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Above samples are wool mordanted in alum white, Symplocos mid yellow, Rhubarb light brown, undyed as yet. The yellow tints are natural mordants.

are a polarising issue. The article below by Alpenglow yarns published 2014 , sums it up so perfectly that I decided to re-post it in its entirety. The author has a scientific background and supports her claims with references. Rather than being emotional about the issue, she delivers facts. I think this makes it a valuable contribution to the discussion and our to decision whether to use chemical mordants or not.

I really cannot add anything other than- enjoy the read. It may change your mind, it has mine!

The link to this blog was given to me by a fellow Instagrammer @becauseofnature, formerly @miss_maizee, my great gratitude goes out to her!

B x

November 11, 2014

Mordants and Natural Dyeing - The Great Debate

By alpenglowyarn

I’ve thought about writing this post for a while. This topic, more than anything else in natural dyeing, brings up emotion and occasional controversy. My goal with this post is to present some facts, with references to reputable sources that you can check and read further. Natural dye books unfortunately tend to be bastions of misinformation, rife with generalities and opinions that are expressed as facts, and I do not consider the majority of them to be reputable sources of factual information about chemicals or chemistry.

I always kind of cringe when someone asks me about mordants and their toxicity. It’s not because I’m reluctant to talk about it, it’s just that it’s a complex subject and I usually don’t have time at a show or in an interview to address the topic well. It’s a difficult one to answer succinctly. That’s a big reason that I chose to write about it here on my blog, since I have the latitude to write a long post, and you readers are used to me going on for a while. :) In fact, I’m going to apologize for the length of this post right now. It’s a beast, I know. But I intend it to be a relatively comprehensive reference for anyone wanting to learn more about the topic.

If there is a short answer to the toxicity question, it’s something like this: In order to dye a wide variety of colors, it takes chemicals. No matter if the dyes themselves are natural or synthetic, they both take chemicals in order to properly bond. They also both take varying amounts of water. They both use resources and leave a footprint. The choice to use natural or synthetic dyes, or the choice of what mordants to use, is primarily a personal decision. Each person makes a different choice based on their goals, experience, and environment. I do not look at this as there being a right or a wrong choice. My goal is to arm you with knowledge, so that you can choose which chemicals you want to use.

To be clear, the health risks of dyeing are to the dyer, not to the yarn consumer. The final yarn itself is not going to be hazardous. The safety risks are due to prolonged exposure to these chemicals, over time, in large concentrations. Dyers should always use gloves when handling mordants, other chemical assists, and wet yarn. Not only may some chemicals cause irritation, but skin is also porous and can absorb chemicals if not protected. Face masks are also recommended for synthetic dyes, or when handling any chemical that is in a fine powder form, and is easily airborne and inhaled. Also, many mordants form acids when dissolved in water, which can be released in gaseous form when the mordant bath heated. ALWAYS mordant in a well-ventilated area, and use lids on your pots to control fumes.

What Is a Mordant, Anyway?

A mordant is a chemical that becomes part of the molecular bond between the fiber and the dye. Primarily these are metal salts. (They are salts in the chemical sense of the word – the hydrogen atom of an acid is replaced with a metal ion. They are NOT edible salts.) You can think of a mordant as a molecular glue. In general, dyes and fibers have a weak affinity for each other. If you tried to dye yarn without a mordant, the color would be very dull, and it would wash out promptly and fade easily. A mordant sticks to fiber well, and it also sticks to dye well. So you essentially “dye” the yarn first with a mordant, then repeat the process with the dye itself. This results in a strong bond between dye and fiber, which is fast to both washing and light exposure (in varying degrees for various dyes and mordants and combinations thereof). The mordant also affects the final color of the dye. Alum and tin are considered neutral mordants, because the resulting color on yarn is pretty much that of the color of the dye bath. Iron and copper are considered “saddening” mordants, because they make the color both darker and either browner, bluer, or greener.

Mordant 1: Alum (aka Potash Alum)

Potassium Aluminum Sulfate or Potassium Aluminum Sulfate Dodecahydrate K(Al)(SO4)2 or K(Al)(SO4)2 * 12(H2O)

Use in Natural Dyeing:

The most common mordant used. It’s considered a neutral mordant, in that it does not result in a color that is appreciably different than that of the dye bath. It is considered to have good color fast properties, though other mordants result in even more color fast shades. It is commonly used in conjunction with cream of tartar, which is thought to increase aluminum uptake and protect the hand of wool fibers, keeping them soft.

Other Common Uses:

  • additive used in water treatment to cause flocculation of impurities. Alum disrupts the electrostatic charge surrounding fine particles and causes them to clump together. Clumped, these particles are easier to filter out.

  • additive for pickling for improved crunchiness. From what I’ve read, this is largely obsolete with quick-process pickling that is commonly used these days. It is no longer recommended, and is no longer included in USDA pickling recipes.

  • the acid in some baking powder to cause CO2 to form. “Alum” is referred to frequently as an ingredient, though sources that use the full chemical name typically refer to sodium aluminum sulfate, not potassium aluminum sulfate.

  • sometimes added to flour, sugar, or salt as an anti-caking agent

  • the crystal in crystal deodorants. This is usually pure potassium aluminum sulfate.

  • astringent in styptic pencils to stem bleeding.

Hazards of Potassium Aluminum Sulfate:

Alum is generally considered the least toxic, or even a non-toxic mordant because it has long been used an additive to both foods and drinking water. Potassium aluminum sulfate does not even have a regularly published LD50, meaning an incredibly high dose would be needed to cause death. However, it does form weak sulfuric acid when dissolved in water. When the water is heated (during the mordant process), this can result in acidic fumes which are corrosive, and irritating when inhaled. Always keep a lid on a hot mordant bath. Moisture from bare skin can cause more concentrated sulfuric acid to form on contact and cause chemical burns, always wear gloves and handle crystals with utensils. Potassium aluminum sulfate is also corrosive to many metals. While it may slightly corrode aluminum pots as well, I always mordant in an aluminum pot to prevent any iron contamination that could occur when using a steel (yes, even stainless steel) pot.

Hazards of Aluminum:

In a World Health Organization report in 2003, they considered studies on the link between aluminum in drinking water and the onset of Alzheimer’s Disease to be conflicting and inconclusive, but recommend minimizing aluminum levels in finished water. A 2011 IJAD article (referenced below) states that aluminum is a widely recognized neurotoxin, and there is increasing evidence that it has a role in the onset of Alzheimer’s. The EPA only lists aluminum in a secondary guideline for recommended maximum contents in drinking water, as 0.05 to 0.2 mg/L (or ppm).

My Conclusion:

Though there is no caluminum crosses the blood-brain barrierlearly demonstrated causality between aluminum and Alzheimer’s, there is certainly evidence that and alters cognition in an undesirable manner. Because of this, I do not consider alum to be a benign chemical. I always handle it carefully (with gloves) both in its solid form and once it’s dissolved in water. I also always mordant outside, and use a lid.


  • International Journal of Alzheimer’s Disease, 2011,

  • Alum on Wikipedia,

  • Clarifying agents or flocculants on Wikipedia,

  • CDC on Water Pre-treatment,

  • WHO on Aluminum in Drinking Water,

  • EPA on the History of Drinking Water Treatment,

  • Alum MSDS, Pro Chemical and Dye,

  • USDA Complete Guide to Home Canning Guide 6,

  • EPA on drinking water contaminants,

Mordant 2: Iron (aka Copperas or Green Vitriol)

Ferrous Sulfate or Ferrous Sulfate Heptahydrate FeSO4 or FeSO4 * 7(H2O

Use in Natural Dyeing:

Iron can be used as a mordant on its own, but it’s generally used as an afterbath, to modify color dyed on fiber that was initially mordanted with alum. It “saddens” color, making it more greenish-brown. Yellows become olive, and pinks become plummy purples. Protein fibers like wool are very sensitive to iron, and too high of iron concentrations or exposure for too much time can damage the fiber and/or make it have a harsh feel. This is why iron is generally used as an after-bath step, to modify color that has already been dyed using alum as a mordant. Exposure can be easily controlled, and the color is already fixed to the fiber with the alum mordant.

Other common uses:

  • dietary iron supplement, used to treat anemia

  • used in combination with tannins to produce historical inks

  • flocculant for treating synthetic dye wastewater

Hazards of Iron and Ferrous Sulfate:

Iron is a main component of the hemoglobin in our blood and necessary for life. Because of this, the body has mechanisms to absorb iron from food, and thus it’s actually possible to overdose. Somewhat paradoxically, this makes it more toxic than many other metals found in mordants, like aluminum or tin. Children are especially susceptible to iron overdoses, and there have been documented cases of accidental death due to them eating adult iron supplements. The EPA only lists iron in its secondary guidelines for contaminants in drinking water (non-enforceable), recommending a maximum of 0.3 mg/L (or ppm). Ferrous sulfate forms weak sulfuric acid when dissolved in water. When the water is heated (during the mordant process), this can result in acidic fumes which are corrosive, and irritating when inhaled. Always keep a lid on a hot mordant bath. Moisture from bare skin can cause more concentrated sulfuric acid to form on contact and cause chemical burns, always wear gloves and handle crystals with utensils.

My Conclusion:

Iron should be used and stored with caution, especially if there are children in the household. I treat it cautiously like any chemical, I wear gloves when handling it and keep hot mordant baths lidded. I use iron very sparingly since I only dye protein fibers, which are easily made harsh with overexposure.


  • NLM, NIH Ferrous Sulfate entry,

  • NLM, NIH Iron Overdose entry,

  • Ferrous Sulfate on Wikipedia,

  • Iron poisoning on Wikipedia,

  • Ferrous Sulfate MSDS,

  • Ferrous Sulfate in wastewater treatment,

  • NLM, NIH abstract on textile wastewater treatment

  • EPA on drinking water contaminants,

Mordant 3: Tin

Stannous Chloride SnCl2

Use in Natural Dyeing:

Tin is considered to be a generally neutral mordant, yet it brightens colors and causes them to pop a bit. It is the only way to get a truly bright scarlet red using cochineal. Cochineal is also the only case in which a single bath method (dye and mordant in the same step) is highly effective and does not cause undue precipitation of the dye. Drawbacks of tin is that (with the exception of cochineal reds) it is not considered to be as lightfast as alum. Overexposure of protein fibers to high concentrations of tin or exposure over long time periods can cause them and become damaged. Tin can be used as a pre-mordant in a separate step, like alum, though care must be taken to preserve the hand of the fiber.

Other common uses:

  • tin-plating steel to make tin cans

  • used as an indicator to detect the presence of gold. A solution turns purple when gold is added.

  • approved US, EU, and WHO food additive, for color retention and anti-oxidation

Hazards of Stannous Chloride and Tin:

Inorganic tin salts are considered to have low toxicity since they’re almost entirely excreted after being ingested. The World Health Organization has determined that it is not necessary to determine a numerical value for allowable tin content in drinking water, which is mirrored by the EPA in that there’s not even a secondary guideline established. Stannous Chloride forms weak hydrochloric acid when dissolved in water. When the water is heated (during the mordant process), this can result in acidic fumes which are corrosive, and irritating when inhaled. Always keep a lid on a hot mordant bath. Moisture from bare skin can cause more concentrated hydrochloric acid to form on contact and cause chemical burns, always wear gloves and handle crystals with utensils.

My Conclusions:

There seems to be a pervasive myth in the natural dye world that tin is “highly toxic.” It’s actually no more or less toxic than alum, and significantly less toxic than iron. I don’t know where the reputation originated, perhaps the misinformed confuse inorganic stannous chloride with organotin compounds, which are completely different and highly carcinogenic. All this said, I only use tin for a few colors, all based off of a cochineal scarlet. I limit my use of tin because it’s relatively expensive (around $45/lb) and it tends to harshen protein fibers. Also, it’s not quite as lightfast as alum, with the specific exception of cochineal scarlet. There’s simply no other way to create that color, and it is incredibly durable.


  • Stannous Chloride on Wikipedia,

  • FDA food additives,

  • FSA food additives,

  • WHO on Stannous Chloride as a food additive,

  • WHO on inorganic tin in drinking water,

  • Stannous Chlroide MSDS,

  • Tin and Stannous Chloride INCHEM report,

  • Tin poisoning on Wikipedia,

  • NLM, NIH abstract on Toxicity of tin and its compounds,

  • CDC occupational health guidelines for inorganic tin compounds,

  • EPA on drinking water contaminants,

Mordant 4: Copper (aka Blue Vitriol or Bluestone)

Copper or Cupric Sulfate or Copper Sulfate PentahydrateCuSO4 or CuSO4 * 5(H2O)

Use in Natural Dyeing:

Copper is used to “sadden” colors, as it tends to turn them more blue-green. Yellows become greens, and pinks become purples. It can be used as both an after-bath to adjust an alum-mordanted color, or it can be used as a pre-mordant on its own. In the pre-mordant case, it’s typically used with citric or acetic acid to aid solubility, create a favorable environment for protein fibers, and increase copper uptake. Unlike iron, copper does not harshen protein fibers. The colors dyed with copper are generally more colorfast than those dyed with alum.

Other Common Uses:

  • herbicide and fungicide

  • “Bordeaux mixture”, an anti-fungal spray for grapes, rules about permissible weather and wind conditions vary according to state and locale

  • controlling root growth near water and sewer pipes, rules also vary about its use

  • controlling algae growth in ponds, rules also vary about its use

  • testing blood for anemia, it causes blood with proper amounts of hemoglobin to sink rapidly where anemic blood will float or sink slowly

  • etching zinc plates for printmaking

Hazards of Copper and Copper Sulfate:

Copper is found in several proteins and small amounts are necessary for proper biological function. Like iron, an excess can cause poisoning and death. Children are especially susceptible. The maximum allowable contaminant level in drinking water is 1.3 mg/L (or ppm). Mostly it is monitored because many plumbing fixtures are made with copper pipe, and the copper in them does leach into water. Excess copper is also detrimental to aquatic life (fish are particularly susceptible), though dilute solutions are used to control algae. Never pour a copper (or any other) mordant bath into a storm drain, or into any waterway or drainage. Copper sulfate forms weak sulfuric acid when dissolved in water. When the water is heated (during the mordant process), this can result in acidic fumes which are corrosive, and irritating when inhaled. Always keep a lid on a hot mordant bath. Always wear gloves and handle crystals with utensils.

My Conclusion:

Simply put, I like using copper. I prefer using it over iron as a saddening mordant, because it does not affect the quality of the wool. It also creates colors which are quite fast – they tend to bleed less when washing and rinsing, and they are more lightfast than colors created with alum. Copper-mordanted yarn is also very effective at exhausting dyebaths, the color uptake is greater than that of alum-mordanted yarn. Many times I’ll throw copper-mordanted yarn into an exhaust dyebath, and the resulting color will be a nicely saturated medium to dark tone. With both copper and alum, I re-use mordant baths and always do my best to exhaust them before disposing of them. I generally will use the exhausted baths to water plants that are acid-tolerant (and sometimes grass), or pour them down the drain (which leads to a sewer and wastewater treatment plant). I handle copper carefully, with gloves, like I do any mordant or chemical.


  • Copper sulfate on Wikipedia,

  • Copper toxicity on Wikipedia,

  • NLM, NIH article on copper sulfate toxicity,

  • NLM, NIH article on copper sulfate poisoning,

  • Bordeaux mixture and copper’s use in agriculture by the Copper Association,

  • EPA on drinking water contaminants,

  • EPA Copper Fact Sheet,

  • National Pesticide Information Center on Copper Sulfate,

  • Copper sulfate MSDS,

Mordant 5: Chrome

Potassium dichromate K2Cr2O7

Use in Natural Dyeing:

Chrome is a mordant that tends to add a golden hue to dyes, and is considered to be quite color fast. In the presences of protein fibers, it is reduced from its highly toxic hexavalent state (chrome-6) to a relatively safe trivalent state (chrome-3), where it then bonds with the fibers and becomes a mordant. It is typically applied as a pre-mordant with formic or tartaric acid. Prior to dyeing, the crystals themselves and mordanted yarn are light-sensitive, both should be stored in dark places out of direct sunlight. Chrome has played an important historical role in dyeing very dark and colorfast blacks in conjunction with logwood. It is not used often now, due to health hazards detailed below.

Other Common Uses:

  • a common ingredient in cement, it helps smooth texture and retard setting photography and screen-printing for its photo-sensitivity

Hazards of Chrome:

Potassium dichromate is a hexavalent chromium compound. It is highly toxic and quite hazardous to health. Small amounts can cause contact dermatitis. It is a known carcinogen meaning it causes cancer. The maximum contamination level of chrome-6 in drinking water is 0.1 mg/L (or 0.1 ppm).

My Conclusions:

This one’s the doozy. It’s the only mordant I won’t use and would actively recommend NOT using. It is carcinogenic in its solid form – the form that’s used to mordant. Despite the fact that it’s transformed to the non-carcinogenic chrome-3 state on fiber, I would never be able to be absolutely sure that there wasn’t any residual un-oxidized excess non-bonded chrome-6. Some textiles dyed with chrome have been found to cause contact dermatitis, which points to the possibility of excess chrome-6. Perhaps it’s possible to use a water-soluble chrome-3 compound that’s not carcinogenic….but for me, it’s not even worth the hassle of researching that. I can create plenty of awesome colors without chrome.


  • Potassium dichromate on Wikipedia,

  • Hexavalent chromium on Wikipedia,

  • Chrome toxicity on Wikipedia,

  • EPA on chrome in drinking water,

  • Walter Myers Gardner, Wool Dyeing Part I, 1893, pg 56

  • Greenpeace on Textiles and Dyeing, pg 19,

Why Natural Dyes

So, if it still takes chemicals that need to be handled carefully to create naturally dyed yarn, and extra processing time, and extra care to prevent fiber damage, why do it? The answer varies for each of us. I like making things from scratch. The way I dye, I use primarily raw ingredients and extract the dye from them to create my colors. I also have a strong background in science, and I like to tinker and I like to learn. Natural dyeing speaks to me because it’s a bit challenging to understand the chemical interactions and different mechanisms for different dyes. Things don’t always turn out how I’d expect them to, and I’m continuously learning more about the dyes, assists, and mordants, and how they interact with fiber.

I also wanted to show people that you don’t need synthetic dyes to create brilliant saturated colors. I get a lot of surprised reactions when people look at my yarns and realize they’re dyed with natural dyes. They’re bright and vibrant, not the subdued color palette that most people associate with natural dyers. Another reason I chose natural dyes is that the dyes themselves are a renewable resource. Some I can even grow myself. It’s pretty inconsequential in the grand scheme of things, but in this one small way, I could choose a natural product over an oil by-product.

It’s also – excuse the pun – a dying art. Not many people use natural dyes, fewer do it in a business and small manufacturing setting. Many cultures with long-standing natural dye traditions are turning towards synthetic dyes because they’re cheap and easier to use. I want more people to learn how to dye with natural dyes, and I’d like to contribute to a resurgence of information about the techniques. I don’t want these skills and this knowledge to decline and be lost.

One last reason that I choose natural dyes is that I have absolute control over the chemicals I use, and I know exactly what goes into each color. This is typically not the case with synthetic dyes. Even ones that use a lot of “green” marketing, combined with scare-marketing hype that proclaims to be “non-chrome” – even these contain small amounts of chrome, lead, and manganese. These are metals which are toxic in smaller qtys and have higher health risks than alum, iron, copper, or tin. To be clear, I am not throwing Greener Shades under the bus here. While I disagree with their marketing strategy, I applaud them for publishing their test results, and I applaud them for trying to provide less hazardous synthetic dyes. Most dye companies do not publish the actual ingredients of their dyes, and even as a dyer, it is rarely possible to obtain this information.

Again, I don’t have a problem with the proper use of synthetic dyes. The final yarn is not going to be toxic or a health hazard. I own plenty (and you fellow yarn hoarders know exactly what I mean by plenty) of yarn dyed with them. I admire the work of all hand dyers and am jealous that synthetic dyers get to employ more varied dye techniques that result in a wider range of final color displays on yarn. I personally don’t want to work with them often, because they’re fine powders that are easily airborne, they do contain small amounts of moderately to highly toxic metals, there’s always a bit of uncertainty as to their exact ingredients, and I’d rather not have to wear a respirator.


  • Greener Shades Organic Compliance Test Results,

  • Greener Shades MSDSs,

  • Manganism on Wikipedia,

  • Lead poisoning on Wikipedia,

The Final Send-Off

Oh man, I don’t want to end this post on a downer! Dyeing is supposed to be fun, have I completely killed it for you? I hope not. By all means, DO go to a dye day. DO take a class on dyeing, natural or synthetic. It’s super fun to play with and create colors! Just be properly informed and properly prepared. Not all dyers treat their chemicals equally cautiously – I’ve seen people stick their bare hands into copper mordant pots, and I’ve seen people use synthetic dyes like fingerpaints without wearing gloves. Be safe. Use gloves, and use a mask when you’re dealing with fine powders. Bring your own if you’re not sure that they’ll be provided. And most of all – have fun dyeing!

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