Exploratorium Part II: Color from Natural Chromophores

Matrix of different natural dyes, different
mordants and different natural fibers

A rainbow of colors produced on wool yarn

Using very simple recipes to make natural dyes, like frozen blackberries (mashed, mixed with boiling water, then strained), and pre-treating the natural fiber yarns with chemical mixtures called mordants from easy to find sources (like alum and cream of tartar, common kitchen chemicals, or even just by using particular metal pots for the boiling/treating, like copper, aluminum, tin or chrome pots), we were able to create a stunning array of colors, like the purple in the picture to the left.


Dyes are typically organic compounds that contain double bonds where electrons can be delocalized, such as a benzene ring or nitro group.  When light hits these chromophores, the energy of a specific wavelength is absorbed, and the rest is reflected, resulting in our perception of a colored substance.  Dyes also have chemical groups (usually charged, like hydroxyl or carbonyl groups) that can bind to a substrate material.  These will form ionic or hydrogen bonds with a charged portion of the substrate, such as keratin in wool or cellulose in cotton.


Mordants are chemicals, usually polyvalent metal compounds like various metallic salts that help fix the dye to the fiber.  Mordants for animal fibers (like wool) are alum (aluminum potassium sulfate, AlKSO4), chrome (potassium dichromate, KCr2), copper sulfate (CuSO4), tin (Stannon's Chloride, SnCl2).  Mordants for vegetable fibers involve tannic acid (or some other source of tannin).    The metal cations from copper, aluminum or tin used as mordants have valencies of +2 or +3 to allow multiple electron donors to bond.  The mordant thus acts as a bridge between the dye molecules and the fabric fiber.  Without the mordant, the dye attaches to the protein of the fiber with hydrogen bonds that break easily.  When the fiber is first heated with the mordant, the metallic salt covalently bonds to the fiber protein and then when you add the dye, the dye also attaches to the metallic salt by a covalent bond.  These stronger covalent bonds allow more dye molecules to attach, resulting in a deeper, longer-lasting color.  The mordant used effects the color achieved with the dye (the same dye will produce different color results with different mordants).  Fastness to washing, light and perspiration totally vary depending on dye used, fiber type, and mordant used.  


Some of my favorite combinations that I did: Cu mordant+carrot-top dye->light green, Al mordant + beet dye->burnt orange, Sn mordant + onion dye->bright yellow, no mordant + indigo->deep blue, Sn mordant + blackberry dye->purple.

The basic solutions with red cabbage indicator juice.

The acids with red cabbage indicator juice.

Some indicator papers made with coffee
filters soaked in various pH sensitive
chromophores (yellow=tumeric, reddish
pink=rose petal, reddish purple=red
cabbage, bluish purple=blueberries)

Anthocyanins are pigment molecules found in vacuoles of plant cells, like red cabbage, violets, blueberries, cranberries, blackberries, concord grapes, that give them a deep red, purple or blue color depending on the pH of their surroundings (redder as environment becomes more acidic). Most store-bought pH indicators change color because of the gain/loss of a H+ which changes the wavelength at which the indicator pigment absorbs and reflects light. Anthocyanins change color because of gain/loss of a hydroxyl ion, OH-, which changes the structure and thus the absorption/reflection of light depending on the acidity of the environment. In their normal function, anthocyanins are thought to act as a 'sunscreen' for the photosynthetic cells of the plant, but absorbing blue-green and UV light (often seen in young trees, shrubs, buds). The reds of autumn leaves are anthocyanins that are synthesized when the plant begins breaking down chlorophyll. Anthocyanins also fluoresce. To extract the anthocyanin from the vacuoles of red cabbage to use as a pH indicator, blend the cut up leaves and strain the juice or just simmer the cut up cabbage in water for 5-10 minutes and strain off the rich purple liquid.  You may need to dilute the dark color of the juice.  Seen in the pictures to the left, we tested the pH of various household items (ammonia, baking soda, vinegar and lemon juice) with the cabbage juice and regular pH indicator strips.  Ammonia at a pH10.0 gave a deep emerald green when mixed with the cabbage juice.  Baking soda at a ph8.5 gave a cyan.  The control cabbage juice was purple in color and had a pH6.5.  Vinegar at a pH3.0 gave a deep red color with the cabbage juice indicator.  Lemon juice at a pH2.5 gave an intense magenta.  A beautiful range of color from the simple cabbage!