Microbial calcification of gypsum-rock and sulfated marble

Abstract

A broth containing the sulfate reducing bacterium Desulfovibrio desulfuricans was used to treat samples of reagent calcium sulfate, gypsum-rock specimens, fragments from a marble monument with a black weathering crust rich in gypsum, and a marble monument with similar crust. Calcite was found to have formed on all treated surfaces suggesting that this microbe has the potential to clean crusted marble monuments whilst also regenerating calcite, the parent mineral of the marble.     

A PRELIMINARY NOTE ON THE USE OF ADHESIVES AND FILLERS IN THE RESTORATION OF ANCIENT MATERIALS WITH SPECIAL REFERENCE TO GLASS

Abstract

We have reacted Georgia marble in sulfur dioxide (SO₂) enriched atmospheres and calculated the reaction rate constant. This constant was derived from surface reduction data, obtained by leaching thereaction product from samples exposed for different periods of time. Application of this constant to marble weathered in known ambient levels of sulfur dioxide (SO₂) and relative humidity, allows the prediction of the thickness of the resulting gypsum crust formed over a given period of time. We have found that the crust grows outwards due to calciumions which migrate from the interior and then react with sulfur dioxide at the sample surface. Thus, anytreatment of the sulfated marble should include the considerations that the crust only masks the marble surface and is not an integral part of the original sculpture, and that cavities exist in the underlying layer of marble from which the calcium ions were selectively depleted.     

The Conservation of Flags In Sweden

Abstract

Optical and electron-beam microprobe examination of pairs of calcite crystals, mounted approximately 50 microns apart and then immersed for various times in solutions containing a barium compound and urea, showed that these crystals were cemented together by the formation of an adherent bond between the calcite substrate and the witherite filling the void space between the crystals. The bonding material was found to be composed of a series of solid solutions of barium calcium carbonate, high in calcium content at the original surface and increasing in barium content as the distance from the original crystal surface increased. The relatively rapid rate of reaction suggests that the formation of the solid solutions proceeds by a co-precipitation mechanism. The formation of the solid solutions is thought to occur most readily in pores and cracks where the calcium ions generated by the dissolution of calcite cannot readily diffuse into the bulk solution. Application of the barium hydroxide-urea treatment to the preservation of decayed calcareous stone is reviewed.     

INVESTIGATIONS OF WEATHERING CRUSTS ON SALZBURG STONE MONUMENTS

Abstract

A series of corroded stone specimens from Salzburg monuments was examined by electron-probe microanalysis. The crusts arose from environmental effects, in the case of marble primarily from SO₂. Deterioration phenomena can be correlated with stone surface features.     

Pattern-welding and Damascening of Sword-blades—Part I Pattern-Welding

Abstract

Marble specimens were impregnated with epoxy resins by first soaking the specimens in a solvent and then treating them, in steps, with increasing concentrations of the resin in the resin-solvent mixture. This resulted in impregnation to several centimeters’ depth into the specimen. Both impregnated and non-impregnated specimens were subjected to 3000 ppm SO₂ atmospheres in a dynamic system. X-ray diffraction and fluorescence studies to determine the deterioration quantitatively, revealed that transformation of the outer layer of non-impregnated marble into primarily calcium sulfite stopped after a maximum of 15 mole percent conversion under normal conditions of humidity. While certain impregnated marble specimens showed significant protection from SO₂ attack, other impregnated specimens revealed more reactivity than the controls. A partial explanation of this has been obtained by scanning electron microscopy which showed that the resin films on the surface were perforated. Coatings of certain acrylics provided some additional protection to impregnated specimens.     

Unusual pigments on a Greek marble basin

Abstract

The examination of the original polychromy of a fourth century B.C. classical Greek marble basin revealed a number of unusual pigments. In addition to natural cinnabar, and Egyptian blue, analysis has shown the presence of wild madder (Rubia peregrina) as the colorant in a purple pigment. This is one of the very few instances in which the use of madder as a colorant in classical painting has been established analytically. The white pigment was found to be neutral lead carbonate (cerussite). The use of this carbonate in painting is extremely rare. The yellow pigment was identified as misy (jarosite). The use of jarosites, iron sulphates, as pigments in painting has not been reported previously.     

Mineralogical changes arising from the use of aqueous sodium carbonate solutions for the treatment of archaeological copper objects

Abstract

Mineralogical changes which occur during the reaction of copper(II) chloride with carbonate in aqueous solution have been investigated, and compared with those involving copper(I) chloride, nantokite, CuCI, and copper artifacts coated with malachite, CU₂CO₃(OH)₂ The reactions observed and the products obtained are seen to have implications with respect to the use of aqueous sodium carbonate solutions to stabilize archaeological copper objects. The frequent observation of the mineralogically rare species chalconatronite, Na₂Cu(CO₃)₂·3H₂O, on the surface of treated objects is readily explained by known reaction chemistry. Its formation can be minimized by washing the object after treatment.     

Re-Discovering a Roman Wall Painting at Harvard: New Research on a Fragment from the Villa at Boscotrecase

ABSTRACT

This article presents historical and scientific analysis, as well as the conservation treatment of a newly rediscovered Roman wall painting fragment, now in the collection of the Harvard Art Museums. Although the piece has not previously been published, it was among a group of fragments removed from a Roman villa near Boscotrecase in southern Italy, an area that has been key to the study of Roman wall painting and other decoration. Technical imaging confirms the use of painting techniques consistent with other high-quality paintings in the area. Materials analysis revealed a palette consistent with published findings of Roman wall paintings, including abundant use of Egyptian blue and green earth. Of interest was the use of Egyptian blue as an optical brightener in select white passages. Despite the high quality of the painting, no cinnabar was present, and all red passages were achieved using hematite. Multiple different white minerals were identified including calcite, aragonite, and gypsum. The widespread presence of gypsum is unusual and may point to alteration.     

Pickeringite: A deleterious salt on buildings

Pickeringite, MgAl₂(SO₄)₄·22H₂O, was observed as efflorescence at the church of Saints Peter and Paul in Largario (Switzerland). The spongy salt crusts formed directly on rusty, deeply weathered gneiss rich in mica. The building stones originate from rocks in the vicinity of the village. On outcrops, similar occurrences were discovered. Ferrous and pyritic gneiss exhibited efflorescences of gypsum, epsomite, potassium alum, and pickeringite in sheltered places. Based on these observations and the known conditions of formation, we conclude that pickeringite resulted as a secondary mineral from weathering of pyritic gneiss. Most likely, weathering happened in an environment where, among other ions, aluminium and magnesium were present, but calcium and carbonate were absent. Such conditions rarely occur. If our hypothesis was confirmed, the fact that an extremely acid, calcium- and carbonate-free weathering system evolves next to lime mortars and plasters would be very surprising. It would imply that restricted zones devoid of a buffering effect develop on a microscopic scale.     

Stone reinforcement by calcite crystal precipitation induced by organic matrix macromolecules

Abstract

The weathering of stone monuments results in the leaching of calcite, with a consequent increase in porosity and decrease in integrity. A new method of consolidation uses organic matrix macromolecules extracted from Mytilus californianus shells to induce the precipitation of calcium carbonate within the pores of the stone. The procedure has been tested in the laboratory on samples of bioclastic limestone and dolostone, by measuring changes in total porosity, amount of water absorbed and superficial cohesion. The results appear to confirm the success of the treatment.     

Some Observations on Methods for Identifying Proteins in Paint Media

Abstract

Fourteen tesserae from the mosaics in the Cathedral of Salerno were examined by X-ray diffraction and by energy-dispersive X-ray analyses. Their constituents were identified and they can be divided into crystalline and amorphous materials. The main crystalline phases are quartz, calcite, magnesium carbonate, magnesium hydroxide and microcline. The amorphous materials are composed of silica containing several elements: Ca, Na, K, A1, Fe, Cr, Mn, Ni, Ti, Cu. For comparison, one tessera from Pompei was characterized. A subdivision of tesserae into three groups, crystalline, amorphous and composite, is suggested.     

Traitement de Conservation des Peintures sur Panneau

Abstract

Samples from the external facade, internal pillars, wall and floor mosaics of the Cathedral of Salerno and mosaic samples from the Cathedral Museum were examined by X-ray diffraction, thermo-differential analysis and by thermo-gravimetric analysis. They were shown to be lime mortars containing varying amounts of an inert fraction such as dolomite, quartz, microcline, sanidine, diopside, analcime and augite. The predominant binder was calcite deriving from Ca(OH)₂ The existence of relicts of Ca(OH)₂ and Mg(OH)₂ of low crystallinity in some samples is discussed.     

THE RESTORATION OF TWO MARBLE STATUES BY ANTONIO CORRADINI

Abstract

The restoration of two early eighteenth-century marble statues by Antonio Corradini is described. Methylene chloride was used to remove plant stains. Sepiolite (magnesium silicate) and water drew out the dirt, and thick incrustations were removed with an ultrasonic dental tool. Incorrectly restored parts were remodelled according to contemporary prints, not in marble but using a casting procedure.     

Preliminary study on the impact of relative humidity on the conservation of jades

Abstract

This paper examines the impact of relative humidity on jades by enclosing the samples under constant relative humidity (RH) of 32, 49, 61.5, and 81.7% at the same temperature of 40 ± 0.1°C for 160 days. The simulated samples were pure and dense tremolite jade and serpentine jade, impure tremolite jade and serpentine jade containing some calcite, and serpentinized calcite, which are all common materials of ancient jade found in archaeological excavations. All samples were characterized through particle-induced X-ray emission and a glossmeter. The degree of deterioration proved to be greater in impure materials than in pure ones. The best RH to conserve various kinds of materials differs: for pure tremolite and serpentine, all of the RH levels are equal; for impure tremolite with some calcite, 61.5 and 81.7% were superior; for impure serpentine with some calcite, the highest RH (81.7%) was best; and as to the serpentinized calcite, there is no clear conclusion as yet and further research is needed.     

When Glass and Metal Corrode Together,VI: Chalconatronite

ABSTRACT

Chalconatronite, Na₂[Cu(CO₃)]₂·3H₂O, is formed as a corrosion product when copper alloys are exposed to sodium carbonate solutions in the air. This also happens when metals come into contact with corroding soda glass which forms alkaline surface films in reaction with humidity from clean air. More often, substantial amounts of formaldehyde are present indoors which react to formate via the Cannizzarro reaction and accumulate over time in the films. Twenty cases of chalconatronite (including two mentioned in the literature) are reported as occurring on heritage objects with glass in contact with copper alloys: Baroque reliquaries with set glass gems, enamel on metal (sixteenth century and a modern replica of intentionally unstable composition), Christmas tree glass baubles with wires, glass buttons with metal shanks, a glass figure with a wire support, miners’ lamps, and a glass framed daguerreotype. These confirmed identifications might help conservators in investigating similar cases to shed more light on the formation conditions.     

NOTES ON THE CONSERVATION OF SCULPTURE,STONE, MARBLE AND TERRACOTTA

Abstract

Sulphuration is the main destroyer of marble, producing imaginary (not true) patina. Other reasons for the discoloration of marbles are discussed. Magnesium silicate saturated with de-ionized water is a safe and simple cleaner. Sintolite is advocated as an adhesive for marble rather than the general more fluid epoxy resins. Solid polyvinyl acetate is strongly advocated as the basis for sma1l repairs to internaIly housed marble. The consolidation of marble is discussed and a method for internal exhibits recommended. Conservation of unbaked clay is touched upon, and a method for the repair of terracotta suggested together with a process for the reproduction of a glazed frieze using a latex mould and epoxy-reinforced plaster casts. A few remarks are made concerning the dangers of cleaning alabaster with water. The commentary is of a general nature and is not intended as a strict blueprint for any particular object.     

Calcium Carbonate on Bronze Finds

Abstract

The removal of calcareous accretions from archaeological bronzes can be a difficult step in their conservation. Chemical cleaning with chelating agents might be an alternative to mechanical methods. In this study the use of the chelating agent sodium tripolyphosphate (STPP) was examined to determine whether it is effective in discriminating between calcium carbonate and copper patina, and non-corrosive to copper alloys in the presence of air. Comparative experiments with STPP and disodium ethylenediamine-tetraacetate (Na₂-EDTA) were carried out on synthetic malachite, cuprite, calcium carbonate, naturally polished malachite surfaces and sheets of bronze and brass. In addition to the properties of the chelating agent, the solubility of the salts and the pH values of the solutions are crucial factors in the removal of compounds of low solubility. The quantity of metal ions dissolved, estimated by atomic absorption spectral analysis and scanning electron microscopy, showed that the calcareous accretions could be removed satisfactorily, but STPP also dissolved constituent parts of the patina, such as malachite and cuprite, and may harm bronze or brass. Compared to Na₂-EDTA, STPP is less effective in dissolving calcium carbonate. It is less harmful to the patina and base metal, but may lead to patination of the underlying metal.     

Decay and treatment of Macael white marble

Abstract

This paper reports the process of deterioration of the white marble used in the Alhambra of Granada, and its consequences. Variations in mineralogical composition, microscopic topology, hardness, compressive strength, propagation of ultrasonic waves, absorptIon and diffusion of water and properties related to the pore structure were studied. The relationships between these factors, the visible deterioration and possible mechanisms are discussed. A preliminary evaluation of some protective treatments was carried out. The results can also be applied to white Macael marble used in other monuments in an environment with wide fluctuations in temperature and relative humidity.     

Ban Chiang bronzes: manufacturing techniques and restoration

Abstract

Original protective renderings have been the focus of numerous studies in recent years. Relevant literature, archival research and archaeological findings reveal that a number of mortar mixes or liquid substances were brushed on to protect and colour-unify the exterior ashlar-stone of many historic buildings. These coatings were of two types: common mortars, particularly improved with organic additives, and water-resistant filmforming substances. Economic cost, local supply, traditional practices and technological development are vital to an understanding of regional preferences for one mortar mix or material over another. The Cathedral of Leon is a mediaeval gothic temple from the thirteenth century, profusely remodelled throughout its history. In the sixteenth century, a new design of the courtyard partially enclosed a fifteenth-century exterior wall. Samples from the enclosed and exposed parts of this wall were examined. Analytical research suggests that the stone surface of this element received a treatment based on gypsum with organic additives. These organic products were fatty materials of the lipid type (tallow or lard) which reacted to form an insoluble, hydrophobic soap, at the same time providing the stone with its yellowish colour. The exceptional amount oflime registered in samples from the enclosed section has been considered a component of this surface treatment.     

A cause of water-sensitivity in modern oil paint films: The formation of magnesium sulphate

Unvarnished twentieth-century oil paintings are often sensitive to aqueous swabbing, a method routinely employed by conservators for surface cleaning. This study proposes a connection between sensitivity and the presence of magnesium sulphate heptahydrate which has been identified on the surface of some of water-sensitive paintings. The probable source of magnesium is magnesium carbonate, an additive in some twentieth-century oil paints, which has reacted with atmospheric sulphur dioxide (SO₂). Films made using modern manufactured paints and formulations made in the laboratory were exposed to gaseous SO₂ and raised relative humidity and examined using scanning electron microscopy with energy dispersive X-ray spectroscopy and X-ray diffraction to characterize the crystalline entities. Films containing magnesium carbonate formed magnesium sulphite and sulphate hydrates. Films containing zinc oxide were also investigated. These formed zinc and sulphur containing salts. Sensitivity to swabbing with water before and after exposure was evaluated. Films that developed salts, demonstrated increased sensitivity to aqueous swabbing after exposure to SO₂. Findings suggest that increased water sensitivity may be due to a combination of the formation of hygroscopic degradation products and to weakening of the paint film due to salt-induced disruption of the surface.