140 THE ORIGIN AND HISTORY OF 



The red sandstone of the Zweibruken furnishes grindstones 

 usually from five to six feet in diameter, with which the agates are 

 ground down ; the stronger and coarser agates are chipped into 

 shape with the hammer, and then ground, the finer ones are cut 

 with circular steel plates with plain discs, covered with oil and 

 emery powder, and for finer work diamond dust ; and polished 

 on rotating wood or lead cylinders, moistened with tripoli. The 

 labour of grinding is severe. " The workman," says Professor 

 Rudler, "lies upon alow wooden grinding stool, specially con- 

 structed to fit to the chest and abdomen, leaving the limbs free. 

 The hands are engaged in holding and guiding the agate, whilst 

 the feet are firmly pressed against short stakes or blocks of wood 

 screwed into the floor ; the reaction enabling the grinder to press 

 the agate with much force against the moving millstone. The 

 millstones rotate at the rate of 180 revolutions per minute, and 

 the friction is sufficient to rasp down the hardest stones. About 

 180 mills, with 850 grindstones, are kept constantly employed. A 

 number of workmen are employed in cutting cameos and intagli 

 in carnelian or sard and onyx. 



The artificial colouring of stones forms a separate branch of 

 the agate industry. The art of colouring stones artificially was 

 known to the ancients and preserved by the Italian cameo 

 lapidaries in Rome, but kept as a secret, and was unknown in 

 Germany until about 1819, when a German lapidary and a Roman 

 stone engraver were imprisoned in the same cell in Paris, and 

 during conversation the Italian let out the secret, which was soon 

 made known and practised in Oberstein, when common grey 

 chalcedonies of no value were stained and transformed to good 

 onyx, thus giving an impetus to the industry. 



The porous bands of the agate may be stained almost any 

 colour by artificial means, while the finer and harder bands, being 

 impervious, are unaffected. One method is to wash the stones 

 and dry them thoroughly, then steep them in oil and honey or 

 some other saccharine substance and water, and after a few days 

 expose them to a moderate heat; then after washing, to place 

 them in sulphuric acid, and again expose them to a gentle 

 heat. The oily and saccharine matter, being absorbed by 

 the porous portions of the stone, is acted upon and decom_ 



