1. iron sulfate, especially when used as a mordant in dyeing or in making ink
Usage: chemistry; historical

Examples

• The dyer added copperas to fix the color in the fabric.
• Medieval scribes used copperas to make iron gall ink.
• Copperas was commonly employed in leather tanning processes.
• The green crystals of copperas dissolved easily in water.
• Apothecaries once sold copperas for various medicinal purposes.
• The textile mill ordered several pounds of copperas for their dyeing operation.