All about Alum Based Mordants

Updated: Sep 10

Aluminium Mordants for every Fiber.



Aluminium is the most used mineral salt for mordanting fibers. We know it as alum (potassium aluminium sulfate) for protein fibers and aluminium acetate is the version used for cellulose fibers (see this blog about mordanting).


Natural alum

Natural alum occurs near active volcanoes or in desert areas. The compound alum has been known by since the 5th century BCE and was used extensively by the ancients for dyeing. Alum quarries shaped the Yorkshire coast, and were the subject of many political and economical quarrels with alum being imported and heavily taxed from Bagdad and Turkey, and then subsequently banned once excellent alum was discovered in Tolfa (Italy). The whole alum trade is fascinating and I highly recommend reading up about it.


Aluminium and the environment.


When we are mordanting we want to take into consideration the following things; how much of leftover mordant are we flushing away, how much of energy is needed to prepare and use the mordant.

In my country there is no longer a limit for waste water containing aluminum. It used to be 3 mg per liter. For environmental reasons, however, care should be taken to ensure that waste water is contaminated with as little aluminum as possible. When mordanting, cold mordanting can save energy and the specific mordant I want to introduce here should also save aluminium in the waste water.


The dye results results should improve using aluminium based mordants in the following order:

For this last one we have to thank Johannes Harborth who developed the mordant to use for felting artists and I will explain about it later on. Not all alum based mordants are supposed to give the same results on the different fiber types. For cellulose based we would use aluminium acetate when for wool we could use plain alum. I will put this knowledge to the test below.

Alum mordants as we know it traditionally;


Alum mordant (for protein fibers).


alum rock

With the standard mordanting process, the fabric is heated in the mordant and loses some of its shine and softness. This is supposedly not a good fit for cellulose fibers.

20% WOF alum (=potassium aluminium sulfate) (you can purchase it in rock form or powder, or even granules) is dissolved in hot water together with 6% cream of tartar. Add cold water. Add pre-wetted fabric. Heat mordant and fibers together slowly, making sure the solution covers the material completely.


Wool and silk can be cold mordanted with alum, I just leave it in a bucket over night. This method is more gentle on the fibers. When it remains in the cold mordant bath for a long time is can become slimy. From my reading up and estimates, there are still plenty of alum ions left in the solution after mordanting, which I then toss out because I can’t estimate how much it is. Some people top up with 50% of the original weight for a second use and so on.

Get potassium aluminium sulfate here.


Aluminium Sulfate.


Instead of alum, it is just as easy to mordant with aluminum sulfate. The mordanting is more environmentally friendly, since with aluminum sulfate only 2/3 of the weight compared to alum is required, so less aluminum ions get into the wastewater. Use at 15% WOF, can be used cold and is also good for cellulose albeit less than aluminium acetate.

Aluminium sulfate is generally more expensive than alum but less that aluminium acetate.


Basic Aluminium Acetate.

Aluminum acetate is more fiber-friendly than pure alum. Wool is less stressed and remains softer.

This is your chosen mordant for cellulose fibers instead of the traditional alum-tannin-alum-tannin method.

When working with aluminium acetate you need to use a chalk aftermath to remove unbound mordant, which is an extra step and extra water. The biggest downside of Aluminium Acetate is that the powder is rather expensive when bought ready made.

I will repeat the recipe for home made aluminium acetate here for 500gr fiber, so 20% WOF. Fit for wool and cellulose fibers.

  • 100 gr alum

  • 50 gr soda ash

  • 600 ml vinegar (5%)

  • 5 liters water

Use a stainless steel pot.

1 - Dissolve the Alum in 500 ml water at a temperature of at least 60ºC.

2 - Add 4 liters cold water to top up.

3 - Dissolve the Soda Ash in a second vessel completely in 500 ml of warm water and add.

3 - Add the Acetic Acid to this mixture.


Add your pre-wetted textiles and slowly start heating to 90ºC. This should take around 90 minutes.

Leave the textiles in this hot mixture at 90ºC for around 90 minutes.

Stir regularly.

Let the fabric cool down in the pot.


Recipe for a gentle aluminium based cold mordant;

100 g alum

365 ml vinegar (5%) or 73 ml vinegar essence (25%)

30 g soda ash

20 gr cream of tartar

5 liters of water

1 - Dissolve the alum in a 10 liter enamel, stainless steel or plastic container with hot water and allow to cool.

2 - Add the vinegar.

3 - In a second smaller enamel, stainless steel or plastic container, the soda ash is dissolved with cold water and very slowly added to the first container and stirred.

4 - Stir in cream of tartar and top up to a total volume of five liters with water.

The mordant can be used immediately cold. Leave fibers in the cold mordant for about 8 hours.


The New Cold Aluminium Mordant


Gentle Aluminum Formate cold mordant according to J. Harborth


Like alum and aluminum sulphate, Aluminium formate contains aluminum compounds that permanently fix natural dyes on protein fibers. It is gentle on wool and has a long shelf life. It can be used for wool, silk and cotton and even linen but the latter will always have a lesser uptake of natural dyes.


Cold mordanting enables the mordanting process with aluminum elements even without the addition of heat - but it takes a little more time. The time issue is not that bog of a deal, because preparing-heating-cool down of the regular mordant would take not that much less but is more hands-on.


The same mordant bath can be used several times until almost all of the aluminum compounds have been used up. What remains is formic acid, which is one of the non-toxic organic acids. Formic acid and its salts are very easily biodegradable and are therefore not a problem for the environment.


Granted; this method of working with mordants goes against all my understanding of how mordanting works but I wanted to test this (see below).


As opposed to the other aluminium based mordants mentioned before, the solution depends on the amount of liquid, so it is very important to squeeze all the excess liquid back into the vessel.


This mordant is best stored in a bucket with a lid at room temperature. Store powder at room temperature.


Pros of the Aluminium Formate mordanting process;

  • Little fiber change due to mordanting at room temperature.

  • The amount of mordant is not related to the dry fiber material.

  • No need to have a specific water to fiber ratio.

  • No specific mordanting temperature and duration is needed.

  • Can be used in any non reactive vessel such as plastic.

  • No disposal problem.

Get Aluminium Formate here.


How to use Aluminium Formate;

Make a 2% solution with (at most) 50 °C warm water. That is 20 grams of powder per liter of water.


Making 10 liter mordant (in a plastic bucket):

10 × 20 g = 200 g of Aluminium Formate are weighed and added to a bucket with warm (max 50ºC) water. Stir.

For smaller amounts (3-5 liter), the powder is dissolved in warm water (approx. 40-60 ºC) and then topped up with cold water.

The mordant can be used immediately.


Time in the mordant;

At 50ºC 1-2 hours. At room temperature 8 hours.

There is no upper limit for mordanting times, so you could keep your fibre in the bucket and leave it there until you need it.


The next batch of fabric can be mordanted in the same bucket. Times needed will extend each time you use up a bit of the mordant. Top up water if needed. According to what I have read, the fibers will absorb what they need and not more, which is very interesting.


Comparing the different aluminium based mordants in order of effectiveness in theory.

For the experiment I had the following samples;


4 sets of textile each 30 grams.

  • Organic pre-washed wool

  • Handwoven matka silk

  • Heavy linen twill

  • Handwoven cotton muslin

Please note these brand new chafing dishes, you can use them on a burner just the same, but easier to store.

Mordant Samples

Set 1; mordanted in 20% alum and 6% CoT heated for 1 hour to 90ºC

Set 2: mordanted in 10% aluminium acetate for one hour at 90ºC

Set 3 : mordanted in 2 liters 2% aluminium triformate solution at 50ºC for one hour (That’s 40 gram of the mordant)

Set 4: Mordanted in aluminium triformate (same batch as no 3, no additions) at room temperature for 6 hours.


The sets were all rinsed in cold water. No calcium carbonate dung baths were used in order not to add more variables to the experiment.

I made a madder dye bath in my XL couscous pan with around 50% madder. If I had planned better I would have let it soak longer in cold water for better color but this is the best I could do for now.

Soda ash added to bring up the the PH, as madder loves alkaline water.

Brought the heat up to 60ºC for an hour and let the fabric cool down in the dye bath for about two hours.










The results, washed, dried and ironed;

Slightly different light so the hues are a bit different.

1 Alum. 2 Aluminium Acetate. 3 Aluminium Formate 4 Aluminium Formate exhaust




Cold mordanting with Aluminium Formate will have my preference from now on, because it saves me from making the mordant for each dye project and schlepping pans with hot liquids. Is it noticeably better in result? On silk I felt it surely was. Especially for protein fibers 'saving' them from being heated in a mordant will maintain the soft feel and lustre of the fabric. Leaving fabric in for prolonged periods is also something that has to be tested more.

The next best bet for me after that would be Aluminium Acetate because it works on everything and I loved the result of it on wool.


Results per mordant based on this experiment;


I will repeat this experiment with other types of silk and wool using different natural dyes.

I understand also that I should do more testing on cotton for optimising the mordants.


To be continued.


Safety precautions;

Chemical powders, cream of tartar, vinegar and soda ash can cause eye irritation.

Wear gloves and safety glasses.

When in contact with eyes: Rinse with water for a few minutes. Remove contact lenses, if possible. Continue rinsing.

Clothes that have come into contact with chemicals should be removed and washed out immediately.

Keep chemicals away from children and pets.


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