I had actually forgotten about this project, hence why I left it for so long with out looking at it. The jar had been sitting outside for 6 months before I came across it while doing some gardening. It contained red onion and a silk scarf I was wishing to colour. Over the six months the onionskins had dissolved completely, making the water a soft ruby colour. This is what the scarf looked like when I took it out:
I have rewrapped the scarf so that other areas of it are exposed and can absorb some of the dye. I may need to add some more onionskins for a stronger colour. I will leave it until I stumble across it again in a few months. As you can see this is a long process, but I think it will be totally worth it.