Alum Mordanting Again

As I have used up almost all my alum-mordanted yarns, it is time to replenish my stock. So this week I have been mordanting wool with alum, following my usual methods. (For details of these, see the post “Alum Mordants”)

As I explained in the above post, I now use 10% alum sulphate without cream of tartar for mordanting wool. (i.e. 10gms alum  – or 2 rounded teaspoons- per 100gms wool.) As I find it easier to measure out liquids rather than powders, I usually make a solution of 100gms alum dissolved in one litre (1000mls) water.  At this dilution, 10% means I use the same quantity of alum solution in mls as the wool weighs in gms. (i.e. to mordant 500gms of wool I need 500mls of alum solution).  To save time later, I tend to make several litres of alum solution at once, as any remaining solution can be stored indefinitely for use at a later date. (See note below for storage advice) The crucial thing to remember when making the alum solution is to make sure that all the alum has dissolved completely. I find it best to add the boiling water gradually, stirring well as I proceed. When I want to use the solution I always shake the bottle well first, just in case some alum has sunk to the bottom.

I have done tests with most dyestuffs and can see little, if any, difference in the depth of colour achieved on wool using a 10% alum mordant, when compared with the results achieved from the same dyebath using wool mordanted with a higher percentage of alum. As one of my aims as a dyer is to reduce as far as possible the quantity of chemicals used, without compromising the quality of the results, I can see no point in using a higher percentage of any chemical than is necessary for good results. However, I would also add that each dyer will tend to have his or her preferred mordanting methods and whatever gives the desired results, without causing harm to humans or the environment, is the best method for the individual dyer in question.  Indeed, the number of different alum mordanting recipes available is remarkable and must be bewildering for a beginner. Dyeing books written in the early to middle 20th century tend to advocate the use of a higher percentage of alum than is necessary or desirable, often as much as 30%, but in more recent books the percentages recommended tend to be lower. In today’s environmentally- and safety-conscious world, where dyers have recognised the need to use the smallest quantities of  chemicals possible, the abundance  of different alum mordanting recipes available would seem to indicate that more people are experimenting to find the recipes that best suit their individual needs.

I don’t work with silk very often, although I do sometimes mordant and dye silk skeins for a friend who is a silk weaver.  My preferred mordant for silk is now 5% alum acetate. In Japan, alum acetate is widely used as a mordant for silk and I find this recipe gives excellent results, although the 10% alum sulphate recipe I use for wool is also effective. 

For vegetable fibres I also use 5% alum acetate, as this is so much simpler than the alum/tannin/alum method I used in the past and gives excellent results.

To return to today’s mordanting: After stirring 600mls alum solution into the water in the pot, I added 600gms of well wetted wool. I started off by raising the heat gradually then simmered the wool in the alum solution for about 45 minutes. Unfortunately, the pot was rather full & the lid doesn’t fit very well, so the mordant bath overflowed during the simmering process and left my work surface swimming in water. After mopping up the liquid and squeezing as much as possible back into the pot, I then turned off the heat and left the skeins to steep overnight. I then removed them, squeezed the excess liquid back into the pot and dried all the skeins except those I want to dye in the next few days. These I left damp in a plastic bag, so I wouldn’t need to wet them out again.

I then added a further 600mls alum solution to the used mordant bath and added a further 600gms of wool. In order to avoid flooding my work surface again, this time I gradually raised the temperature to simmering point, keeping a close watch on the pot all the time, then immediately turned off the heat. I left this batch of yarns to steep in the mordant solution for about 36 hours, moving the yarns around gently from time to time. It doesn’t really matter which method one uses, as both are equally successful. In fact, it’s also possible to apply an alum mordant to wool fibres without heating the solution at all, as long as the fibres are allowed to steep in the cool solution for 3 to 4 weeks and are moved around in the pot regularly. However, when mordanting wool, I prefer to apply heat, at least to start with, mainly because wool tends to absorb liquids better with heat and I feel the results are therefore likely to be more reliable. This is also why a hot indigo dye vat is recommended for wool, whereas cotton, linen and other vegetable fibres can be successfully dyed in a cool vat. However, if wool is left for a longer period in a cool vat, it will gradually absorb the colour. Silk, whilst technically an animal fibre, seems to fall between the two fibre categories and tends to react equally well in both hot and cool vats.

Storage advice: Alum solutions can be stored in glass or strong plastic containers, with well-fitting lids.  However, I don’t recommend the rather flimsy opaque plastic milk containers as they may absorb, rather than store, some of the solution. I know that when I used these milk containers to store woad solution, I ended up with dark blue containers & rather less blue pigment for dyeing than I had expected.

I use the strong plastic gallon/5 litre containers in which things like cider, apple juice, vinegar, detergent etc are sold & have found these ideal. (My husband tells me he thinks these are actually made from some sort of nylon, which is non-absorbent.) Otherwise, the clear strong plastic bottles in which water & juices are sold are also suitable.  Containers for household bleach would also be OK, as long as they are thoroughly washed out first, and they have the added advantage of child-proof tops. Large glass bottles which contained pickled onions, gherkins etc would also be fine & if the neck is too wide, I use a funnel to make pouring easier. As long as the containers are well washed out, it doesn’t really matter if they still smell of whatever they contained, as any aroma is unlikely to be transferred to the materials being treated in whatever solution was stored inside them.

I usually store all my solutions in a brick-built outbuilding, which doesn’t get much natural light. In general, store solutions in a cool place, away from direct light & make sure they are clearly labelled, preferably in indelible pen. For extra security I usually fasten a plastic label round the neck, as I’ve found that snails tend to make their way into the outbuilding and sometimes eat away at my paper labels.

3 replies
  1. Helen
    Helen says:

    Hi Jenny

    the mls to grams ratio for measuring the wool and alum makes complete sense now with this easy to understand information, a huge thank you for putting things down in plain terms as it is much easier to follow and understand.

    One thing I am not so clear on though is what type of container is best to store the mordant liquid solution once it is made up for use at a later date? I recall reading in one of your posts about woad not storing so well in plastic milk containers because the cheaper plastic tends to soak up the majority of the blue pigment not leaving much in the solution for dyeing much else. So I am now wondering whether these cheaper types of plastic containers are suitable for use with the dyes and mordants at all or whether I should be looking at saving darker glass containers instead.

    Any advice on storage (probably worthy of an entry in its own right thinking about it) would be gratefully received for a beginner like myself 🙂

    Thanks for keeping up this blog as I am finding it a wealth of information and loving seeing all your photographs.

    Keep in touch!


    Helen x

  2. Bettina
    Bettina says:

    thanks for the extensive info – always appreciated! esp. when I agree totally with the contents – I have been using progressively less alum during the last few years and found no difference, be it in take-up of dyes or in fastness! I don’t see why I should put in more than I need – it’s better for the environment – and my pocket!!

  3. Helen Melvin
    Helen Melvin says:

    Hi Jenny I have been writing on mordanting too! (On the 22nd January) We are sort of saying the same things but from slightly different angles.

    I always found when I made an alum solution that it would gradually come out of solution and form a thick crusted layer on the bottom of my bottle. Any idea why this should be? bw Helen

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