BARK TANNING & HOW TO AVOID WOOL-DYEING
Most bark-tanning creates leather with a red or purple-ish hue (and modern chrome-tanning goes to great lengths to mimic this, as we discussed last HTOTM). The tanning agent in bark-tanning is tannic acid + the singular molecule is called a tannin. Each tannin is associated with its own unique colour. And most of them are red or purple-ish. Additionally, there are other dye molecules present in bark + plant material which will move into the hide during bark-liquoring. There's no stopping a hide from picking up the pigment of tannins.
→ When a tannin finds a protein in a hide, it binds to the protein. The colour is fastened to the hide just as much as the tannin is.
→ But for hair-on/wool-on leathers + rugs, we don't need the tannins to interlock, and therefore we don't need the hair or wool to remain dyed.
Read on.
How to Avoid Wool-Dyeing in Bark-tanning
1. Start with the animal: Be intentional about wool or hair type
The primary factor in how much dye remains in wool or hair is the type of animal. With sheep, the finer the wool, the more permeable; the coarser the wool, the more resistant. Working with ancient breeds like Jacob or Karakul = less dye transfer to the wool. The sheepskin in the photo above is a Jacob cross, with very minimal pigment picked up in the wool. With other animals, you can tell when hair is coarse or fine pretty easily. Racoons will pick up dye from tannins much more easily than deer will, for example.
2. Then, the pre-tan work: Acidify your hide
I learned this trick from Andy Widjaja a few years ago when we were collecting hides in Montana together. Typically, I wouldn't acidify my woolllen sheep rugs before bark-tanning; it's something I reserved for hair-off leathers and denser hides like deer. Acidifying our hides does a lot of different things for barktans, and turns out it does one helpful thing for hair/wool: because acidification has a constricting effect, it will help repel dye molecules (which are fairly large) and resist pigment.
To acidify a hide before bark-tanning, dissolve 3oz citric acid + 1 pound salt in a gallon of hot water - then times this recipe x5 or so depending on much solution you need to cover your hide; soak your hide in this solution for at least 48 hours before bark-liquoring.
You can also acidify your hide using vinegar (this is what I use for hair-off leathers) but it will retain a scent of vinegar for a while after tanning is complete.
3. The bark-liquoring: Choose your tannin intentionally
The type of tannin matters. Hydrolyzable tannins are less pigment forward than catechol (condensed) tannins. Strong barks like Hemlock are mostly catechol tannins and will strongly dye the hair/wool. Other barks like Willow contain a mix of both tannins and we can use warm (not hot) water to extract the hydrolyzable more than the catechol tannins. Still other barks like Myrobalan contain mainly hydrolyzable tannins and create a soft yellowish hue of leather, with the same minimal effect on hair or wool.
The only material I know of that is known as a “clear tannin” is Oak Gall. This isn't a bark per sae but a diseased growth on an Oak tree. Tannins accumulate int he area to help heal it. Once the spot is healed, the gall remains, and so do the tannins.
4. After bark-liquoring: rinsing tannins from hair and wool
Once a hide has turned into leather, tanning is finished. But our hidework is not. We rinse the hide with fresh water to remove any excess tannins that are stuck to the outside of the leather. We also can remove tannins and dye molecules from the hair or wool of our new leather rug + make sure those microscopic molecules don't stick around. Even if they aren't absorbed now, they could slowly absorb into the strands with time.
Wash tannins from hair or wool using a strong detergent like Orvus Paste or a wool soap like Eucalan. You can wash the leather by hand in a watertight tote or put it in a washing machine. You can hang it on a fenceline and hose it down, suds it up, and hose it again. There's no right or wrong way to wash it, but make sure the hair or wool is washed thoroughly + rinsed well.