Logwood gives shades of lavender and purple and has historically been very important in dyeing a true black, especially for Victorian mourning clothes. At one time in history, dyeing with logwood was outlawed because it was considered to not be lightfast enough, but even then it was still needed and needed for dyeing a good black. These days, we do not wear our clothes as hard or long as they did in the past, and windows block out a good bit of uv rays, so the beautiful colors that can be obtained with logwood are sufficiently lightfast for many purposes.
The Logwood tree has been used in Central America for centuries and in Europe since the 17th century as a natural dye source, and still remains the best source of Haematoxylin, which is used in histology for staining. In its time, logwood was considered a versatile dye, and was widely used on textiles, paper and leather. The dye’s color depends on the mordant as well as the pH. It is reddish in an acid environment, and bluish in an alkaline environment.
Mordant cellulose fibers with aluminum acetate, and wool and silk with alum, or try aluminum acetate for a brightened effect on wool.
The recommended amount of aluminum sulfate to mordant wool is between 10-15% of the weight of fiber, but I’ve read that for improved lightfastness with Logwood, you can use 22% alum.
An easy thing to remember, for me to mordant at 10-15%, when mordanting wool with alum, is to just use 1 scant Tablespoon for 100 grams (3.5oz skeins) or 1 good Tablespoon for 4oz skeins of fiber. You can go up to 20% of the fiber weight by using 1 Tablespoon + 1 teaspoon per 100 grams (3.5oz) of fiber. More mordant results in deeper shades with many of the colors, especially the red dyes, or in logwood’s case, increased lightfastness.
Cream of tartar should be 6% of the weight of fiber, or 1 1/4 teaspoons for 100 grams (3.5oz) of fiber. You can use aluminum sulfate and cream of tartar together, the cream of tartar brightens many colors and helps keep wool fibers soft.