1Funori is a polysaccharide extracted from the mucilage of Gloiopeltis Furcata, G. Complanata and G. Tenax. These three types of seaweeds grow up in the waters off Japan, Korea, South China and North America. The extract of funori seaweed is used in Japan since the XVII century. Thanks to his particular optical properties it is used until today to produce, and now also to restore, paper and fabric artefacts. The centuries-old tradition of his use is another good reason to take in serious consideration this adhesive/consolidant as a valid alternative to other aqueous consolidants for the treatment of cellulosic fibres in restoration of paintings on canvas. A recent study published in 2005 (Geiger and Michel 2005) from the Swiss Federal Institute for material Testing and Research demonstrated this potentiality, testing funori capacity not to modify its mechanical properties and its molecular composition after accelerated ageing. Assessed its usability, another important consideration is the similar chemical nature of Funori and cellulosic fibres that make the textile support of paintings. This observation made ideally funori particularly suitable for the consolidation of depolymerised fibres.
2Nowadays the most used procedure, and probably the only way right now, to give the structural role back to the canvas highly depolymerised is the lining. If the use of Funori could hypothetically give an alternative to line, at least in less serious cases, it would be a step forward for the restoration of canvas paintings.
3The chemical structure of funori is in the middle between agar and carageneean, two of the three classes in which are subdivided Rodoficee, a family of red seaweeds. Infact, like agar, funori contains the monomeric unit 3,6-anhydro-α-L-galactose but, on the other hand, its structure is sulphated like the carrageenan (Swider and Smith 2005). So the chain is composed, like agar, by the two following types of cyclic galactose (Galactopyranose) regularly repeated: β-D-galactose and 3,6-anhydro-α-L-galactose. The two monosaccharide combined together through a condensation reaction and the result is a polymer: agarose. But recent studies show, as was said before, a structure more sulphated than agarose, so in the end we can ideally consider funori as a 6-sulphonated-agarose (Geiger and Michel 2005) .
4The chains align to each other in an helical structure and just the presence of lateral sulphate groups conditions the different ability of gelation (Swider and Smith 2005): agar, without sulphated groups, gel at low temperature and concentration, thanks to the ability of helical structures to be pressed togheter for lack of steric and electrostatic hindrance; on the contrary carrageenans and funorans contain sulphated groups that induced mutual repulsion between helices, making them unable to gel readily (Horie 1992);
5In Japan funori is traditionally used, since 1673, for its adhesive and consolidating properties, in particular for the starching of fabrics and as a plaster additive. It was added to the chalk for the finishings of internal and external mud walls of houses or to cover with paper the mud walls of the room of the tea ceremony to protect them from abrasions. In that case funori was considered better than wheat starch glue, another type of glue used a lot in the Japanese tradition, because funori shrinks less than starch glue avoiding, in that way, the detachment of papers.
6His production was initially in relation with silk products: its commercial production is coeval with the beginning of silk production, during Heian period. It was used for the starching and dyeing of Kimono-Yuzen (Noriko et al 2004).
7Funori was used until today as a detergent for clothes and as a shampoo. But this seaweed is also an edible one and it is traditionally eaten in Japan: “Hegi-soba” are typical Japanese noodles made of funori, eaten in the province of Niigata. Also raw funori can be eaten, maybe in soup or salad or as a “Sashimi” decoration. As the other red seaweeds it is also used as a stabilizer and thickening agent in foods, cosmetics and pharmaceutical products.
8The use of funori as a consolidant started in the field of papers and fabric restoration, used as an adhesive or to bring the correct viscosity to other product such as rice starch and animal glues. Used for the first time three hundred years ago, funori is still produced and considered a good alternative to other traditional consolidants and synthetic ones.
9In the last twenty years funori has become known outside Japan because of its numerous advantages compared to traditional aqueous consolidants. In particular the advantage that makes it known in conservation is its capacity not to modify the appearance of the painted surface, that is to say the index of refraction of the paint layer . It is an uncommon property: commonly used aqueous consolidants such as gelatine, sturgeon glue and the cellulose ethers Klucel E and Methocel MC often alter the aesthetic of the object by producing undesired gloss, darkening or tide lines (Geiger and Michel 2005). So in the last twenty years Funori has been used by conservators as a suitable consolidant for matt paint or powdery paint layers (Finozzi et al 2012).
10But this is not the only property that made funori a so interesting molecule: because of the similar chemical nature of funori and cellulosic fibres that make the textile support, conservators have supposed funori can restore depolymerised fibres, although scientific research about this hypothesis does not exist yet. I just started from this assumption and I tried to verify it.
11The seaweeds are harvested in summer and then are washed and bleached with a sodium peroxide solution. So funori changes colour from a red to ayellowish-brown. Before sodium peroxide was introduced in the production process, the softening and the bleaching were obtained by fermentation. After the bleaching funori is washed in water for two times and then poured into a mold, similar to the one used for the preparation of paper, to form a sheet. The sheets are then spread on rice mats and dried under the sun. When they are dry it’s necessary to spray them with water again to soften them and to finish the bleaching by sun. The complete process lasts three or four months during the summer (Masuda 1984).
12So funori is sold in sheets of dry seaweeds. There are many ways to prepare the consolidant starting from that sheets (Finozzi et al 2012). In particular it can be prepared by heating or at room temperature. An interesting study about the extraction methods of the polysaccharide was published in 1978 by the University of Tokai (Noriko et al 2004). The authors compare the solution obtained by the extraction at room temperature and the one obtained by heating as it was the traditional procedure, testing the viscosity, the separation of the solution with centrifuge, the molecular weight by GFC (Gel Filtration Columns), FTIR analysis, Biuret test and peeling test. The results show that the first solution has a lower viscosity (11,5 mPa-s) then the others obtained by heating (121,3 mPa-s). With the GFC they demonstrated that molecules form links by heating and as a consequence the molecular mass increases. Moreover the solutions extracted by heating present a greater quantity of precipitated components that is to say that the substances extracted by high temperatures are hardly soluble in water at room temperature and have a greater molecular weight. FTIR spectra show different absorption bands in the two cases, in particular the consolidant extracted by heating contains new components not founded in the solution extracted without heat that, on the other hand, reacted positively at the Biuret test. Finally the solution obtained by the extraction at room temperature has a greater peel strength then the others extracted by heating.
13Taking into account the lower viscosity and the slightly greater strength of the solution the choice was to extract funori at room temperature. To obtain the mucilage, the sheets were broken into pieces and soaked in water for one night. Then the solution was filtered and so it was ready to use.
14The experimentation started from the assumption that the similar chemical nature of funori and cellulosic fibres can allow the consolidant to cure physical breaks of damaged structure of the fibres. This possibility was still taken in consideration by some restorers but scientific research about this hypothesis does not exist yet.
15In particular, the study case from which the research began is a canvas painting that is the altarpiece of the parish church of Rondissone, a small city near Turin in Piedmont (Italy). The fibres which compose the canvas support were overviewed with the Scanning Electron Microscope. They seemed very damaged in their micro-structure and, as a consequence, the mechanical tests shown the lack of resistance of the fibres.
- 1 In the previous phase of the restoration, the transportation of the painting from the attic of the (...)
16Considering the large losses of painting layers that characterized the conservation status of the object (more or less 30% of the painting surface), the idea was to realise an “archaeological restoration”, not filling paint losses but leaving exposed the fabric of the support. The final destination of the painting was the just- renovated church St. Francis and Catherine of Alexandria in Rondissone, so the climatic parameters allowed such a restoration. So the first trouble was to identify a consolidant for the paint layers that was able not to modify the appearance of the canvas support, in order to preserve the original tone of the fabric and to avoid the typical saturation of colours given by the majority of consolidation materials.1 For this reason funori was chosen to fix the fragments of the paint layer that were not completely adherent to the support.
17The other reason that suggested the use of funori to consolidate paint layers was its potentiality to cure the structural breaks of the fibres observed by the use of SEM. If this cure would be true the possibility to avoid the lining process could be possible and so the project of a minimum intervention could be realized. The evaluation of this possibility at first was carried on with some samples. Three different funori solutions have been prepared, each one with a different concentration: 0,5%; 1%; 1,5%. Then some samples of depolymerised flax fibres from a clipping of a seam of the canvas were taken and experimented with the three different concentrations of funori (Andrina 2013). To help the funori solutions to penetrate in the structural breaks of fibres, in order to obtain the maximum penetration, low pressure table was used as it is the normal procedure to allow the evaporation of water when a paintings consolidation treatment with an aqueous consolidant is performed. Then the samples were placed to the low pressure table between a blotting paper and Melinex. Then the low pressure was started. After five minutes the blotting paper was replaced by a new one and the samples were again put under pressure for ten minutes to dry them completely, an essential condition not to have any problems during the overview with the microscope. By observing these samples with the Scanning Electron Microscope before and after the treatment it was verified that the consolidant was able to fill structural breaks of deteriorated fibres already at 1% (w/w)consolidant solution. In particular, the sample treated with funori at 0,5% didn’t shown the presence of the consolidant at the morphological observation. Probably the low concentration of the solution combined with the high porosity of the yarn caused the completely penetration of funori solution. In the second sample, treated with the solution at 1%, the overview by SEM has shown two different behaviours of funori consolidant. It fills structural breaks of the damaged fibres. Moreover some parts of the fibre are covered by a thin film of consolidant which follows the trend of the surface. The third sample confirmed the behaviour overviewed in the second one.
18The consolidation of the canvas painting dating back to the XVII century from which the research was started seems to confirm the validity of the treatment. This research represent a starting point and it is hoped that future research can verify more thoroughly the effects of the consolidation on the mechanical properties of fibres.
19The similar chemical nature of the filler, the capacity not to modify the index of refraction of the layer, the good stability when exposed to UV radiation and variations of humidity and temperature, no higher viability against biological attack than the other consolidants make Funori an interesting consolidant maybe useful to stem the problem of cellulosic fibres depolymerisation in canvas paintings.