A scrap of alumed silk fabric and thread, soaked in the above for a day.
I know from experience that this beautiful blue is fugitive, and not light sensitive. When I tried this last year it faded prettily through shades of green and violet to just a suggestion of grey when left on a sunny windowsill in a matter of weeks. It doesn't make me like it any less though.
:: If you would like to spend a day learning about a very colourfast blue, there are spaces available on my Indigo Day on 20th July at my home workshop. We will be using a fresh leaf technique with the plants I have been hovering anxiously over since February, and also have a vat for some deeper blues. More info here.