This question arrived by emailed today:
"Does the tannin have a short shelf life, once it's mixed with hot water? I have a silk shawl that is not accepting the tannin. The tannin was mixed with hot water several days ago and has worked well on other cotton and silk pieces, but it is now cool and several days old. Do you think if it is heated again it will dye better—or should I just dump it and mix a fresh batch?"
When tannin is mixed with iron oxide you get black.
It is available at the store at Rust-Tex.Com
I have not noticed tannin going bad in 2 or 3 days!!! I put it in a bottle and let it sit on the deck until it grows moldy. That is a sure sign to toss it. My experience is that it works best, (read as fastest) in hot weather. If you are doing this outside in cold weather it will take longer to work. Try letting it sit longer than overnight.