How does logwood produce black and gray shades for silk and leather?

Logwood produces rich black and gray shades through a chemical process involving its primary colorant, hematein. While the heartwood of the logwood tree initially yields hematoxylin, this compound oxidizes into hematein during the extraction process.1

To achieve black or gray tones on silk and leather, the hematein must be combined with a metallic mordant. Specifically, the use of chromium salts allows the dye to produce various shades of charcoal, gray, and black. Additionally, a rich black can be formed on silk by mordanting the material with a salt of iron before applying the logwood dye. This interaction between the plant-based hematein and the metal ions is essential for fixing the color and developing the desired dark hues.12