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The article presents information on calcium phytate in the treatment of corrosion caused by iron gall inks. Iron gall inks, developed later than carbon ink, became steadily more common and widespread during the middle ages. The carbon inks in use up until that time had the invaluable quality of not being reactive, due to the stability of carbon. They were not subject to chemical alteration and did not contain any substances that would damage the support. It seems that the instability of carbon ink may have encouraged the practice of adding small quantities of ferrous sulphate.