FEEDBACK FRIDAY: This Week in Natural Dye Questions

This week: What’s the shelf life of mordants and is cold indigo leaf dyeing stronger or weaker than hot?

Each week, we are emailed with questions from our natural dye community asking simple and complex questions that we thought might be worth sharing. Here are a handful from this week answered by natural dyer in chief, Kathy Hattori, Founder of Botanical Colors:

I had purchased 250g of aluminum acetate awhile ago. My question is: does it have a shelf life and/or does it go bad?

Most of the minerals and assists are stable for a number of years. As long as they are kept dry and away from heat and excessive moisture, the aluminum acetate should be good for at least two years. A few notable exceptions are: iron is susceptible to moisture from the air and should be kept in an airtight package and aluminum sulfate gets clumpy or “rocky” if exposed to moisture but it still works fine once it’s broken up again.

I’d love to hear more about how cold indigo leaf dyeing process generally compares in color intensity to hot methods (like simmering the leaves and doing the usual aeration and reduction).

They are very different. Fresh leaf dyeing is generally a light to mid-tone greenish blue or “teal” shade.  It is possible to achieve a slightly darker shade with repeated fresh bath dips and a higher intensity of fresh leaves, and the bath is never reduced, nor is the pH raised to achieve these shades. It’s simply cold water and ground up indigo leaves.

The color intensity with fresh harvested indigo that is “fermented” in a warm solution, then aerated can be very dark, depending on the type of indigo used. It is a more involved procedure, sometimes taking a few days to extract the pigment and then create the vat. Those lucky enough to live in an area where it’s possible to grow Indigofera tinctoria have been able to achieve some lovely colors, very similar to a vat made with powdered indigo.