Readers of my latest book “Colours from Nature” (details on this blog under “Publications”) will probably notice that my recipe for mordanting animal fibres with alum has changed slightly. I now use 10% alum & no cream of tartar, instead of 8% alum + 7% cream of tartar. The reasons for this are as follows: cream of tartar, potassium hydrogen tartrate not the culinary sodium pyrophosphate, was difficult to obtain here for a while, so I got used to mordanting without it. After some experiments, I found that alum used alone at 10% (i.e. 10gms alum per 100gms fibre) seemed the lowest % of alum that gave good results, so I adopted this as my method of mordanting animal fibres. When I started using cream of tartar again, I found that for some reason it wouldn’t dissolve properly & remained as a sludge at the bottom of the pan. So I decided it was simpler (& less wasteful) to use alum alone. 10% alum works well & is very convenient if you make it into a solution as I explain in the book. With a solution of 100g alum dissolved in 1 litre of boiling water, when you use 10% alum, you use the same quantity of alum solution in mls as weight of yarn in gms. E.g. To mordant 450gms wool you need 450mls alum solution. This seems so much simpler & the results are just as good, in my experience. In fact, with some dyes, such as madder, I think the results are better. And the alum solution can be stored indefinitely in a glass or strong plastic container with a well-fitting lid. Also. it seems much easier to measure out a liquid than a powder. But of course, if you have your own preferred recipe for alum mordanting, then that’s the best method for you.
Recently it has become possible to obtain aluminium acetate in the UK & this has made mordanting vegetable fibres, & also silk, so much easier. 5% alum acetate used in a single process can replace the more time-consuming tannin & alum mordanting processes commonly used for vegetable fibres. Aluminium acetate is widely used in Japan as a mordant for silk & I have found it to be excellent for both silk & vegetable fibres.
PS One last comment: Before I throw away the used 10% alum mordanting bath, I usually add one final skein to it & mordant it as usual. I label it “alum exhaust”, just in case there wasn’t enough alum left to have even a tiny mordanting effect, & then use it with a dye, such as madder, which will fix both with & without a mordant.