Restauration and consolidation of the limestone blocks engraved with the four Greek Hymns of Isidoros
By and large, the limestone blocks engraved with the four Greek Hymns of Isidorus show a homogeneous structure and similar features of composition. They are white, but differ with respect to the shades of colour. These differences reflect the differences in the lithoid materials used for the blocks. Two types of lithoid materials can be singled out:
jone is mostly white shading into greyish, the other is white shading into yellowish.
The preservation status of the blocks looked particularly critical. In particular, the yellow lithoids were crucially endangered: they were undergoing phenomena of extreme decohesion, exfoliation and chalking of surfaces. Even the white ones, however, displayed serious fractures, flaking phenomena, and the loss of some parts.
The engraved surfaces have been found covered with a fine cotton gauze. It was attached by means of an acrylic resin approximately ten years ago, when the preservation status of the blocks was perceived of as particularly critical.
Moreover, both the engraved and the non-engraved surfaces were covered with a layer of vinylic adhesive. This treatment was presumably carried out before the gauze was applied in order to consolidate the lithic structure, but it turned out to be extremely damaging: the outermost layers, full of waterproof adhesive, behaved differently from the innermost layers, and ended up detaching from them.
The first and most urgent restoration operations involved the fixing of slices of rock which were semi-detached and of areas of the surface that were lifted up. Next, the protective gauze was removed along with the vinylic layer, using dimethyl-ethyl-ketone.
When the engraved portions were uncovered, it was possible to evaluate directly not only the status of the engravings but also the damages caused by the use of acrylic and vinylic polymers. These were left on the surfaces for an extremely long time, while the blocks remained unsupervised, exposed to repeated cycles of rain and sun drying. The most visible damages included chromatic alterations of the outermost surfaces consisting in localized stains, a dangerous process of detachment of the external surfaces, the almost total erosion of some portions of the text, and the consequent loss of definition of the engraved characters. These damages were particularly evident in the yellow blocks.
Unfortunately, the rate of engravings in a good state of preservation – i.e. those portions of the text that were easily readable thanks to the remains of red and black pigments inside the engravings – turned out to be rather modest.
The restoration work involved the ablation of the altered adhesives, of the pollutants of biological origin, of the deposits of particles, and of the remains of cement mortar on the surfaces. For this purpose we used de-ionized water, acetone, potassium nitrate diluent, soft brushes and a surgical knife. Before consolidating the blocks with ethyl silicate, we removed soluble and hygroscopic salts (sulfates and chlorides detected by means of micro-chemical analyses) using compresses soaked with deionized water.
In order to consolidate the blocks we used products containing ethyl silicate with different concentration gradients. A first treatment with FUNCOSIL 100 – a low-viscosity ethyl silicate, necessary in order to obtain the maximum degree of penetration of the consolidating agent into the rock – was followed by a second treatment with FUNCOSIL 300 KSE – an elasticated ethyl silicate with a higher concentration gradient. This procedure, which we devised on the basis of the petrographic data collected during the mineralogic and petrographic analyses, was fully confirmed by the technicians of the German company “Remmers”, which synthesizes this ethyl silicate.
When the white lithoid type (Picture 1) and the yellow one (Picture 2) are observed through the polarizing microscope they look very similar, as they are both composed by a fine organogenic detritus rich in foraminifers, among which fragments of big globigerines are also recognisable. In the shell detritus, remains of lamellibranchs, gastropods, and echinoderms are mainly recognisable. The embedding element (orthochemical according to Folk) is formed by a matrix with a very fine mesocrystalline texture (microsparite): the two rock samples are therefore definable as fine-grained calcarenites from the point of view of texture, and as biomicrosparites from the point of view of composition. The difference in colour shades turn out to be directly correlated with the different rate of limonitic matrix (which is higher in the yellow lithoid type).
“Foreign” materials on the engraved surfaces
The analysis of micro-samples drawn from the surfaces through the polarizing microscope has highlighted serious phenomena of gypsification, involving the embedding and lifting of the outermost lithic portions (Picture 3).
The analysis also revealed the presence of bleaching materials containing portland cement and plaster agents such as fragments of gypsum and cemental minerals (Picture 4).
Moreover, the gauze soaked with adhesive material turned out to be a fertile culture medium for micro-organisms (Picture 5).
Some letters contain black (Picture 6) and red (Picture 7) pigments. The analysis through the polarizing microscope and the electron microscope revealed the use of coal black and red ocher.