Impregnation of leather during "freeze-drying"

Freeze-drying is a recognized method for the preservation of waterlogged objects. Naturally, freeze-drying has also been used for waterlogged archaeological leather often after treatment with Na2.EDTA and impregnation with PEG; but the treated leather sometimes suffers from “excessive drying” becoming too stiff and brittle.
The aim of this study was to examine the effect of a conventional freeze-drying method against an alternative freeze-drying method that preserves the natural moisture content of the leather. Both new and archaeological waterlogged leather were included in the study and the leather samples were treated in one of four ways: pre-treatment with Na2EDTA, impregnation with PEG 400, pre-treatment with Na2EDTA followed by impregnation with PEG 400 as well as no treatment. After the treatments, the leather samples were freeze-dried either by the conventional or by the alternative method. When using the alternative method, the process was terminated, when the samples had a moisture content of ~14 %, similar to the water content of new leather at 20 °C 50 % relative humidity. Finally, the samples were evaluated with respect to their flexibility.
The results of the present work suggest that the process which takes place within the leather during the freeze-drying in not actual freeze-drying, but rather a sophisticated way of distributing the impregnating agent. The pure ice phase freezes out, but the impregnating agent remains liquid as the temperature does not become low enough for the PEG 400 to freeze. Further, as the ice sublimates, the impregnating agent will gain access to sites, previously occupied by ice, thus impregnating the leather during “freeze-drying”