802 CHROMIUM, BLUE OXIDE OF 



prepared by boiling chromate of potash, dissolved in water, with half its weight of 

 flowers of sulphur, till the resulting green precipitate ceases to increase, which may be 

 easily ascertained by filtering a little of the mixture. The addition of some potash 

 accelerates the operation. This consists in combining the sulphur with the oxygen 

 of the chromic acid, so as to form sulphuric acid, which unites with the potash of the 

 chromate into sulphate of potash, while the chrome oxide becomes a hydrate. An 

 extra quantity of potash facilitates the deoxidation of the chromic acid by the forma- 

 tion of hyposulphite and sulphide of potash, both of which have a strong attraction 

 for oxygen. For this purpose the clear lixivium of the chromate of potash is 

 sufficiently pure, though it should hold some alumina and silica in solution, as it 

 generally does. The hydrate may be freed from particles of sulphur by heating 

 dilute sulphuric acid upon it, which dissolves it ; after which it may bo precipitated, 

 in the state of a carbonate, by carbonate of potash, not added in excess. 



By calcining a mixture of bichromate of potash and sulphur in a crucible, chromic 

 acid is also decomposed, and a hydrated oxide may thus be obtained ; the sulphur being 

 pirtly converted into sulphuret of potassium, and partly into sulphuric acid (at the 

 expense of the chromic acid), which combines with the rest of the potash into a 

 sulphate. By careful lixiviation, these two new compounds may be washed away, and 

 the chrome green may be freed from the remaining sulphur, by a slight heat. 



Another method of preparing green oxide of chromium, is to mix intimately 45 parts 

 of gunpowder with 240 parts of perfectly dry chromate of potash, and 35 parts 

 of chloride of ammonium (sal-ammoniac), reduce to powder, and pass through a 

 fine sieve ; fill a conical glass or other mould with this powder, gently pressed, and 

 invert so as to leave the powder on a porcelain slab of any kind. When set on fire 

 at its apex with a lighted match, it will burn down to the bottom with brilliant corus- 

 cations. The black residuum, being elutriated with warm water, affords a fine bright- 

 green oxide of chromium. 



Preparation of Green Oxide of Chromium for Calico-Printing. The following direc- 

 tions are given by Do Kerrur. At the commencement of the process the green 

 hydrate of the oxide of chromium is first prepared by dissolving 4 kilogrammes of 

 bichromate of potash in 22 litres (39 pints) of boiling water. Then into a boiler or 

 vessel containing 108 litres (24 gallons) of boiling water, 4 or 5 kilogrammes (8 or 10 

 Ibs.) of pulverised white arsenic are thrown, and boiled for 10 minutes : a precipitate 

 will be formed, and must be allowed to settle : the clear liquor is then run off, and 

 immediately mixed with the solution of bichromate of potash, stirring all the time : in 

 a short time the mixture acquires a green tint, and the hydrated oxide of chromium 

 will be formed and precipitated. After being several times well stirred, and allowed 

 to cool, the whole is thrown upon a filter of white wool, and the hydrate of chromium 

 remaining on the filter is carefully washed with boiling water. It is then dried, and 

 ready to be employed for the preparation of the chloride. In order to obtain that salt, 

 hydrochloric acid of 22 Baume is diluted with water, until the acid no longer gives 

 off vapour. It is then heated , and whilst hot, as much of the hydrated oxide of 

 chromium, prepared as above, is added as will saturate the acid and leave a slight 

 excess of the oxide undissolved. The whole is then left to settle, and the clear liquor 

 is decanted from the dissolved matter. In this state the solution of chloride of 

 chromium still presents some traces of free acid, which would act injuriously upon the 

 fibres of the cotton. To remove this, and to obtain the product in a neutral state, 

 potash-ley (marking 36 Baume) is poured in very gradually, until the oxide of 

 chromium begins to be precipitated. The solution of choride of chromium thus pre- 

 pared, and which is of a dark green colour, is evaporated until it marks 46 Baume ; 

 after cooling, oxide of chromium of the finest green colour is obtained. This prepara- 

 tion is sold under the name of Sea-green. 



This oxide may also bo prepared by decomposing, with heat, the chromate of 

 protoxide of mercury, a salt made by adding to nitrate of protoxide of mercury (mercu- 

 rous nitrate), chromate of potash, in equivalent proportions. This chromate has a fine 

 cinnabar red, when pure ; and, at a dull red heat, parts with a portion of its oxygen 

 and its mercurial oxide. From M. Dulong's experiments it would appear that the 

 purest chromate of mercury is not the best adapted for preparing the oxide of chrome 

 to be used in porcelain painting. lie thinks it ought to contain a little oxide of man- 

 ganese and chromate of potash to afford a green colour of a fine tint, especially for 

 pieces that are to receive a powerful heat. Pure oxide of chrome preserves its colour 

 well enough in a muffle furnace ; but, under a stronger fire, it takes a dead-leaf 

 colour. H.M.N. 



A hydrated sesquioxide of chromium is used as a green pigment, under the name of 

 GUIQNET'S GHEEN. 



CHROMIUM, BLUE OXIDE OF. The following directions have been given 

 for the preparation of a blue oxide of chromium : The concentrated alkaline solution 



