too little alum is used, marbled designs made with watercolor or acrylic pigments cannot bond to the paper. Western-style papers used in the European marbling tradition are sized to improve their durability and slow their absorbency. Alum bonded to the surface of mordanted paper is the chemical helping hand essential for linking marbling pigments to the paper fiber, and stabilizing the colors for lightfastness in the long term. Any marbler who has accidentally printed on the unmordanted back side of a sheet is familiar with the disappointment of watching their design wash down the drain with the rinse water, having no alum to hold it fast.

However, if too much alum is used, crystals may build up on the surface of the paper without actually adhering to to the paper fibers. In this case a bit of the excess alum will dissolve into the size as each print is made. If the size becomes polluted with alum, it causes the paint to clump together in a very frustrating way, and must be replaced with fresh size.* If you can feel a buildup of powder on the surface of your paper or see white streaks on colored papers, you are using too much alum.

Additionally, alum is acidic, and using a lot will both waste alum and make the paper overly acidic. Over time, this causes the paper fibers to become brittle and crack apart.

 Watercolor Pigments

Watercolor Pigments

Testing Alum Quantity

These samples, excepting sample A, were coated with a wash of aluminum sulfate dissolved in warm water:

  • A no mordant
  • B 1.5 teaspoons/pint
  • C 1 tablespoon/pint
  • D 2 tablespoons/pint
  • E 3 tablespoons/pint

As can be seen in both the watercolor and acrylic tests, unmordanted sample A is comparatively very pale. This was not a surprise, as it has no mordant at all. I was surprised, however, to find no discernible difference the in strength of the colors in samples  B-E. Any quantity of alum from 1.5t - 3T per pint acts as a successful mordant.

 Acrylic Pigments

Acrylic Pigments

My recommendation: 1.5 teaspoons alum per pint water

I will continue using 1.5t/pint to mordant papers in future, the amount recommended in the first marbling class I took with Stephen Pittelkow. As alum is slightly acidic, I prefer to put as little as needed on my paper to get the job done. Pre-nineteenth century marblers used potash alum, but nowadays you can mordant with either potassium aluminum sulfate or aluminum sulfate - just mix it with warm tap water to ensure the crystals are fully dissolved. Some marblers soak their paper in a mordant bath, some apply the alum solution with a natural sponge or a spray bottle; I use a wide foam brush. As each sheet is coated I flip it over and pencil an X on the back. The whole stack of wet papers is weighted with a heavy board to stay flat. They can be printed in about half an hour when still slightly damp, or kept up to 4 days. I prefer to print papers within one day of mordanting.

If you've prepared your papers with mordant but the marbled design doesn't adhere, you may be using archival paper coated with calcium. Choose a different paper stock and try again.

*Of course, any mistake can become an interesting pattern when repeated intentionally! Mixing a little bit of alum into your gall water or one color of paint can give an intriguing, crunchy texture to a marbled design.

on alum

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.

marbling tools.jpg

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.
alum solubility chart

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.