2.1. THE WOOD MATERIAL: GENERAL ASPECTS

Before speaking of the different conservation and treatment methodologies of the waterlogged wood, it’s necessary to treat about the structure and the age of the material to be treated. So this chapter is dedicated to the comprehension of the wood structure; it will possible to understand the differences among the different wood species and to understand why the wood deteriorates and how the wood becomes waterlogged.

2.1.1. Fresh or green wood

The wood is the result of the growing of a young plant of tree, and to understand the structural organization of the wood is necessary to tell something about its genesis and the chemical reactions that happen inside the structure: the photosynthesis.

2.1.2. From photosynthesis to the wood

The photosynthesis is the totality of the chemical reactions thanks to which the sun energy is caught by the chlorophyll and then converted into chemical energy used to synthesize the carbohydrates, with the glucose first, starting from the molecules of CO2 and water. The photosynthesis implies two types of reactions that happen inside the chloroplasts: the reactions of the light and the reactions of the obscurity.

2.1.2.1. The chloroplasts

The chloroplasts is the part of the cell in which are stocked the enzymes polymerasis of DNA, RNA, the ribosome, and all the necessary factors associated to the plant. The chlorophyll is the green pigmented molecule contained inside a membrane (thylakoide) that catch and adsorb the light constituents (the photons) for the photosynthesis.

2.1.2.2. The thylakoide membrane

The thylakoide membrane constitutes a very complex part of the chloroplast, forming flat bladders organised in sections:

✔  in which many strata of membranes are piled one on the other to form the lamellas (grana or lamellas grana) that contain the proteins complex (cytochromes B6/f and PSII);
✔  the membrane appears as a unique strata the stromal lamellas that contains the PSI, B6/f and ATP.

The internal space entirely occupied by the thylakoide is called lumen, and the external space inside the thylakoide membrane and wrapping the chloroplaste is called stroma.

2.1.2.3. Photosynthesis and formation of carbohydrates

The light energy adsorbed by the molecules of the thylakoide membrane is used to carry the reactions that generate the 
NADPH and the ATP, that are the reactions made by the light.

For the reactions that occur in the obscurity, the ATP and NADPH procure respectively the energy and the necessary reduction to allow the synthesis of the carbohydrates starting from CO2 and H2O molecules. The glucose, the first to be synthesized, diffuse normally in different parts of the plan. If the glucose formation exceed the rhythm of this diffusion, the excess will be transformed into carbohydrates of bigger molecular size as the wood cellulose.

Among the synthesized carbohydrates there are the sugars and the starches. The glyceraldehyde – 3 phosphate and its 
isomer, the dihydroxyacetone-3phosphate are the substrates for the synthesis of sugar and starch. The synthesis of the sugar 
is happens in the citosol of the cell, while the starch synthesis happens in the citosol, starting from the same substrates.

The starch synthesized in the chloroplast is often stored in the stroma as granulates The stroma, that is the place where the synthesis enzymes of the starch are, transform the enzymes of elaboration of the fatty acids and of assimilation of inorganic nitrogen and of sulphur.

The sugar and starch quantity inside the cells are regulated by metabolic needs of the plant. The excesses are accumulated 
in the grains, in the roots and the leaves of some species.

2.1.2.4. Wood appearance

The proteins, codified in the genome of the new cells are synthesized in the cytoplasm and then imported in the chloroplast.

The unicellular eukaryotes can contain from one to many chloroplasts, while the vascular plants can contain hundreds of thousands of them. Immediately after the cell division, the cell start to become longer and to increase its diameter and the cell wall forms.

The lignification starts in a successive moment coming out from one side of the cell and it starts developing from the medium lamella among the cells and then from the primary and secondary wall cell. The development of the fibres or trachides, after the cellular division till its death lasts about 14-21 days. Only the cells of the parenchyma are alive in the trees: the wood is done essentially of the dead part inside the trunk plant.

2.1.3 General wood structure

The traversal cut of a trunk of a plant can show different parts depending on their function or from the essence of the tree. 
From the inner to the external part it’s possible to observe the heart, the alburnum and the phloem:

✔  the heart, that can be called also duramen, contains only dead cells. It’s normally darker as is rich in polyphenol substances; these cells are useful as defence against the predators (micro-organisms, fungi and insects) and from the climatic variations. The wood of the heart is characterised by a inferior water quantity and by a better durability compared to the other parts of the wood. The annual rings are thinner compared to the alburnum✔  the alburnum, sometimes called young wood, is the external part of the wood that is still constituted by the alive cells of the wood.

Often a transition zone between the heart and the alburnum which seems to be clear sometimes in the transversal section of the wood.

The phloem forms different cells that have specific functions:

✔  the conductor cells of the sap, that communicate through the plasmoderm that are more or less large forming some small grouped perforations (the pores, 0.3 – 2.5 micron of diameter) that are called cribs;
✔  the cribrose pipes that have the holes larger than the cribs;
✔  the contact cells that are made with cribrose elements;
✔  the parenchyma that can accumulate both reserve of sugar and starch;
✔  the secretor cells of pigments, terpens and different resins;
✔  the lignified or not lignified cells depending on the pant species.

If the tree is cut to produce different objects, the wood is usually bereft of the phloem. Only the hear and the alburnum, that form the wood in the real sense of the word, are generally exploited because of their hardness in comparison to the rest of the tree. Even though there are differences in the cell function, the structural organization of their parts doesn’t seem to be very different.

2.1.3.1. Constitution of the wall cell

The wood cells are grouped in elementary organs called fibres or tracheides depending on species. Many studies have been developed to determine the structure of the wall cell of the plants. The results are in accordance as concern three macromolecules: the cellulose, the hemicelluloses and the lignin.

2.1.3.1.1. The cellulose

The cellulose is the most abundant organic macromolecule renewable in nature and it constitutes the main part of the wood.The wall cell of the fibres of tracheides is constituted by wrapped capillary of cellulose molecules grouped in elementary fibrils. 
Each of them is constituted by about forty cellulose molecules linked among them by hydrogen bindings, are ordered in microfibers that are developed in a lignin and hemicelluloses matrix. The dimension of the right sections of the elementary fibres have been in this way evaluated. The are about 3.5 nanomicron, for multiples of 3.5 nanomicron to 4 nanomicron; the multiples depend on the space.

Even if there are some differences with the identification of the intra and inter molecular links in the cellulose structure, the authors are unanimous about its constitution. It has a linear polymeric structures formed with unities of D-glucopyranose linked with bonds of the type ß (1-4). The molecules of D – glucopyranose have a chain conformation. The hydroxyle of the cycle, the hydroxyl-methyl and the oxidic bindings are in equatorial position where all the hydrogen are axial.

The nature ß of the bond implies a rotation of 180°C of one unity of glucose every two, of cellobiose, base for the cellulose. The ß nature of the of the bond give the molecule a structure strongly consolidated among the intramolecular hydrogen binding, as known between the hydroxyl carried by the C atom n. 3 and the H atom intra-cycle of near the glucose unit.

In the molecular chains forming the elementary fibril, the crystalline ordered regions (about 300 nm of length) are separated by amorphous regions, or the molecules are less ordered. In these areas the water can circulate among the elementary fibres. Again in these regions the degradation caused by acids or heat can be produced.

Even if there are three hydroxides groups on which chain there is the anhydroglucose molecule, the cellulose is completely insoluble in the water (cold and warm). Moreover, its state of polymer super macromolecular in linear chains, formed from 6*103to 104 unities ß 1-4 D – glucopyranose help the formation of many hydrogenous binding intra and extra molecular. The numerous hydrogen binding that link the chains together can’t be broken by water.

Only strong acids and strong bases, the salt concentrated solutions and of different reagents can swell, dissociate or dissolve the cellulose, causing the partial fracture of the very ordered crystals.

This is the strongly ordered crystalline structure that prevent obstruct the access to reagents and enzymes.

The tax of cellulose inside the wood, starting form the multiplicity of the species, is about 40-50% of the dry mass of the wood. The residual 50-60% consist on lignin, hemicelluloses, not structural organic constituents and mineral salts.

2.1.3.1.2. The hemicelluloses

The hemicelluloses are structural polysaccharides of the cell wall different from cellulose. The difference is justified because, even if insoluble in water, the hemicelluloses are soluble in the strong alkaline and acid solutions.

This property has been taken into account to improve the separation methods for hemicelluloses from the mass of carbohydrates called holocelluloses (cellulose + hemicelluloses). Moreover it’s possible to distinguish them for the sugar components different from glucose. These are polysaccharides formed from 5 to 6 sugars different from glucose, as the xylose, arabinose, galactose, mannose. These sugars are bound to cellulose fibres for accretion. The hemicellulose have a more amorphous structure than the cellulose, and a smaller molecular mass. This explains the high solubility and the sensitivity to hydrolysis compared to real cellulose. They have a polymerisation degree that stabilize themselves between 100 to 200 glucose unities.

The hard woods contain about 30%, the soft ones about 20-25% of hemicelluloses in the cellular wall.

2.1.3.1.3. The lignin

The lignin is a complex macromolecules made of many hydrocarbon and non hydrocarbons constituents and bound inside the wood. It’s mainly localised inside the medium lamella of the cell wall of the trees. It exact structure can’t jet be completely identified formally.

All the experimental studies are in accordance on the fact that a great part of lignin molecules have a three-dimensional from formed by amorphous aromatic polymers.

The lignin is a macromolecule starting from the polymerisation of cynnamile alcohol, p-coumaryle alcohol, coniferyle alcohol and synapyle alcohol. It derive from a phenyl-propane unity, linked in many ways with chemical bindings. Four types of bonds corresponding to characteristics groups form the lignin structure:

✔  the saturated aliphatic groups;
✔  the carbonyl group;
✔  the aromatic group formed by aromatic rings;
✔  the hydroxyl group.

Some of the bonds are of carbon-carbon type, particularly the guaiacyl and syringylpropane units.

These parts seem to guarantee the non complete degradation in case of depolymerisating reactions.

The lignin doesn’t solubilize at low temperature during the alcoholic hydrolysis. Vice versa at high temperature the great part of lignin is solubilized, particularly if appropriate solvents are used. (this property, of essential importance, will be better deepened in the part of the impregnation of wood with starch).

The lignin represent the 20-30% of the constituents of the plant, depending on species and the age of the plant itself.

2.1.3.2. The other wood constituents

The wood contains, in addition to the above mentioned constituents, some non structural organic constituents and mainly mineral substances.

2.1.3.2.1. The non structural carbohydrates

They seem to be the main constituents of the wall cells because they’re at the base of its constitution through the metabolism. Among the most important constituents there are the starch and the sugars (normally the saccharose). They are polysaccharides for reserve stoked generally in the wood parenchyma. The quantity of starch in some species is the 5% of the dry matter of the wood, with the highest concentration in the cambium.

2.1.3.2.2. The mineral salts

The quantity of mineral salts varies in function of the kind of plant, the kind of species. The mineral salts are usually considered as foreign bodies because are only present because they have been extracted from the soil with water by the roots of the plant. These salts are among the extractable components of the wood.

2.1.3.2.3. The extractable of the wood

All the plant constituents that can be separated from the structural substance of the wall cell are insoluble and extractable.There is a noticeable variation among the repartition of the extractable constituents from the wood. This variation is clear if you notice the longitudinal and transversal variation of the stalk, the great part are in the wood heart.

The extractable elements include a great amount of chemical substances, among which the most common are terpens (sterol and resins) and essential oils, the fatty acids, the nitrogenous constituents, the not saponifiable constituents, the aromatic compounds, (tannins, quinines, flavonoides, aldheides, alcohols, dimmers phenil-propans) and the colouring pigments.

The colouring components give an aesthetic value to the wood, as the phenolic compounds that give the wood the resistance to:

✔  attacks of predatory organisms as fungi, insects, xylophages grubs, wood fungi, decolouration fungi;
✔  enzymatic degradation of the hemicelluloses and partially of celluloses in sugars and alcohols;✔  climatic variation effects.

Moreover we must add to the extractable components also the structural polysaccharides as the hemicelluloses and also the not structural polysaccharides as the starch and the sugars and the mineral salts. Finally the water represent an important part of the compounds of the fresh wood. It occupies, depending on the essences, the 40-75% of the total weight of the wood inside the fresh pulled down wood

2.1.4 Differences in the woods

The physic and anatomic differences among the different wood species allow to classify them with criteria more empiric and scientific.

2.1.4.1. Physical differences

The literature distinguish two wood groups: the soft wood and the hard wood. Anyway this subdivision doesn’t always mean that the hard wood will be the most durable.

In fact, in a study on the structure and chemical constitution of the wood, Thomas (1977) warned against this classification. It seems that some wood, generally classified as hard, are softer than other classified as soft. Another empiric classification is the one between hardness of wood and possibility to loose their leaves in winter. A correlation has been observed between the softness of the hardness of the wood linked to the preservation of the leaves all the year. In fact the woods considered soft are constituted by wood that tend to maintain their leaves on, while hard woods seems to come from plants that loose their leaves in winter to renew them in springtime.

Moreover, even if there are some differences from the physical point of view between hard and soft wood, from the structural organization the authors are in accordance.

2.1.4.2. Anatomic differences and wood density

For the soft wood, the fibres are used as medium for the translocation of the sap. They are interconnected through open spaces called punctuation. For this structural different attributed to the fiber, in the soft wood they are called tracheides. So the tracheide is useful as conducing pipe, because of its lumen, and as support because of its wall cell. The tracheide dimensions varies from 1,5 to 9,5 mm of length and from 3.0-4.0 mm to 20-80 µm of diameter for the commercialised soft woods. In the case of the soft wood in addition to the tracheides also the parenchyma assure the stocking f the nutritive substances.

On the contrary for the hard wood the term “fibre” has a real meaning, because the wall cell are thicker , and they have mostly the role of support. The hard wood has four kinds of fibres:

✔  the vessels work for the transport of nutritive substances;
✔  the fibres for the support;
✔  the axial parenchyma;
✔  the transverse parenchyma for the stocking of the nutritive reserves as the starch and the sugars.

Most part of the studies underline that the main difference between hard and soft wood is the presence of vessels (called pores) in the hard woods. The soft wood practically don’t have them. The conductor vessels bring the sap to the alive part of the tree and have a great variety of dimensions, forms and engagements. In fact the fibres are a little smaller (0.50-3 mm of length and 10-20 µm of diameter). The vessels, or more precisely the segments of vessels, are usually 1.3 mm of length and 20-330 µm of diameter.

Also the volumic mass (or density) has been deeply studied. The most part of the results gave medium values for the volumic mass, that for the soft woods can be about 0.36 (from 0.25 to 0.60) and for the hard ones about 0.50 (from 0.30 to 0.80).

Moreover it has been studied the density of the different constituents of the cell wall: the cellulose of the wood has a density of about 1.55 while the lignin is about 1.33 and the amorphous cellulose about 1.50. So the medium density of the cellular wall can be of 1.50, and it’s almost the same for all the species.

The density of the wood is greatly influenced by the medium volume of the cellular wall of the fibres or of the tracheides compared to the free space, so is influenced by the porosity of the wall cell. For the resinous pine, the hydrated layer of the cellular wall of the tracheide contains about the 25% of free space. The water contained in the capillaries of the wet walls of other plants are over the 40%, with about the half of the free space. There are some free spaces between the wall cells inside the capillary vessels.

Some other studies have been conduced on the capillary vessels dimensions. The transversal section is from 16 to 60 Å. The results demonstrate that sonly the molecules of adequate dimension could diffuse in the cellular wall inside the capillary vessels and act as filling agents or to modify some wood properties, as the dimensional stability, the consolidation and the durability. Moreover the inorganic salts dissolved in the sap could diffuse in the free spaces and stay there. They have been recognized for their presence in the ashes of burnt wood.

The circulation of the sap in the wood can happen because of the difference of the pressure in the capillary vessels. It starts thanks to the photosynthesis energy with the presence of the liquid and gas phase that separate.

In the soft woods, the tracheides are in communication by small holes called punctuations, naturally positioned in the interstices of the microfibrils (fibrils) and of the diaphragms. So in the hard wood the different kind of existing vessels comport anyway the punctuations and sometimes the perforation of the diaphragms with a even more complex structure.

There are some holes in the different adjacent segments, the vessels, that allow the dislocation of the sap from a vessel to another. This communication system, vessel-punctuation-vessel, realise a certain type of long piping through which the sap is transported on the totality of the tree thanks to the capillary pressure.

Into the punctuations, the diaphragms can move thank to an internal suction of the wall cells, to block the holes. This can happen in case of injury of the wood and this can cause a separation of the nutritive substances from the cell walls. For this structural organization and the blocking movement of the holes, the diaphragms prevent the air to come in and to stop the circulation of the fluid. The shutting of the punctuations thanks to the diaphragms happens especially when the wood cell are loosing the sap. It’s the same when they loose water as during a desiccation operation.

There are some realized researches that compare the relation between the permeability (to water and nitrogen) and the porosity of different wood species: the results of this study show that the desiccation imply a growing aspiration by the punctuations till the 42-57% in the alburnum and till the 60-74% in the heart of the wood. The authors deduced than because of this permeability reduction it’s necessary a strong pressure gradient to separate the water through the structure of the wood samples. Similar conclusions were found by other authors that said that the penetration of liquid substances before its desiccation is very difficult to be realized. Moreover the most important factor for a good penetration of liquid substances and for the maintenance of this substance inside thee wall cell before the preservation treatment is the texture of the cell wall. Other results were obtained studying the distribution of the mass through the cell wall, or by tincture absorption and the conclusions are similar. The external part of the wall fibres of the wood, constituted by the external stratum of the secondary wall, the primary wall and the medium lamella, has a inferior density and so a higher porosity than the internal part.

These anatomic study reveals the complexity of the structural organism of the fresh wood. It’s a first evaluation of the complexity of this matter and so on the difficulty of the preservation and conservation of these materials. But before arriving there we want to see who it’s possible to bypass from the fresh to the waterlogged wood.

2.1.5 From fresh to waterlogged wood

The wood is naturally a biodegradable material. Immediately after its abatement it starts to get some progressive alteration on the superficial part till to reach the internal part. These alteration are caused on one side by the conjunct effects of the temperature, the ultra violet rays and visible rays, of the pH, water content, oxygen content in the environment, and also of the action of the different micro organisms population (fungi, bacteria, actinomycetes, mould) that produce different enzymes and organic acid that allow to decompose the main wood constituents.

These effects must be summed to the ones caused by the xylophages effects, especially the termites.

From the ancient times the men tried to elaborate some methods to preserve the wood from the rapid progression of these deteriorations. In fact, in the ancient Egypt, the people used to spread oils on the sarcophagi to preserve it for longer times. It’s common to find wood objects that could be preserved extremely well.

Especially during the natural catastrophes, or during some shipwrecks in the sea or on the lakes, the alteration on the wood are more critic and at the end they stabilize themselves. In fact, after their long permanence in the water, in the lakes, in the sea or under the ground, the wood boats or objects absorb the water and become what we call waterlogged wood. This denomination means a more prudent definition of archaeological object made with a wood that is waterlogged. Of course this is valid also for wood elements not worked by men but also natural elements submerged by waters or ground (as trees, pieces of wood) a long time ago.

It’s note that the changes that lead to the water absorption by the wood are caused by deterioration of variable duration in function of:

✔  the environment characteristics (pH of environment, oxygen content) in which the wood object finds itself that are activated by the environment temperature;
✔  the wood species and the part of the exposed wood that could be marked because of the resistance difference between the heart and the alburnum.

✔ the use condition before its immersion.

As we have said before the wood degradation starts form the wood abatement. Then it goes on depending on the age of the wood before its immersion, and the kind of environment in which the wood was used, the mechanic solicitation and their duration, the time of use before the immersion, and so on, are other factors that influence the state of the wood deterioration when the object is excavated. Also the anaerobic micro-organisms and the sub marine insects have a particular action on the wood deterioration; the hydrolysis linked to salts and the diffusion of the soluble matter in case of wet environment, and eventually also the slow oxidation. The wood constituents effectively hydro soluble are the first to be diffused in the environment, especially the hydrolysed compounds, the hemicelluloses and partially the cellulose. The bacteria implied in the decomposition of cellulose are mainly the clostridia . The reduction of the crystalline area of the cellulose will be the main cause of the big of the shrinkage and of the weakness of the waterlogged wood cells.

The lignin is hydrolysed by the extra cellular enzymes, by fungi (white-rot fungi) and bacteria.

There are many species of white-rot fungi that can degrade the lignin but the worst ones are the species of streptomycin, bacillus, norcodia and pseudomonas. The main consequence of this degradation is the loss of the wood solidity of the different objects. 

Anyway after a long time the wood object stabilise a relative equilibrium with the environment. This equilibrium state, that was created in centuries or in thousands of years, is suddenly broken when the object is extracted from the water. This sudden change of environment needs absolutely of a conservation procedure otherwise the wood objects becomes too fragile and the pushed degradation make them disappear definitively. Such a disappearance will bring to a loss of the cultural heritage, and the historical witnesses of shipwreck and all the other human activities conserved in the wood would disappear forever. It so necessary to find these wood heritage and to expose them in museums to let them know by the population. For this reason it’s indispensable to treat and restore the wood object.

The literature on the treatment procedures show that there are different techniques depending on the wood varieties and on the variable degradation. So the treatments must follow this variability. Implicitly there are many efforts that have been made to characterize the waterlogged wood objects.

2.2. STATE OF ART OF THE WATERLOGGED WOOD CONSERVATION METHODS

2.2.1. Introduction

The literature provides us many different treatment methodologies already experimented for the waterlogged wood restoration and conservation. The first documents for the conservation treatment of the waterlogged wood are dated at the end of the fifties (1959-1960). Anyway in Europe, that was the first continent in which the problem of wood conservation was observed and studied for the safeguard of this heritage category, since 1859 the first companies treating the waterlogged wood with a mineral product (alum and other variants) were set up. Many of the methods that we’ll observe now have been inspired by application to fresh wood.

Stamm, that realized many investigation on fresh wood, stated that there are four possible ways for obtaining a dimensional stabilization in the wood. They are:

✔ depositing a protecting film on the wood surface;
✔ decreasing the hygroscopicity of the wood with different methods;
✔ producing some links of hydroxyls groups of the close structural units;
✔ finally depositing neutral substances non volatiles inside the capillary walls of the wood.
This last possibility is the best solution to consolidate the fragile structure of the waterlogged wood. On these basis, numerous are the investigations executed by different authors, as:

✔ definition of the restoration-conservation methods depending on the different kind of waterlogged objects at disposition;
✔ verification of the efficacy of new methods developed by comparison with other methods ; the literature is quite rich for this category;
✔ modification of the existing methods case by case.
When adopted the idea of the consolidation by introducing an adequate material to reinforce the waterlogged wood, many different impregnation methods have been experimented till now.

2.2.2 Impregnation methods

Many different kind of materials have been used to try to restore the ruined structure of the waterlogged wood objects of archaeological interest.

Among these materials, at the first place we can find the polyethylen glycol (PEG).

2.2.2.1 Impregnation with polyethylen glycol (PEG)

The ethylene glycol is a synthetic resin obtained starting from the reaction between the ethylene oxide and the water molecules, with this chemical reaction:

H2C-CH2 + H2O -> HOCH2CH2OH (monoethylene glycol).

Combined n times (n>1), the ethylene glycol molecules associate themselves arriving at the big family of Polyox of the general formula (HO-[CH2CH2O]n-H). According to to the bond conformation, it’s possible to obtain both Polyethylenes Glycols (HOCH2CH2-[OCH2CH2]n-CH2CH2OH), both to Polypropylenes glycols (H-[CH2-CHR-O]n-H) by addition of methoxyls [R].

The PEG is used in many industrial sectors: pharmaceutical, textiles, leather objects, elastic sector, ceramic sector, cosmetic sector and for the wood preservation, particularly in the waterlogged wood sector. The PEG impregnation is the best known one and very used since it was experimented the first time in Sweden in the ’50. The investigation was done on many compounds of this polymer, on different molecular mass (from PEG 200 to PEG 660, soluble in water) to resins of high molecular mass (from PEG 1.000 to 4.500-5.000), solid resin but soluble in warm conditions.

The PEG has been used as simple solution mono dispersed or mixed and so poly–dispersed. Sometime it is used firstly with a starting impregnating solution with PEG of low molecular mass (soluble in cold water), then with a second impregnation using of PEG of high molecular mass at different concentration to ameliorate the wood impregnation.

The PEG has been also mixed with completely different products as the D-mannitol, the tertiary butanol and the sugar.

The advantage of the use of polyethylene glycol is that it is soluble in cold conditions when its molecular mass is low, and at warm conditions when the molecular mass is high.

Its consistence varies, is slightly toxic and has properties similar to a solvent : this is the reason why it has so many applications. The impregnation with PEG for the waterlogged wood needs a long time, from some months to many years. Unfortunately the results obtained are not so satisfactory, as the PEG has a hydrophilic nature. It’s well known that at high humidity condition, the PEG is exudes from the treated wood. Moreover the treated object become dark with PEG treatment, that can be considered the worst inconvenient of this method.

The Polypropylene glycol (PPG) impregnation has been as well used in the conservation of the waterlogged wood objects. It has been mixed with PEG because it has an anti-bacteria effect.

Andersen (1990) tested PPG 400 mixed with PEG 4.000: so it has obtained a good protection against the micro organisms development. The author nevertheless said that the PPG 400 produced the whitening of the treated waterlogged wood. It’s important to underline that, on the contrary of what could seem, the PEG doesn’t preserve the wood against its predators.

To minimize the inconvenient related to the use of synthetic resins, other alternative method have been proposed. Above all, the use of sugar will be examined.

2.2.2.2 Impregnation with sugar

Many different sugars have been used (saccharose, glucose, sorbitol) for the consolidation of the waterlogged wood. But the use of sugar in the wood sector is not new. It was used in the past to stabilize dimensionally the fresh wood in substitution of the PEG. The sugar has the advantage to be cheaper than the PEG, not toxic and to have a low molecular mass (MW= 342.3 g). Is highly soluble both in hot and in cold water and, because it has a great similarity to the structural and chemical structure of cellulose, it perfectly enter the ultra-structure of the wood. The experiences realized by Hoffmann (1990), Parent (1985), Tiemann (1951), Stamm (1937) showed that the ordinary sugar penetrate in a good way the cambium of the most part of trees and the hear of some of them, allowing a reduction of the shrinkage of the treated wood. Wieckzorek et al. used a variation on the sugar impregnation to conserve about 300 small art wood objects and 10 m3 of objects of 1.5 – 5m of length. The sugar has been mixed with warm linseed oil and epoxydic resin. Among all the tested sugars (sorbitol, saccharose and mannitol) the best one has been the saccharose.

Combined with PEG for the wood impregnation, the sugar gives results that can be considered satisfactory.

The sugar impregnation method has, as main inconvenient, the fact that the ideal environment for the micro organism proliferation is created, and the duration of the drying of wood object treated with sugar is very long both on room conditions and with the classical methods.

For reasons linked to their affinity to the water, the sugar and the PEG can not avoid by itself the drying shrinkage of waterlogged wood. Thus, some substitution techniques have been experimented. This is the case of the impregnation processes with acetone-colophon.

2.2.2.3 Acetone – colophon impregnation

Of Danish origin, this method has been used mostly in Switzerland. It’s the result of some researches for methods destined to the dehydration of the wood without provoking any deformation before the real impregnation. Effectively, many studies demonstrated that it’s impossible to get the water from the wood without causing deformations that will influence badly the wood quality.

The dehydration-impregnation methods consists, in a first phase, in the substitution of the water with an alcohol (tertiary butanol TBA, methanol), or with the diethyl ether or with acetone.

Often it’s necessary to repeat the operation substituting the solvent with new one to obtain the maximum water extraction.

The amelioration is obtained with the azeotropic extraction. The impregnation is finally done with a resin that can be both synthetic (PEG, isocyanate) or natural (colophon) that is put inside the substitution liquid.

The objects treated in this method can resist in an atmosphere with a relatively high humidity. The colophon resin which main constituent is the abietic acid, replaced the PEG in this method because of the great disposability but especially because the PEG 4000 solubility in acetone is very limited.

The acetone-colophon method will protect the samples against the accelerated ageing and against the biodegradation.

The main inconvenient are the toxicity and the high cost. Moreover we must add the negative reaction of acetone with the wood constituents. In fact, acetone is for its nature efficacious with the vegetal lignin (it extract it!).The colophon resin is very often sticky, it has a low melting point (80°C) and a high acidity index ( I.A.= 160). Finally it becomes old quickly because it oxidizes in the open air.

2.2.2.4 Alcohol-phenol impregnation

This Byelorussian method has been particularly used by Kazanskaya et al (1990) and by Wieckzorek et al. (1990). The oligomeres of phenol alcohol combined with sugar, glycerine, ethylene glycol, urea or with the formaldehyde have all been tested.

The aim of these chemical combination is the formation of binding agents within the waterlogged wood during the impregnation. The reaction between the formaldehyde (obtained by catalytic oxidation of methanol) and the urea or the phenols bring to the production of a resin (urea resin-formaldehyde, phenol-resin- formaldehyde). They constitute the basic products for the wood adhesives.

One of the variation to this methodology consists in alternating, from six to eight times, a cold bath (20°C) to a hot one (100-103°C). The cold bath is made of alcohol phenol in water solution in concentration of 50%. The hot bath is realized with 50% of sugar in water acidulated with lactic acid introduced progressively till the reaching of a pH of 3.5.

The authors conclude that this kind of treatment ameliorate the mechanical stability of the treated objects. This allowed to treat for the conservation more than 6.000 art wood objects, for a total volume of 60 m3. Among other alcohols the tertiary butanol (TBA) and the ethanol alone or mixed with other products as xylene, have been used to impregnate the waterlogged wood objects.

The results of these tests vary a lot with the different type of treatment applied, the different wood species and the degradation level of the wood.

2.2.2.5 Glycerine impregnation

The glycerine and the ester of boric acid have been used at the end of the ’80 The procedure was patented in 1982. In the modification of the previous method, the glycerine has been compared to sugar. The main modification occurred at hot bath level.

The cold bath is still constituted by alcohol-phenol in aqueous solution at 50%. So, the results obtained with glycerine (103°C) instead of sugar (102°C) showed that:

✔ The solidity of treated object with this method is highly improved;
✔  During the water immersion of the samples treated with glycerine, the water adsorbed by the samples is inferior to the ones treated with sugar;
✔  The humidity adsorption of the wood treated with glycerine in controlled atmosphere is sometimes higher than the one of the samples treated with sugar.
The method seems to have been abandoned because of its high costs.

2.2.2.6 Lactitol impregnation

The lactitol, proposed to substitute the sugar in the above mentioned impregnation, derives from lactose. Its chemical structure is represented by the conformational scheme of type 4 – O (β-D-galactopyranosyl) – D glucitol. Its chemical formula is C12 H24 O11, and its molecular mass is 344 that is very similar to the traditional sugar, the saccharose (342,3). Less soluble at low temperature than the saccharose, it becomes more soluble at 70°C and is less expensive. Its main advantage compared to the traditional sugar is to be more stable against the micro-organisms and to present a low fermentation degree. It’s very stable to the heat. The lactitol has a low hygroscopicity compared to PEG and traditional sugar.

2.2.2.7 Other polymers impregnation

Many other polymers have been tested, with different resins, or simply combined with sorbitol, with sugar or with mannitol., with Arigal C, with Lyofix DML, with butylmethacrylate, or more with the epoxy resins, with polyvinyl alcohol and with isopropanol.

In example, Arigal C is a melamine-formaldehyde substance that has a resinous form in the water. The worst inconvenient of the application of these resins is the low wood absorption. The epoxy resins are known to be used as adhesive for the glass, the ceramics or the metals. They are obtained starting from the reticulation reaction as the creation of bridges in the polymers chains based on salts of bis-phenol, of epichlorydrine and of a tri-ammine.

All the reticulation reactions give a high molecular weight to the finished product, but this is a negative property for Arigal C because a heavy finished product is what a correct impregnation should never provoke.

Recently a series of products composed by four synthetic polymers from the fluorine-polymers class have been experimented. The authors obtained the best results with aqueous solution of epoxy resins on the soft wood, while the aqueous emulsion of Tecnoflon TN latex gave good results both for the soft and hard wood.

Other non organic compound were tested to try to consolidate the waterlogged wood, for example the metallic salts as silicate and chromates.

To end the presentation of the state of art of the consolidation phase it’s correct to mention that none of the methods used until now gave completely satisfying results. Sometimes the waterlogged wood objects have been impregnated using high cost treatments and the results obtained show worst aesthetical properties (i.e. using the PEG) or the impregnating agents used can’t guarantee the needed stability under exposing conditions (case of sugar and acetone-colophon).

After the impregnating phase for reinforcing the wood, the consolidation agent must be hardened inside the wood, particularly in the cellular wall of the fibres. The main methodologies used for this purpose are the air dying (at room temperature and hot), the continuous vacuum drying, the freeze drying and the polymerisation with gamma rays irradiation.

2.2.3 The polymerisation methods

2.2.3.1 The hot air drying

The hot air drying is the main procedure to polymerise the used materials for consolidating the waterlogged woods. This treatment is relatively easy set up if compared to the other drying methods. So to reduce the humidity of the impregnated woods, the convection drying method has been experimented both under normal ventilation, both under forced ventilation. The natural ventilation has actually been abandoned because it’s impossible to manage any drying parameter. With the forced ventilation, the tensions linked to water evaporation, are causing great deformations and cracks that can arrive till a complete transformation of the treated objects. The hot air drying is a process not only slow but also hardly tolerable by the treated objects. It’s convenient for the desiccation of the more resistant fresh wood, able to sustain the above mentioned consequences with a reduced degradation process.

2.2.3.2 Freeze drying

The aim of the application of the freeze drying is the suppression of the shrinkages typical of the traditional hot air dying and especially those caused by the drainage of the soluble products. Effectively the hot air evaporation mechanism causes strong tensions on the cell walls of the wood. The action of these strengths, it has been demonstrated, acts both collapsing the structure, and causing its shrinkage. With waterlogged wood, notably more vulnerable, these phenomena are even more visible.

To contrast these difficulties the freeze dying, known for its action in the field of the structural preservation, has been introduced in the drying of waterlogged wood. The impregnated and non impregnated wood have been dried by freeze drying at low temperatures (between -30°C and -50°C) and low pressures (from 5 to 0.5 torr). These conditions are the most tested and mentioned in literature. Some freeze drying tests at room pressure have been performed but for the moment abandoned. This could be a really interesting treatment, tanks to the reduced investment cost needed to realize it, but according to the results obtained in the preliminary tests carried out the atmospheric freeze drying has been considered absolutely not efficacious.

2.2.3.3 The gamma rays irradiation freeze drying

This method was set up in Switzerland and the first publication speaking of it is dated 1967. Its application for the moment has been done for the dehydration-impregnation method (or substitution with acetone) already described. For this procedure it’s necessary to use a polymerisable resin. For this purpose the PEG has been mostly used. The applied gamma rays have been obtained starting from the Cobalt 60 or with Caesium 137. The gamma irradiation method is used both for the polymerisation (drying, here) and for the modification and sterilization (of the food products as starch). The gamma rays allowed both a hardening of the resin and to provide a sterilization to the treated objects.

The main inconvenient of the application of this system is the fact that the treated object become darker and heavier, moreover the installation of a irradiation plant is very expensive and the dimension of the objects to be treated are still small. The treatment chamber of these plants has dimension of 4 m x 4 m. Moreover the main disadvantage of this method is the diffidence and scare of the population against it.

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