It is a water-soluble ink made by blending ingredients derived from plants such as tea leaves with iron. It penetrates the paper and solidifies due to oxidation; therefore, it is suitable for preserving the documents for an extended period of time as it is highly resistant to light and water. Due to such characteristic, the ink color that is bright when you start writing becomes gradually black as it gets oxidized with time, which lets you enjoy the progress of changes in color.
