A new alum mordant?

Recently I was given a link to a website in Germany, which was offering something called “Kaltbeize AL” or “cold alum mordant”. Naturally, my curiosity was aroused, especially as I had never come across a reference to this anywhere else. So I decided to find out more about this alum mordant, which would appear to be something new.

According to the information on the website (www.textiles-werken.de),  this form of alum mordant is suitable for wool and silk and is applied cold. Once made, the same mordant solution can apparently be used many times. The instructions seem to imply that the mordant is only exhausted when the liquid level has dropped too low to enable the fibres to be immersed in the solution.

I ordered some of this alum mordant from the website and decided to do some tests to compare it with my usual alum mordant.  I planned to follow the method given on the instructions accompanying the cold alum mordant and also to try out the same method with the aluminium sulphate I usually use to mordant wool.

As far as I was able to ascertain, this mordant would appear to be aluminium formate – something I’ve never come across before. It is apparently made using formic acid, rather than sulphuric acid or acetic acid – hence the name. But that is all I know about its chemical composition so, if anyone knows any more, I’d be grateful for further details.

Unlike with other alum mordants, where the amount of alum used is based on the weight of fibres to be mordanted, with this cold mordant it is the ratio of mordant powder to quantity of water added that would seem to be crucial. So once the solution has been made, no more than 20% more water should be added at any stage. The recommended proportions are 20gms mordant powder per litre of water, so to start with I made 5 litres of solution, using 100gms of Kaltbeize AL.  The alum powder is first dissolved in hand-hot water, then added to the pot, which is filled up with cool water to the appropriate level. According to the information given, 5 litres should be sufficient to mordant at least 1 kilo to 1.25 kilos of fibres in total, which is virtually the same as 10% alum – the percentage of alum sulphate I generally use. The fibres can be added in several batches. When the fibres are removed from the mordant, the advice is to squeeze any excess liquid back into the solution, so that as little liquid as possible is lost. (Actually, I found that, after mordanting about 1 kilo of fibres in three batches without adding any more water, the level of the liquid had dropped only slightly. So I would question whether the level of the liquid is really a reliable indicator of the continued viability of the mordant. I would be inclined to assume that after mordanting about 1 to 1.5 kilos per 5 litres of solution, the mordant is probably exhausted. But I may be wrong.)

The first batch of fibres should remain in the solution for at least 4 hours and subsequent batches should remain in the liquid for longer periods. I entered three batches, each weighing about 350gms, and the first batch steeped for 8 hours, the second batch for 12 hours and the third batch for 24 hours. However, the instructions are not specific as to timings and only give general guidelines. There is no limit to the length of time fibres can be left to soak, so they can be left in the mordant bath for several days or even weeks.

I followed exactly the same procedure with an aluminium sulphate solution, dissolving 100gms alum sulphate in hot water then topping it up to 5 litres. I mordanted three identical batches of fibres in this cold solution, so I could compare the results with those from the Kaltbeize AL mordant.

I dyed skeins from each batch from each of the two mordant solutions and the results were interesting. There was hardly any difference in depth of colour achieved from brazilwood when the two mordanting methods were compared. The colour from the Kaltbeize AL mordant bath was very slightly deeper on the fibres from the first batch, but after that the colours from each of the two mordant solutions were virtually identical. These experiments were very useful, as they illustrate clearly that alum sulphate can be used cold, providing the fibres are left in the solution long enough. I think in future I would probably leave the first batch of fibres in an alum sulphate solution for at least 12 hours to start with.

The question then was what to do with the remaining mordant solutions. Might there be some mordant potential left in them? As I’m the type of person who finds it difficult to throw anything away, especially if it might still be useful, I decided to add a further 500gms fibres to each solution. I left these fibres to soak for 36 hours. I then tested a sample of each in several different dyebaths and was surprised to find that they dyed well. This indicated that there had been enough alum remaining in both mordant solutions to mordant the added fibres perfectly satisfactorily. Now I have to decide whether to continue adding even more fibres or whether to assume that the alum is exhausted.

There are still some aspects of this cold mordant I’m not sure about. For example, would there be any difference in the results between 1 kilo of fibres mordanted for 4 hours in a fresh solution and 250gms of fibres mordanted for 4 hours in a fresh solution? Is it possible that the 250gms batch would have absorbed more mordant because the alum had to be divided between fewer fibres? Would my results have been different if I had added all my 1kg of fibres at the beginning, rather than dividing them into three batches?  The instructions suggest that each solution is sufficient for about six batches of fibres, but the weight of the batches doesn’t seem to matter. So does this mean that a 5-litre solution would mordant six batches weighing 1 kilo each? How does this mordant work if there seems to be no relation between the weight of the fibres added to the solution and the weight of the alum dissolved into the liquid? Why is the ratio of alum powder to water so important? Surely the mordant must be exhausted before most of the water has evaporated away? If anyone has answers to these queries, I’d love to read them.

If I lived in Germany and had easier access to this new Kaltbeize AL mordant, and if I knew exactly what it is and how it works, I would certainly consider it as an alternative to alum sulphate. However, the high cost of postage to the UK, added to the cost of the mordant, makes it rather expensive. And if I can get similarly good results from alum sulphate used cold in the same way, there would seem little point in incurring the extra cost. But I’d still like to know more about aluminium formate.