Oxalic

Oxalic acid, or "oxalate," is a naturally occurring plant chemical which, in the human diet, can react with calcium, iron, and other minerals to inhibit mineral absorption. Foods such as tea, beets, rhubarb, strawberries, spinach, and chocolate are known to increase oxalate in the urine and may contribute to kidney stone formation.

If not sufficiently purified, a little hydrochloric or oxalic acid, or caustic potash may be put in the water, according as the stains are from grease or from ink.