Calcium hydroxide nanoparticles from solvothermal reaction for the deacidification of degraded waterlogged wood

https://doi.org/10.1016/j.jcis.2016.03.038Get rights and content

Abstract

Hypothesis

A combination of acid and iron ions inside the wood has been corroding the cellulose matrix of the Swedish warship Vasa, imposing its deacidification. Past deacidification treatments displayed poor penetration inside the wood matrix with limited efficacy. A vacuum assisted treatment of wood using newly developed calcium hydroxide nanoparticle dispersions represents a possible candidate for the treatment of acidic waterlogged wood objects such as sculptures and decorative artifacts.

Experiments

A solvothermal process was used for the synthesis of calcium hydroxide nanoparticle dispersions. Before the application on waterlogged wood, the physico-chemical characterization of these systems was carried out using several techniques. The efficacy of the deacidification treatment of wood samples from the Vasa was assessed by determination of pH and Differential Thermal Gravimetric (DTG) measurements.

Findings

The proposed solvothermal reactions can be used to produce stable and highly concentrated calcium hydroxide nanoparticle dispersions in alcohols, needing no further purification before the application. This process has also the advantage to be upscalable to industrial level. Both pH and DTG measurements showed that the newly developed dispersions can homogenously penetrate inside the wood up to 20 cm, neutralizing acidity and creating an alkaline buffer inside the wooden matrix, to hinder the degradation of residual cellulose.

Introduction

In 1628, on the day of its maiden voyage, the Vasa sunk into the brackish and polluted water of the Stockholm harbor, from which it was recovered more than 300 years later. In order to prevent the shrinkage of the wet wooden structure, PEG aqueous solutions were sprayed on the shipwreck for 17 years. Nowadays, the preservation of the Vasa is threatened by the concomitant action of acid-catalyzed hydrolysis and metal-catalyzed oxidation (Fenton reactions). Acidic compounds include sulfuric acid, mainly present in the upper layers, and organic acids, such as oxalic acid, whose concentration increases with depth. Oxidative degradation is promoted by iron salts present inside the wood [1], [2], [3], [4], [5], [6], [7]. For the extraction of iron ions the usage of aqueous solutions of chelating agents (e.g. ethylenediimino-bis(2-hydroxy-4-methylphenyl) acetic acid, EDDHMA) has been proposed and tested [8], but the preservation of the Swedish warship Vasa is still a challenge for wood conservators. The Vasa is indeed a unique conservation challenge, but there are several other waterlogged ships and objects, such as, for instance, the English carrack-type warship Mary Rose or the Norwegian Oseberg burial finds, whose preservation is threatened by acidity and oxidation.

Hydrolysis and oxidation are the two main degradation processes that affect cellulose-based works of art. Since hydrolysis and oxidation usually act in a synergistic way by creating the so-called spiraling effect [9], both a deacidification intervention and an antioxidant treatment should be applied. Several groups have indeed proposed antioxidant materials to be combined with the commonly used deacidification treatments [10], [11], [12], [13], [14]. Recently we proposed a new method, based on the use of nanoparticles of calcium or magnesium hydroxide [15], [16], able to stop cellulose degradation mechanisms induced by metal ions and acidity. The method takes advantage from the much slower pace of the catalytic activity of metal ions around neutrality [17]. Conservation of cellulose can be achieved in a single-step pH-controlled deacidification treatment to inhibit the acid-catalyzed hydrolysis and the metal-catalyzed oxidation, prolonging the useful life of the artifacts.

Calcium hydroxide nanoparticles, obtained from homogeneous-phase reactions, were originally designed for the consolidation of wall paintings [18], and subsequently used for the deacidification of cellulosic materials, i.e., paper, canvas, wood [19], [20], [21], [22], [23], [24], [25]. However, the required purification and the small amount of particles obtained from the homogeneous-phase pathway made this procedure time-consuming. Preparation pathways based on the grinding and dispersion of slaked lime have been also considered as potentially rapid and simple ways to obtain high amounts of particles [26], [27]. The size of particles obtained with this method is in the range from 300 nm to 700–800 nm, which can result in limited penetration when the porosity of the substrates is lowered by the presence of protective coatings or consolidants. An improvement was achieved using a thermo-mechanical treatment for hydration of calcium oxide [26], [27], but the need of highly reproducible particles’ size distributions led to the development of different strategies. In particular, a bottom-up synthetic procedure for the “scaling down” of particles size is presented here, and the obtained particles penetrate more homogenously through low porous materials, i.e. PEG-consolidated archeological wood.

Calcium hydroxide nanoparticles were synthesized starting from metallic calcium and short-chain alcohols, i.e., ethanol, and n-propanol. At high temperature and pressure, calcium alkoxide is formed; the subsequent hydrolysis of this compound yields nanoparticles of calcium hydroxide already dispersed in the appropriate solvent, and the system can thus be directly applied for conservation purposes because no purification step is needed. It is worth noting that the usage of non-aqueous medium for the synthesis hinders the growth of nanoparticles due to Ostwald ripening [28], [29], [30], [31], [32]. Besides the decreased particles’ dimensions, the advantage of this new synthetic procedure, based on the widely used solvothermal process for oxide nanoparticles syntheses [33], [34], [35], relies in the feasible upscale of the process to industrial level, with great benefits in terms of costs.

Calcium hydroxide nanoparticle dispersions obtained from the solvothermal reactions were characterized by Attenuated Total Reflectance Fourier Transform Infrared Spectroscopy (ATR-FTIR), X-ray Powder Diffraction (XRPD), High Resolution Transmission Electron Microscopy (HR-TEM), Selected Area Electron Diffraction (SAED), Dynamic Light Scattering (DLS), surface area analysis, and turbidimetry measurements, before being applied to hardwood samples from the Vasa warship. The efficacy of the deacidification treatment was assessed by determination of pH and Differential Thermal Gravimetric (DTG) measurements.

Section snippets

Chemicals

Ethanol absolute (99.8%, Fluka), n-propanol (99.5%, Sigma-Aldrich), and metal granular calcium (99%, Aldrich) were used for nanoparticle syntheses. Highly pure water (resistivity >18  cm, by a Millipore Milli-Q UV system) was used.

Nanoparticles synthesis

The hydroxide nanoparticle syntheses were performed in a high-pressure reactor (Parr-instruments) by a one-pot reaction consisting of two steps:Ca+AlcoholCa(alkoxide)2+H2Ca(alkoxide)2+H2OCa(OH)2+Alcohol

During the first step of the reaction, the alcohol (ethanol or n

Synthesis and characterization of calcium hydroxide nanoparticles

The deacidification treatment of wooden artifacts must fulfill some applicative requirements: (i) the treatment must not induce any changes in the appearance of artifacts; (ii) in order to achieve a highly homogenous treatment, dispersions must be stable for a considerable amount of time, i.e. from several hours up to days; (iii) the final pH of samples should not be excessively basic because, in the presence of oxidized cellulose, high pH values may induce depolymerization due to the β-alkoxy

Conclusions

The major limitation in past deacidification treatments of archeological wood relied in the poor penetration of the deacidification systems [20], [21], [50], [51] or, in case of alkaline solutions, in the low amount of deacidification agent that could be uploaded into the wood [52]. A mild vacuum assisted treatment of wood using newly developed pure and highly crystalline calcium hydroxide nanoparticle dispersions was tested. Both pH and DTG measurements on Vasa waterlogged wood showed that

Acknowledgment

The authors would like to thank Ingrid Hall-Roth and Emma Hocker (from Vasa Museum) for providing wood samples from Vasa and Lars Ivar Elding (University of Lund) for helpful and stimulating discussions. Thanks are also due to Maurizio Ferretti and Alessandro Toccafondi for the preparation of Vasa wood sample (woodcutting). CSGI and European Union (project NANOFORART, FP7-ENV-NMP-2011/282816) is gratefully acknowledged for partial financial support.

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