Determination of Maximum Freeze Drying Temperature for PEG-Impregnated Archaeological Wood

Abstract

A series of experiments, cooling aqueous solutions of polyethylene glycols (PEGs) with molecular masses ranging from 1000 to 10000 to full solidity followed by heating to full liquefaction, combined with measurement of the physical stability, was performed. The experiments were carried out using everyday metal thermos flasks, a domestic freezer and a simple temperature data logger. These experiments gave standard curves for eutectic temperatures, thawing temperatures and strength properties as a function of the molecular mass of the PEG. The experiments showed that the softening and thawing temperatures for aqueous PEG solutions are higher than the solidification and eutectic temperatures. These key properties can be used to find the highest temperature at which PEG and water are still solid, thus minimizing the time and energy spent on the freeze drying process, which requires that no liquid phases are present in order to prevent collapse. The methods, based on simple procedure and equipment, can be applied in the conservation workshop to determine the above key properties of the solutions used to impregnate waterlogged wood.

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