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It is well known that the sesquioxide of chrome imparts a green colour to fluxes and especially to glass The bichromate of potash possesses the same property as it is decomposed by heat into oxide of chrome and neutral chromate of potash The latter salt is in its turn decomposed by the silica whence result oxygen silicate of potash and oxide of chrome Hence in the presence of silica all the chromic acid of the alkaline bichromate passes into the state of oxide of chrome which remains in the glass If the proportion of the salt is small the glass is transparent perfectly homogeneous and of a green colour with a yellowish tinge But if it is larger in a certain proportion spangles of the sesquioxide of chrome are found in the glass My experiments were made with the following proportions which have always been the same the proportion of the chromate alone being variable
- Sand 250 parts
- Carbonate of soda 100
- Calcareous spar 50
First Experiment With 10 grms of the bichromate the glass melts refines and anneals well It is homogeneous transparent and green with a tinge of yellow
Second Experiment With 20 grms of the bichromate the glass is worked and annealed as easily as in the preceding case its colour
is very dark green and minute spangles of the sesquioxide of chrome are easily recognized in it Third Experiment With 40 grms of the bichromate the fusion is decidedly more difficult and the glass is filled with extremely brilliant crystals Those persons who saw specimens of this glass at once compared it to Venetian aventurine and called it chrome aventurine which name I propose to retain FourthExperiment With 50 grms of the bichromate fusion is extremely difficult The glass is filled with a confused mass of spangles and has no longer the lustre and beauty of the preceding Hence the best proportions for the preparation of the chrome aventurine are these
- Sand 250 parts
- Carbonate of soda 100
- Carbonate of lime 50
- Bichromate of potash 40
The resulting glass contains from 6 to 7 per cent of oxide of chrome nearly half of which is combined with the glass the other half remaining free in the form of brilliant scales or spangles The green aventurine is much more easily prepared than the Venetian It exists during the fusion of the glass while the latter is only formed during the annealing I may refer those who are interested in this question to Fremy and Clemandot's paper upon artificial aventurine in the Comptes Rendus for 1846 p 341 Chrome aventurine sparkles in the sun and in strongly lighted places in this respect it is surpassed by the diamond alone It is harder than common glass which it scratches and cuts easily and is especially harder than the Venetian aventurine hence its greater value Among the irregularities presented by the outlines of the spangles disseminated in the chrome aventurine M Daubree has recognized with certainty the form of the regular hexagon which belongs to the crystalline system of the sesquioxide of chrome Moreover the spangles have the greatest resemblance to certain varieties of laminated oligistic iron ore such as are met with in certain lodes especially the specular variety and it is well known that oligiste iron is isomorphous with the sesquioxide of chrome The colour of the new aventurine is that of the third yellowish green the thirteenth shade in M Chevreul's chromatic circle The lapidaries who saw my first specimens of the new aventurine and who cut some of them agree in stating that it forms an important acquisition to their branch of manufacture for which reason I have thought it worth making public Comptes Rendus October 1 6 1865 Intelligence
The London, Edinburgh and Dublin Philosophical Magazine and Journal of Science. By M.J. Pelouze. p455. Google libros Descargar el pdf.