How to Mordant Cellulose Fibers with Tannin and Alum
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WHAT IS MORDANTING?
Mordanting is the most important process of preparing fibers to accept color. This is not an optional step but there are many mordant variations, and indigo as a vat dye does not require a mordant. Using a mordant helps to ensure the most durable and long-lasting colors. This mordant method is an alternative to using aluminum acetate, and is easy to use.
A tannin bath before the alum step helps increase lightfastness and all baths can be reused for multiple mordant sessions.
How to mordant using tannin and alum
This is a room temperature procedure but you do need very hot water to dissolve the tannin and the alum.
Before you start, weigh the dry fibers you want to mordant. Write down the weight of the fiber (WOF), and calculate 10% tannin on that weight.
Weigh the tannin and dissolve it in a cup with very hot water. It will be a murky tan color. Add this to a larger container that will comfortably hold your fibers, like a small bucket.
Fill the bucket with enough water so that the goods can move easily in the liquid. Wet your fibers first, then add them to this container and gently submerge them, pressing out air bubbles.
Let the fiber remain in the tannin liquid for at least 1 hour and as long as overnight. Rotate the goods in the tannin bath occasionally, keeping them submerged. This tannin bath can be reused - keep it covered and labeled in between uses.
Next, calculate 12% alum on the WOF, and measure and dissolve the alum in a cup with very hot water. It will appear clear. Calculate and measure 1.5% soda ash and dissolve separately. Carefully add both ingredients to a separate container that will comfortably hold your fibers. The mixture will bubble vigorously so make sure you have enough headspace in the container for the reaction.
Fill the container with enough room temperature water so that the goods can move easily in the liquid and mix well. Add the tannin-soaked fibers (do not rinse beforehand) and gently submerge them, pressing out air bubbles.
Let the fiber remain in the alum liquid for 2 hours, or as long as overnight. This alum bath can be reused - keep it covered and labeled in between uses.
Once the fibers are mordanted, you may remove them from the alum mordant bath and dye them.