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Copper mordant. Why not? Plus it’s something I haven’t tried until now for dyeing wool. Alum and cream of tartar is my go to mordant, being easily available and non toxic. I have used copper as an afterbath occasionally, but only on previously mordanted yarn where the change to the colour compared to using iron is minimal.

Time to give it a go.


I have several jars of copper bits and pieces topped up with water that have been festering for literally years. They have turned a beautiful blue.

Basically this was guess work. For alum mordant I weigh it out (8g per 100g of yarn so 8%). But for this copper mordant experiment I had no weights or measures. So I went through the same process as usual, with more care when it came to wearing gloves and with good ventilation. A copper mordant can be toxic at high concentrations, though this probably wasn’t that strong. I filled the big pot with water and added the liquid from one jar after a good shake up. Then I added one 100g skein of yarn which had been soaking in water until thoroughly wet. The pot was heated to steaming point and then left to steep at low heat for about an hour before leaving overnight to cool. Needless to say, none of the pots I use for dyeing are ever used for food.

The yarn turned a delicate pale blue green. Unlike alum mordant, a copper mordant will colour the yarn.


I cooked up a pot of nettles, bashing them about a bit to get out as much colour as possible. Then I left it to cool overnight. Next morning I strained it off into another clean pot, many of my pots are grubby inside from using iron afterbaths and I didn’t want any of that residue to adulterate the results.

The yarn went in, it was heated to boiling, steeped and then left to cool. It was interesting. The colour was decidedly different from using alum mordant, a much more golden tone. However the dye take-up wasn’t as complete. There was plenty of colour left in the dyebath. Not wanting to waste any dye I decided to use that for a skein of pre-mordanted yarn (with alum) with an iron afterbath.

Flush with success I repeated the copper mordant and nettle dye process with more yarn (4 x 100g skeins this time). The concentration of the mordant (and dye) was lower, although the colour was still decent enough.


Clockwise from top left:

No 1: The first skein with copper mordant.

No 2: The repeated (less concentrated) copper mordant.

No 3: Alum mordanted yarn using the exhaust dye from No 1 with an iron afterbath.

No 4: As a control. A skein of my ‘regular’ yarn – alum mordanted, nettle dyed and iron afterbath.


A copper mordant is definitely worth a go. Although those jars have been sitting for an age to achieve that amount of leaching. I will add a dash of vinegar (acid) to help the process along.

I am always on the hunt for more dye options which don’t cost £££.

Copper also has the advantage over iron as it doesn’t damage the yarn. Iron eats into the wool fibres like nobodies business, requiring a good soak in fabric conditioner at the end to make it workable.

In short – yes I would do it again.