Testing Ammonium Citrates for Enhanced Washing of Paper

Ammonium citrate was introduced to paper conservation as an additive for what might be called “enhanced” washing, viewed as a more powerful brightening agent than washing alone and less aggressive than bleaching. We tested its effect on cellulose using two historical papers and one filter paper immersed in solutions of ammonium citrate (pH 5.5; pH 8.5), citric acid (pH 1.8), and ammonium and calcium hydroxide (both at pH 9.0). One set of papers was washed exclusively in these solutions, a second set subsequently received an alkaline reserve. We determined the CIE L*a*b* values for all samples before and after accelerated aging to capture color changes, and a selection of 36 samples underwent molar mass determination. Testing revealed differences between the samples treated with citric acid and ammonium citrate compared to the other treatment solutions: both historical papers brightened and maintained their brightness after accelerated aging. Molar mass data indicate that deacidification following the treatments helped to maintain the cellulose integrity of the tested papers during accelerated aging. Our findings indicate that the application of ammonium citrate may be considered as a washing additive if the treatment solution is thoroughly rinsed, and an alkaline reserve is added using calcium hydrogen carbonate.