Abstract
Gaji is the Georgian name both for a gypsum-and-clay-containing soft rock found in eastern Georgia, and for the plaster traditionally derived from it. Gypsum-earth plasters have a long history of use across Caucasia, central Asia, and the Middle East. Gaji has been used in the preparation of mortars, plasters and decorative finishes, and as a support for Georgian religious and secular wall paintings together spanning the 9th – 20th centuries. Little research has been conducted into its properties, however, and within the context of wall painting conservation neither its influence on condition nor its implications for treatment have been adequately explored. Using the Church of St. Demetrios of Thessaloniki, rediscovered in 2015, at the monastic complex of David Gareji as an example, this paper explores the use of gaji plaster technology and the conservation challenges it presents.
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Notes
- 1.
It should be noted here that “gaza”, as mentioned in documentation of the St. Petersburg Cement Plant as a component of Portland cement [7], does not refer to a gypsum-clay material, but rather to a loose clay limestone.
- 2.
Three monasteries are located in present day Azerbaijan – Udabno and Chichkhituri in part and Bertubani in its entirety.
- 3.
Personal communication with master craftsmen who, before gas-fired factory production became the norm, used to process gaji using traditional methods.
- 4.
Environmental monitoring (RH and AT) was undertaken using U12 Hobo internal sensors and UX23 Pro V exterior sensors.
- 5.
There were no visual signs of salt efflorescence or crystallisation within the painting stratigraphy. Preliminary salt (anion) analysis was performed using MQuant ion test strips on stone, stone masonry, brick, and plaster samples. Chloride was below the level of detection in all cases; sulfate was found in plasters and mortar consistent with containing calcium sulfate; nitrate was found at moderate concentrations in some plaster samples and in one stone masonry sample.
- 6.
S/2015 samples derive from the clearing of the church in 2015. The interior of the church of St Demetrios Thessaloniki was found filled with earth to a height of 1.5 m above the floor. Unfortunately, while removing the soil, some original plaster fragments fell from the dado level, which were then stored by church authorities in the niches of the church.
- 7.
Raw gaji samples C1 – Didostati Ltd and C2 – Ginu Ltd were collected from production plants. The plants and quarries are located very close to each other (ca. 10 km) in Garadabni municipality near villages Axal Samgori and Gamarjveba, Georgia.
- 8.
Granulometric analysis was not undertaken on painted sample S07 to protect the painting surviving on the fragment nor on sample S05, due to potential interference of primary support (natural rock) within the sample.
- 9.
Historic gaji plaster samples, as well as commercial gaji samples (C1 and C2) prepared as plasters by mixing with water, were hot-mounted (150–200 ℃) in adhesive (colophony resin and/or epoxy resins) and then made into thin-sections using a grinding and polishing machine. Thin-sections were examined under a polarizing microscope ‘Optika B-383POL’ (Italy). This method was adopted due to resource constrains, though we note that the high temperatures involved in preparation of thin sections may influence the hydration state of calcium sulfates within the samples; alternatives methods will be pursued in future.
- 10.
The decohesive nature of these sample prevented their preparation as thin-sections with the methods available.
- 11.
Chemical analysis was conducted according to USSR standard: 1. Moisture content GOST 22688–77, gravimetric analysis; 2. Crystalisation water GOST 23789–79, gravimetric analysis; 3. AIR GOST 23789–79, gravimetric analysis. 4. LOI GOST 5382–91 P.7, gravimetric analysis, 5. CaO GOST 5382–91 P.10, volumetric analysis, 6. SO3 GOST 5382–91 P.9.3, gravimetric analysis. The historic sample S07 was ground and analysed, while the commercial processed (unhydrated) gaji samples were already in powder form. The data of the given chemical analysis was used to calculate the percentage composition of gypsum, bassanite, clay content, calcite, and insoluble fraction of the samples.
- 12.
For XRD analysis, the historical plaster sample was ground to a powder and the modern gaji sample was analysed in raw powder form. Analysis was performed using diffractometer
with copper anode and nickel filter, U-35kV, I-20mA, scan rate 2 degree/min. λ = 1.54178 Å. - 13.
It is currently unclear whether the charcoal was a deliberate addition to the plaster or present as a consequence of the firing process, due to the accidental inclusion of natural vegetation during quarrying of the raw materials, or derived from fuel used for firing the gaji oven. In February 2020, members of the research project interviewed the director of ‘Ostatis Saxli’ Ltd. (gaji producer). According to the source, the historic firing process of gaji obtained from the quarried rock involved the construction of ‘ovens’ from the raw material. The structure contained holes into which wood fuel was placed. After the gaji was fully burnt it was crushed together with any remaining fuel or charcoal and then sieved.
- 14.
For samples S05 and S07, the aleuro-pelitic structure was evident through petrographic analysis as granulometric analysis was not undertaken (see footnote 8).
- 15.
The plasters made from commercial gaji powder contained unexpectedly high proportions of bassanite. The reason for this remains unclear and will be investigated in future work. This plaster was prepared one month before the thin-section preparation and so we expect the bassanite to have largely transformed into gypsum. However, it is possible that some of this gypsum (only in the modern plaster and not the historic plaster) was transformed due the high temperatures used in the thin section preparation (see footnote 9). We note that other possible sources of bassanite include “over-burned” bassanite that hydrates very slowly.
- 16.
Ref. 18 identifies an organic binder in the plasters; this possibility has not been investigated at St Demetrios church. Organic additives to plasters are historically common and are worthy of future investigation.
with copper anode and nickel filter, U-35kV, I-20mA, scan rate 2 degree/min. λ = 1.54178 Å.References
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The authors would like to thank the International Education Center Alumni Association, Georgia for its support. This paper presents the early stages of a project that was curtailed due to the pandemic.
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Sagaradze, M. et al. (2023). Gaji, a Gypsum-Earth Plaster in the Wall Painting Technology of the Church of St. Demetrios of Thessaloniki, David Gareji, Kakheti, Georgia. In: Bokan Bosiljkov, V., Padovnik, A., Turk, T. (eds) Conservation and Restoration of Historic Mortars and Masonry Structures. HMC 2022. RILEM Bookseries, vol 42. Springer, Cham. https://doi.org/10.1007/978-3-031-31472-8_21
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