Dyes are used in papermaking to provide the right tint of paper or to produce coloured grades, as measured by the L*a*b* value.
Besides carefully regulating the dosage of dye to achieve the target colour, it’s important to choose a dye that doesn’t bleed, doesn’t vary with pH or heat, is safe to handle, is in compliance with regulations (e.g. for food contact grades) and doesn’t fade on exposure to light (called lightfastness). You’ve probably noticed when taking down artwork made with construction paper that the colours fade, even indoors. The lightfastness of printing inks is also important for printed products such as posters, billboard ads and other printed products exposed to sunlight. You may have also noticed that red inks fade faster than blue or yellow ones as shown in this poster for Canadian Pacific. This is because red dyes absorb short-wavelength, high-energy light in the UV region much better than other colours such as yellow and blue, and the absorbed light can accelerate the breakdown of many dye molecules. The history of developing lightfast red dyes and inks has a fascinating history going back over two thousand years, as I recently learned while travelling in Peru!
On a hike in the Colca Canyon, our local guide pointed out what looked like white powder on prickly pear cactus pads near the trail. In fact, this type of damage is caused by a small insect called a cochineal, a parasite that lives on the surface of the pads. The adult females, about 6 mm long, use their mouths to hook in and hold onto the pads, staying there for the rest of their lives (3 to 4 months), eating, ballooning in size and producing eggs while producing a whitish-grey wax that coats their bodies and prevents them from drying out. Their bodily fluid, called hemolymph, is a rich crimson, consisting of ~20% carminic acid, responsible for the red color. To demonstrate, our guide picked off one of the insects and crushed it in his hand, producing a red stain.
Later that day, we passed through a village where harvested cochineal insects were being dried on a mat outdoors. It takes about 150,000 cochineals to produce 1 kg of red dye. Peru is currently the world’s biggest producer, and others include Mexico and the Canary Islands. Many other countries have tried and failed over the last few centuries, because cultivating cochineals is very difficult and labor-intensive. Furthermore, the whole cochineal industry was almost wiped out by the invention of synthetic dyes in the late 19th century.
Indigenous people in Mexico and Peru cultivated cochineal long before Spain made it a global commodity in the 16th century, importing it from their Spanish colonies and using it to dye textiles mostly for aristocrats, since it was too expensive for the common people. Prior to importing cochineal from the Americas, Europeans had other sources of insect-based red dyes in Europe, including “St. John’s blood” and “Armenian red”. But it was discovered that cochineal had several advantages over these two sources. First, the Armenian red insects contained up to 50% lipids: oily, fatty, waxy compounds that melted in the dye vat and coated the fibers being dyed, interfering with absorption. Second, cochineal could be much more efficiently harvested, several times a year instead of annually. Third, cochineal’s colour was much more potent: up to ten times as much dye per ounce as St. John’s blood and 30 times more than Armenian red! Furthermore, cochineal is one of the few water-soluble colorants that can resist degradation over time. It is one of the most resistant of all natural colorants to light exposure and more stable than many synthetic colorants. Rembrandt used cochineal in his paints, producing reds that have not faded in over 300 years.
In the 1850s, William Perkin, based in the UK, discovered a purple dye that could be made synthetically. Based on his discovery, over the next few decades a whole new industry developed, mostly based in Germany, to make synthetic dyes that replaced more expensive natural dyes. In the 1870s and 1880s, there were reports of rashes by wearers of garments containing these new dyes, and this was traced to the use of arsenic in the production process, but better washing was able to remove the arsenic. The market for cochineal red became a smaller specialty market for certain users in food, cosmetics, medicine and art for the next few decades. However, beginning in the 1970s, it was discovered that certain synthetic dyes, especially those derived from coal tar, were carcinogenic, and this led to a renaissance in cochineal production in the 1990s. Even though the structure of carminic acid is now known, it is more expensive to synthesize it than to collect it from cochineals in the same way that it’s been done for two millennia.
By the way, you’ve probably eaten cochineal, since it is used as a food additive, where it’s labeled “carmine,” “cochineal extract,” “E120” or “natural red 4.” Today’s product is well-filtered during processing to remove any insect parts.
The fascinating story of cochineal-based trade, chemistry and politics is recounted in the book “A Perfect Red”, published in 2005. I highly recommend it if you want to learn more!
Martin Fairbank, Ph.D. Martin Fairbank has worked in the forest products industry for 31 years,
including many years for a pulp and paper producer and two years with
Natural Resources Canada. With a Ph.D. in chemistry and experience in
process improvement, product development, energy management and lean
manufacturing, Martin currently works as an independent consultant,
based in Montreal. He is also an author, having recently published
Resolute Roots, a history of Resolute Forest Products and its
predecessors over the last 200 years.
Martin Fairbank Consulting
Industry Experience
- Pulp and Paper Technology
- Materials Recycling
- Biorefinery Development
- Manufacturing
- Government Subsidy Programs
Services
- Technical Writing
. White Papers
. Grant Applications
. Explain technical concepts - Scientific Editing
. Review of articles for publication - Project Assessment
. Evaluation of Technologies
. Project evaluation for funding agencies - Pulp & Paper
. Conventional and emerging technologies
