Black crust

Zinc exhibits a valence of +2 in all its compounds. It also is a highly electropositive metal. It replaces less electropositive metals from their aqueous salt solutions or melts. For example, a zinc metal bar put into Cu2+ solution acquires a brown-black crust of copper metal deposited on it. At the same time the blue color of the solution fades. Zinc reduces Cu2+ ions to copper metal. The overall reaction is ... 

For all comets visited by spacecraft, the measured albedo is <5%, making comet nuclei among the darkest objects in the solar system. This coloration is due to black crusts that cover most of the nuclei. The materials now comprising the crusts were presumably spewed out of the interior at speeds below the escape velocity, so they can be considered lag deposits. The parts of surfaces covered by such deposits are inert, and active areas releasing gas and dust constitute only a small percentage of a comet s total surface area. 

Compounds of arsenic are extremely widespread. The black crust often found on native arsenic is a mixture of arsenic and arsenious oxide, AsaOs. This oxide frequently accompanies other arsenical minerals and occurs in two crystalline varieties, arsenolite (cubic), usually as minute octahedra in capillary crystallisations or in stellar aggregates, and claudetite8 (monoclinic), in thin plates resembling selenite. Both forms are lustrous and may be transparent to translucent, colourless to white. 

Since copper carbonate decomposes at a lower temperature than copper nitrate and yields an impalpable powder instead of black crusts, it is advantageous in this preparation to first precipitate copper carbonate (actually a basic carbonate) from the copper nitrate solution and after drying it, to heat the carbonate. 

Ostracitis, thin, black, crusted with burnt earth. 

If you put a strip of zinc metal in a solution of Cu ion, the blue color of the solution fades as a brown-black crust of Cu metal forms on the Zn strip (Figure... 

This important metal is sometimes found as metal, but it chiefly occurs in cop er pyrites, the sulphuret, and in blue copper ore, or malachite, which is carbonate of copper. The latter ore, heated with charcoal, yields the metal most easily/ It is distinguished from all other metals, except titanium, by its red colour. It melts in a strong red-heat, and has the Sp. G. 8 667. It is both ductile and malleable, and has a high degree of tenacity. It is hard, elastic, and sonorous. Heated in the open fire it absorbs oxygen, and produces a black crust of oxide... 

After complete bromination, the bromide is sublimed in a pure Ng stream at 270°C, passing through glass wool plug b and the constriction c into section d. Then, a stream of high-purity Hg is introduced and NbBrs is allowed to sublime slowly into tube section e, kept at 500° by means of furnace og. It deposits on the tube wall as a shiny black crust and as a black cone on the cold finger. The tube is opened and the product is removed imder a protective Ng blanket. The outer crusts are not air-sensitive and are insoluble in HgO. On the other hand, the cones deposited in the inner part of the apparatus decompose rapidly in the presence of moist air. 

Black crust with a metallic luster, or red-brown powder. Insoluble in water and other solvents and in concentrated oxidizing acids. Slowly hydrolyzed by water. The above product does not correspond exactly to the composition given by the formula. 

Figure 5. Field Museum of Natural History, Chicago. Scanning electron micrographs of Georgia Marble with black crust. Showing gypsum at surface and in the intergranular space. Figure 5B is an enlargement of a portion of Figure 5A.

Gypsum and magnesium sulfate form due to atmospheric SO2 reaction with the parent calcite and dolomite CaMg(C03)2 of limestone, marbles, and the lime-mortars. The black crusts on surfaces of most ancient buildings consist of gypsum which has Incorporated soot in the process of transformation of the parent calcite. 

Scrappings from black-crusted surfaces 3. Grains from naturally cleaned surface. 

Black crust developed on a column of limestone, Lincoln cathedral, Lincoln, England. 

Clinical symptoms can be reported as follows after an incubation period of 5-7 days, a papule appears and rapidly becomes vesiculo-pustular, haemorrhagic and, later, ulcerated. Later, ulceration is covered by a hard black crust, surrounded by a zone of erythema and oedema. Lymphangitis and lymphadenitis are usual (Baxby et al. 1994). Healing takes place in 3-4 weeks. Treatment is limited to prevention and/or care of secondary infection. 

If you put a strip of zinc metal in a solution of Cu ion, the blue color of the solution fades as a brown-black crust of Cu metal forms on the Zn strip (Figure 21.4). Judging from what we see, the reaction involves the reduction of Cu ion to Cu metal, which must be accompanied by the oxidation of Zn metal to Zn ion. The overall reaction consists of two half-reactions ... 

Figure 5. Images of black crusts a) black crusts on a granitic pilaster (note the distribution pattern in relation to front face b) detail of black crusts on granitic stones and concrete (b).

Many interesting studies have been published on the effects of a polluted atmosphere on stone with emphasis on the more chemical aspects [39,40,41]. The physical-chemical analytical techniques employed in the study of building materials provide very accurate qualitative and quantitative results on the alterations related to the patina or crust as well as the bulk chemistry of the exposed stone. Scanning electron microscopy (SEM), Electron probe X-ray microanalysis (EPXMA), Fourier-transform infrared analysis (FTIR), X-Ray diffraction (XRD), energy dispersive X-Ray fluorescence, Ion Chromatography, are the most used techniques for the studies of sulphate black crusts as well as to evaluate the effect of exposition time of the sample stone to weathering[42,43]. 

Romarchite is a tm(II) oxide mineral with composition SnO. It is a secondary mineral which occurs as thin soft black crusts with metallic lustre, or as microscopic laths, formed as a weathering product of tin compounds. It was first reported by Organ and Mandarine (1971) as a corrosion product of tin pannikins which had been immersed in water at Boundary Falls (Ontario, Canada). Romarchite is named after the acronym for the Royal Ontario Museum of archaeology, and is often found in association with the hydroxyl-bearing form hydroromarchite, which also has tetragonal symmetry. 

Fig. 1.6 Darkish colouration related to the presence of black crusts (a) with gypsum aggregates that contribute to fixate atmospheric particles (b) Scanning electron. Studies performed at CEMUP laboratory (University of Oporto, Portugal)...

Conserv Manage Archaeol Sites 6(3) 155-165. doi 10.1179/135050304793137801 CappiteUi F, Toniolo L, Sansonetti A, Gulotta D, RanalU G, Zanardini E, SorUni C (2007) Advantages of using microbial technology over traditional chemical technology in removal of black crusts from stone surfaces of historical monuments. Appl Environ Microbiol 73(17) 5671-5675. doi 10.1128/AEM.00394-07... 

Elevation of St Matthew s R.C. Church, Newtownards Rd, Belfast (1881-3). Note the catastrophic material loss from what was originally an extensively black-crusted wall. The roughly dressed stonework is a variety of Scrabo stone, witi the window dressings and string course in a red sandstone, possibly of Scottish origin. 

Elevation detail on sandstone of Lyttle s Warehouse, Victoria St, Belfast (1866-7). A uniform black crust is observed, but is noticeably more severe along mortared joints. These are a potential calcium source for reaction with sulphur pollution to form gypsum - the main crystalline component of black crusts. Note, also, the blistering due to the crystallisation of soluble salts within the stonework. 

Severely flaked Scrabo sandstone on a buttress corner. All Souls Church, Elmwood Ave, Belfast (1895-6). Flaking is active at different depths and has aided wind erosion in the removal of a large amount of material. A blistered remnant of black crust can be seen at the left of the photograph. Note, too, the apparent soundness of the stone immediately above and below the flaked block, illustrating the variability of Scrabo. 

Characteristic blistering on a sandstone black crust, Lyttle s Warehouse, Victoria St, Belfast (1866-7). Note more severe material loss in region of mortared joints (see also Figure 6D). 

Gypsum is also the main component identified in black crusts, and an analysis is presented in Figure 7. Other characteristics of crusts are illustrated in Figure 5D, 6C and 6D. These crusts exhibit many of the characteristics observed on crusts from limestone... 

Figure Seven XRD analyses of a black crust on Scrabo sandstone at All Souls ...

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