COB

COB

( 309 )

■finely powdered, azure blue, is a very deep blueglafs, made of the calcined ore of cobalt andlome vitrifiable ingredients, which is ufed as acolouring matter for a variety of purpofes. *The proportion of the vitrifiable balls to thecobalt depends on the eflimated goodnefs of thelatter and the refult of fmall trials. On anaverage, equal parts of the calcined cobalt ore,of potafli, and of ground flints are taken. Thefeare firft fritted and then made into glafs, in potsfunilar to thole of the glafs-houfes, requiringfrom 8 to 12 hours of fufion. When the blueglafs is perfeft, it is taken out with iron ladlesand dropped into cold water to crack it in everydirection and make it more eafily reducible topowder, which is afterwards performed in amill made of very hard ftone inclofed in awooden cafe. At the bottom of the glafs-potsa quantity of regulus of bifmuth is alwaysfound, (the ores of bifmuth and cobalt beingnaturally mixed) and above it is a mixed alloyof arfenic, iron, and copper.

The grinding the blue glafs is a work oflabour and fome difficulty. Several degrees offinenefs are prepared by means of grinding andwafhing. Thel'e are all known by the generalterm of fmalt , or, when in very fine powder,azure. The term azure however is alfo appliedto lapis lazuli blue, a totally different fubftance.

As a colouring matter fmalt is very valuable,both on account of the fine intenfe blue whichit produces, and for its comparative cheapnefs,and becaufe it is not altered by any heat. Inthis laft refpect it is fuperior to lapis lazuli, thecolour of which is entirely and permanentlydeftroyed in a red-heat, but fmalt will not mixwith oil colours, and therefore can only be verypartially ufed. Smalt is ufed when mixed withftarch to give a flight blue to linen, or ratherto correct the tendency to yellow which linenacquires when worn.

Zaffire is prepared alfo in Bohemia , Wirtem-berg, Silefia, Lorrain, and fome other parts ofthe Continent , but the Saxon is preferred, andyields to the proprietors an annual revenue of200,000 crowns'.

Cobalt, when rendered as pure as poffibleby the method above-defcribed, has a Heel-greycolour, not very refplendent, when flowly cooledappears reticulated in its texture, is brittle, andfufible at a heat about equal to that of caft-iron. However carefully it has been freedfrom iron, it remains magnetic, and thereforethere is reafon to fttppofe that this is a propertyinherent in the metal itfelf.

K Encyd. Arts & Metiers, art. Saffre.. 1 <

Cobalt burns in oxymuriatic acid gas with abright white flame, but in a common fire itonly oxidates when heated ftrongly in contactwith air. In this cafe the oxyd is black. Ahundred grains of reguline cobalt yield by thisprocefs 118 grains of the black oxyd, ra andhence ioo grains of the oxyd contain 84.75cobalt and 15.25 of oxygen When it containsarfenic the oxyd becomes fomewhat reddilh.

Cobalt or its oxyd is foluble in molt acids.

The fulphuric acid difiolves cobalt with diffi-culty, but better its oxyds. A fulphat of cobaltis formed by digefting the black oxyd, or com-mon zaffre, or the precipitate from nitrated,cobalt by carbonated potafli, with four timesas much fulphuric acid, fomewhat diluted, orbetter, by diltilling the acid from the cobalt todrynefs and adding warm water to the refidue.This folution evaporated flowly affords by cool-ing cryftals of fulphat of cobalt, which are of afine red when the metal is pure, but greenifhwhen it contains nickel. n Thefe cryftals whenflowly depofited are often aluminiform. Theyrequire 24 parts of cold water for folution, butonly 15 parts when heated to 145 0 .

Nitric acid diffolves both cobalt and its oxydscopioufly and with eafe when gently heated anddigelted. The folution is red, or claret-coloured,or if holding iron, approaching to yellow. Byevaporation it fcarcely cryftallizes, but leaves afaline deliquefeent mafs. This when furtherheated parts with all its acid, and leaves a darkred or violet-coloured oxyd.

Muriatic acid adds with great difficulty oncobalt, and only by repeated evaporation to dry-nefs and adding frefh acid. But the oxyds ofthis metal it diffolves with much more eafe.The folution is of a rofe-red, but when heated,and much concentrated, it affumes a beautifulblue green, more approaching to blue as thecobalt is more free from iron. It is neceffarythat the folution be concentrated to producethis effeft, and the dry fait fhews it {fill moreperfectly. This fingular property of the muriat.of cobalt was firft ufed by Hellot in making -.vbeautiful Sympathetic Ink. If characters be traced-with this folution fufficiently diluted’ on paper,they are fcarcely vifible when coldj hut whenheld before the fire they ipeedily affume a verybright beautiful blue-green. On cooling theyagain become invifible, and'may at pleafure bemade again to appear and depart by alternately-warming and cooling. The paper ffiould be nomore heated than is neceffary to produce the.full effetft. Any other folution of cobalt tQ.

ierhard. »Klaproth. 0 Leochardi.