Dying vegetable fibers is more involved then animal fibers. This is because they are not made from proteins, which take on color easily. Instead, plant fibers are made of lignin or the cell wall. For this reason it is usually advisable to use a mordant to prepare the fiber for the dye.
A mordant is decried by Jenny Dean in the book Wild Color as "a substance that has an affinity with both the materials to be dyed and the natural plant dyestuff. Acting as a bond between the two, a mordant helps the dye become permanently fixed to the fiber."
After putting the shirts in a pot of water and heating it we used a per-mordant to raise the Ph. This helps the plant fibers maintain there strength. We used a solution of oak galls, 4oz per one pound of fiber and let the shirts soak over night (more then 8h is ideal). Then, we used the mordant alum, Aluminum Sulfate, again letting the shirt soak (8h+). While this is not something you want to swallow its less toxic then many other mordants. The alum solution was done proportionally 4tsp of alum to every 4oz of fiber and 1 1/2tsp of washing soda to every 4oz of fiber.
Tonight I add the shirts to the dye. I'll let them step over night and then we'll see how they turn out in class. Exciting!