NOTES ON ALUM & OTHER MORDANTS
Recently I was asked some rather probing questions about the various names given to alum and the other chemicals used for mordanting. So since then I’ve been working on supplying some answers but, as my understanding of chemistry is limited to what I need to know as far as dyeing is concerned, I realise how many gaps there are in my knowledge. However, I hope the following information may be helpful.
For a detailed explanation of the history of alum production I’d recommend Dominique Cardon’s book “Natural Dyes”, published by Archetype Publications. This is a huge, scholarly work and, although it is costly to buy, it is well worth the expense if you are looking for an all-encompassing reference book.
Alum is not poisonous but it should be handled with care and not ingested. The 4 types of alum used by dyers are: aluminium sulphate, potassium aluminium sulphate (or potash alum), ammonium aluminium sulphate (or ammonium alum). – (Note: sometimes these last two are written with the word “aluminium” first) – and aluminium acetate. As far as I understand it, the different names refer to the different methods used in the processing of alum to refine it. In the early days of production, refining was necessary to make sure the alum was free from iron contamination and this was mainly done using either potassium or ammonia (or stale urine which contains ammonia). Pure aluminium sulphate (without the potassium or ammonia) contains the highest proportion of aluminium of the 3 sulphates, but by only a small margin, and is a later 19th century development, resulting from improved methods for removing the iron from alum shales. All the three aluminium sulphates mentioned above can be used for mordanting, although ammonium alum is less commonly used as a mordant. Aluminium acetate is used to mordant vegetable fibres and silk only.
The alum available in the past in chemists’ shops in the UK tended mainly to be ammonium alum, used, I think, for pickling and for applying to the skin in poultices. The aluminium content of ammonium alum is slightly lower than that of the other two aluminium sulphates, but not sufficiently lower to make a significant difference. The other alum sulphates may occasionally have been available in chemists’ shops but alum in any form is rarely available over the counter in chemists or pharmacies in the UK now. It may sometimes be possible to buy aluminium sulphate in garden stores as a soil treatment and as long as it doesn’t have any iron contamination (usually in the form of specks of brown in it) it is fine to use as a mordant.
The difference between aluminium sulphate and aluminium acetate is that the former is processed using sulphuric acid and the latter using acetic acid. Alum acetate is usually used for mordanting vegetable fibres and silk only, not animal fibres such as wool, mohair etc.
I think the alum sulphate most suppliers sell for mordanting is probably potash alum, although it is often simply described as alum. In the 1990s, when I had my mail-order business selling dyes and mordants etc, I supplied aluminium sulphate (iron free), and not potash alum. The reason for this is simple – when I took over the business, the previous owner recommended that I should continue to supply granular aluminium sulphate (iron free), purchased from the wholesalers in 50kg sacks, as this is what she had supplied. So this is what I did. Since then, I have always used aluminium sulphate myself, so when I mention alum sulphate in my writings that’s what I mean, not potash alum. But it really doesn’t matter which of the three forms of alum sulphate a dyer uses. However, it does matter that dyers understand the difference between alum sulphate & alum acetate & what each is used for.
Cream of tartar: the cream of tartar used together with alum in some mordanting recipes is potassium bitartrate (or potassium hydrogen tartrate). It was originally made from the salt deposits that formed on wine casks & could be either white or red, depending on the type of wine for which the cask was used. The culinary cream of tartar sold in supermarkets may sometimes be sodium pyrophosphate, which is slightly less effective than potassium bitartrate but still fine to use. Some dyers mordant wool and other animal fibres using alum without cream of tartar, although in some sources an alum + cream of tartar mordant is recommended as preferable to get good strong colours from cochineal & other insect dyes. I have found that mordanting animal fibres using 10% alum sulphate, without cream of tartar, gives me good results with all dyes, including cochineal, but the mineral content of my local water may also play a part in this.
Washing soda (or soda crystals) is sodium carbonate. Soda ash is also sodium carbonate but with the water content largely removed. However, as washing soda can be bought in supermarkets and hardware stores, it may be easier to obtain than soda ash and is equally effective. In all the recipes in my books I use washing soda/soda crystals rather than soda ash.
Iron is harmful if ingested and should be handled with care. It is usually supplied in the form of ferrous sulphate (or iron sulphate), which is a green powder. (Incidentally, when it gets damp it turns brownish but this doesn’t seem to make any difference to its viability). It may also be called copperas or green vitriol. Iron sulphate is also sometimes available in garden stores as a treatment for plants showing iron deficiency and it is sometimes cheaper to buy it this way, if you can find it. Home-made iron water (called iron liquor in the USA) is technically probably ferrous acetate, because it’s made using acetic acid, but I avoid the chemical term, as I’m not sure how accurate it is when used to describe a home-made product. Home-made iron water can be used instead of ferrous sulphate as a mordant or colour modifier and some writers suggest that iron water may be less corrosive on fibres than ferrous sulphate solutions.
Copper is toxic and should be handled with care and not ingested. It is usually supplied as copper sulphate, a blue powder, and is also known sometimes as blue vitriol or blue copperas. Home-made copper water (copper liquor) is technically probably copper acetate (see comments on iron acetate above) & is occasionally called verdigris. Verdigris is the green deposit that sometimes forms on copper piping etc and I think this is what is dissolved into the solution when making copper water. Copper water can be used instead of copper sulphate as a mordant or colour modifier.
Before disposing of any liquids containing chemical residues, make sure they are well diluted with water. Iron and alum are the most common elements in the earth’s core and disposing of mordant or dye bath remains containing these chemicals should not present any problems, especially as they are used in very small quantities and most of the chemical content should have been absorbed by the fibres anyway. I usually pour solutions containing small residues of iron and alum around acid-loving plants, such as broad-leaved evergreens or blueberries. Otherwise remains may be poured down the drain or disposed of on the ground in an isolated spot, away from children, pets and septic tanks. Copper is toxic so should be handled with particular care but it should be safe to pour residues containing copper on the ground in an isolated spot as described above. Some dyers prefer to avoid using copper because of its toxicity and it is certainly possible to achieve excellent results and a wide colour range without the use of copper.
I do not use chrome or tin as they are toxic, difficult to handle safely and disposal of chrome and tin residues may present problems.
For more information, Helen Melvin (Fiery Felts) has written at length about mordants and alum on her blog. (www.growingcolour.blogspot.com) However, wherever one goes for information, one tends to find different mordanting & dyeing recipes, as dyeing can be a highly personal craft. So in the end the choices are individual ones. My aim is to make good, reliable results as simple to achieve as possible – probably because I’m an impatient person & I don’t see the sense in making things so complicated that one is deterred from even making a start.
But that doesn’t mean that my methods are better than anyone else’s, so try out different recipes and select the ones that suit you best and give results that you like.