MANGANESSE, or MAGNESIA NIGRA; a mineral substance used in the tinging of glass. It is dense, ponderous, and heavy; in its purest and finest pieces approaching greatly to the colour of lapis hematites, being composed of regular parallel strata, diverging from a centre to the circumference. There is another kind, however, more common, of an iron-grey colour, and irregularly streaked like the steel-grained lead-ores.
But the most common manganese is entirely of an irregular structure. It is very heavy, moderately hard, and of a deep dusky grey, approaching to black; tho' sometimes of a ferruginous brown. It is found in many parts of England and Germany, in large masses, of a rude, rugged, and unequal surface. It is commonly supposed to be an ore of iron: but the experiments of Mr Pott and Mr Cronstedt shew, that this stone contains little or no iron; and therefore the latter author has made a distinct order of this earth, which he calls terra magnesia. It has the following properties: 1. It does not effervesce with acids, tho' they dissolve some part of it, especially when it is calcined: spirit of vitriol acquires from it a rose-colour: aqua regia also acquires colour, especially from the black kind. From these solutions fixed alkalies precipitate a white earth. 2. A small quantity of this earth mixed with glass frit, gives red or purplish colours to the glass: larger quantities give a deep purple, or even a black. Dr Lewis tells us, that preparations of iron, whose colour in glass, in a dilute state, is sometimes yellow, and sometimes greenish or bluish, are always of a dark brown or black when the glass is overdosed with them: hence many of the ferruginous earths and stones melt into a black glass; as the coloured clays, several slates, and the stone called wobynn-stone. Black glasses or enamels made on this principle, have, however, like the concentrated vegetable liquids, one imperfection; that though of a deep black colour when in masses of any considerable thickness, yet, when spread thin, they always betray some of the original
colour, or of the particular hue which they would have if the colouring matter was in less quantity. To this inconvenience the black glass made with manganese is likewise subject: and therefore the best method of obtaining a perfect black, is by mixing two or more of the abovementioned darkening materials; and instead of taking colourless glass or enamel for the basis, to use fragments of different coloured pieces, or compositions which have been spoiled in trying to tinge them of other colours.—The common black glass of which beads are made, is coloured with manganese only; and hence, when powdered, it looks of a dirty purple: the most perfect black used by the enamellers is composed of manganese, zaffre, and scales of iron. Manganese is also used to give a glazing to pottery. 3. Fused with nitre, or with fixed alkali; it gives to warm water various colours, green, purple, red, or blue; which change by agitating the water. 4. Cronstedt affirms, that it deflagrates with nitre; Pott says it does not. 5. Cronstedt says, that he has sometimes extracted a small quantity of tin from manganese. 6. The same author affirms, that the colours given by manganese to glass are easily destroyed by arsenic, or calces of tin. 7. Dr Lewis suspects, that it may increase the fusibility of glass. An ingenious friend, he tells us, observed, that in making impressions in different kinds of glass, he found this black sort to be by far the most fusible of any. 8. When manganese is fused with glass, a strong effervescence ensues, whence it may be presumed that these two substances act violently on each other.