なるほど、外皮に豊富に含まれるタンニンか
Walnut Ink: Botanical Inks: History of Walnut Ink - FelinFach https://www.felinfach.com/pages/walnut-ink?srsltid=AfmBOorVxImyheHpYkpkOMbv-55RVwzPwEciW1bGPcajzdREnxTqP-fI