This will probably be much more than you wanted to know, but that’s the way it is for me once I get started on something I find interesting
The various kinds of plant pigments differ in their solubilities in different kinds of liquids, which can include “organic solvents,” alcohols, oil, water. They also vary in stability as a function of light, pH and other variables. It’s definitely complicated and many doctoral dissertations and scientific papers have been written on plant pigments and dyes by scientists in the dye, food and wastewater treatment fields.
While chemically synthesized pigments are comprised of the purified compound (just the molecules of the pigment), dyers have a long history of finding ways to extract naturally occurring pigments from various plant materials to dye fabric. My kitchen is not a laboratory and my methods are crude, so anything I’m extracting in a jar in my kitchen or in a pot on the stove is going to be a mix of plant pigment molecules and other things that are soluble in alcohol and/or water. I also end up with very fine to coarse plant particles that don’t dissolve, which I can filter out, let settle to the bottom of a jar, or include (if they’re not scratchy!).
As soon as I can pull myself away from making soap I’m going to work on a reference table that I will post for anyone who might be interested. I got started down this road because of the color changes with chlorophyll and because I had no luck whatsoever getting my madder powder to infuse into oil. While some pigments, like the carotenoids in annatto seeds and paprika, seem to be readily soluble in oil, my madder powder hasn’t noticeably given up any color to the oil it has been sitting in for the last couple of weeks. Based on observations only, chlorophyll, which is a primary pigment in green plants and algae, seems to be at least somewhat oil soluble. But, unlike many of the carotenoids, it is a very unstable molecule. Soapers know that because the green colors fade or turn brown. For that reason, it’s probably better to use plant material ground into a very fine powder to get the green color. I also have an idea about trying to protect very finely ground plant material a bit from the ravages of lye by soaking it in a glycerin and alcohol mixture and then evaporating off the alcohol before using the glycerin infused powder in a recipe. Another experiment in the making! The primary pigment in Spirulina is phycocyanin, which I have not had a chance to research.
Indigo dye is especially challenging because it is insoluble. Dyers have to use a special process to make it “stick” to fabric. I just recently found a great article on using indigo in soapmaking. It’s by Kevin Dunn and is available on the WSP website,
here. This is the key part as it pertains to using indigo in soap.
“Indigo is sold in two commercial varieties. The traditional one is a dark blue powder that is insoluble in both oil and water. It can be mixed with a little oil to make a slurry and then added to your oil as with any other solid soap colorant. The newer variety has been pre-treated with lye. While it is sometimes described as “soluble” or “pre-reduced,” it is actually neither of those in the strict chemical sense. It forms an opaque suspension in water (not a transparent solution), and may be added to your lye portion. Either variety may be used in soap, but they should be used sparingly to avoid staining hands, sinks, and countertops. One gram of indigo per kilogram of oil produces a dark grayish-blue color in cold process soap.”
I’ve had some success using indigo from Brambleberry, which I ended up mixing with oil because it really did not want to mix with water. I have not tried adding it to my lye water.
