Tannins

Tannins are a family of chemical compounds found in plants. Different plants (and different parts of the plant) have different concentrations of tannins. E.g., galls > outer bark > sapwood. When mixed with iron, tannins form new chemical compounds (mostly importantly, ferric-polygalloyl glucose complexes). The chemical compounds are black, bond with paper, and are (relatively) waterproof after a few days.

By adding gum Arabic, we protect these chemical compounds from degrading while in solution.

Tannins fall into two groups of chemicals:

Hydrolysable tannins

• Bonded sugar and water.
• Breaks down readily in water.
• Yellow-brown without modifiers.
• Black when iron is added.

Condensed tannins

• Stronger bonds including carbon.
• Requires strong acid or alkali to break down.
• Reddish tones.
• Dark brown when iron is added.
• Cooking in alkali yields redder browns.

Where to find tannin-rich plants

Year-round / by chance

• Bark found on forest floor:
  • Spruce bark
• Culled / cut trees:
  • Logs for log burner
  • Storm damaged trees
  • Garden clippings
• Oak galls (only if gall has hole)

Technically, walnut’s rich brown is due to juglone, which is part of the quinones, but it does have tannins as well.

According to season

Spring

• Bramble tops

Summer

• Nettles

Autumn

• Rowan berries
• Sumac drupes (red ones, not the white ones)
• Haws (berries of hawthorn trees)
• Alder catkins
• Conker husks
• Acorn caps
• Pine cones (and cones from other conifers, such as spruce)

Winter

• Holly berries (caution holly berries are toxic)
• Autumn harvest items may still be available depending on the year and where you live

In the kitchen

The following grocery-store-friendly plants are also a good source of tannins.

• Avocado pit
• Pomegranate
• Sumac