TIMBER 1051 



an inferior pigment, called ultramarine asJics. The best ultramarine is a splendid blue 

 pigment, which works well with oil, and is not liable to change by time. 



Analyses of lapis-lazuli ultramarine will be found in the following article. 



UI.TRAlVia.RIWE, ARTIFICIAL For many years every attempt failed 

 to make ultramarine artificially. At length, in 1 828, M. G-uimet resolved the pro- 

 blem, guided by the analysis of MM. Clement and Desormes, and by an observation 

 of M. Tassaert, that a blue substance, like ultramarine was occasionally produced on 

 the sandstone hearths of his reverberatory soda-furnaces. M. Gmelin, of Tubingen, 

 published a prescription for making it ; which consisted in enclosing carefully in a 

 Hessian crucible a mixture of 2 parts of sulphur and 1 of dry carbonate of soda, 

 heating them gradually to redness until the mass fuses, and then sprinkling into it by 

 degrees another mixture, of silicate of soda and aluminate of soda ; the first con- 

 taining 72 parts of silica, and the second 70 parts of alumina. The crucible must be 

 exposed after this for an hour to the fire. The ultramarine will be formed by this 

 time ; only it contains a little sulphur, which can be separated by means of water 

 M. Persoz likewise succeeded in making an ultramarine, of perhaps still better quality 

 than that of M. Guimet. Lastly, M. Kobiquet has announced, that it is easy to form 

 ultramarine by heating to redness a proper mixture of kaolin (China clay), sulphur, 

 and carbonate of soda. It would therefore appear, from the preceding details, that 

 ultramarine may be regarded as a compound of silicate of alumina, and silicate of soda, 

 with sulphide of sodium, and that to the reaction of the last constituent upon the 

 former its colour is due. 



The constituents used in the different methods of making ultramarine vary in 

 character and in quantity. It is said that a good mixture may consist of : dried 

 kaolin, 100 ; calcined Glauber salt, or sulphate of soda, 41 ; calcined soda, 41 ; pul- 

 verised charcoal or coal, 17; and sulphur, 13. When such a mixture is heated 

 without access of air it yields a product from which a white substance may be obtained, 

 known as white ultramarine. Calcined in crucibles at a high temperature, with a very 

 limited supply of air, the mixture affords a semi -fused greenish mass termed green 

 ultramarine. By carefully roasting this with sulphur at a low temperature, with free 

 access of air, the ordinary blue ultramarine is obtained ; and this when powdered, 

 lixiviated, and dried, is ready for the market. 



It appears that potash-salts cannot be substituted for soda-salts in the manufacture 

 of ultramarine, but it is said that those of baryta may be so employed. In some 

 cases, silica is added to the ultramarine mixture in the proportion of from 5 to 10 per 

 cent. 



Both native and artificial ultramarine have been examined very carefully by several 

 eminent chemists. The following are a few specimens of these analyses : 



Green. 



25'5 



30'0 



39'9 



4-6 



0-9 

 0'4 



Silica ....... 47-306 



Sulphuric acid ..... 4 % 679 



Resin, sulphur, and loss . . . 12-218 



Notwithstanding the many investigations which have been made of ultramarine, its 

 chemical composition is by no means thoroughly understood ; and the German Asso- 

 ciation of Ultramarine Makers have recently (1874) offered a prize for the best essay 

 on this subject, from which, it may be hoped, more light will be thrown upon the 

 constitution of this compound. 



UIiVA. A seaweed used in the preparation of Green Lavcr. See ALG;E. 



UMBER. A mechanical mixture of limonitc (brown haematite) and hydrated oxide 



