Agate-staining In The Early Part Of The Century

(via The Journal of Gemmology, Vol.XIV, No.3, July 1974) M J O’Donoghue writes:

In 1913 Dr O Dreher published a book entitled Farben des Achates, in Idar Oberstein. Long out-of-print, there is no copy in the library of the Gemmological Association nor in the British Museum. However, in his recent book, Gemstone & Mineral Data Book, John Sinkankas summarizes a number of Dr Dreher’s findings.

For obtaining a red color Dr Dreher recommends a dye solution with the following composition: ¼ kg iron nails dissolved in 1 kg concentrated nitric acid. When the liquid is clear the agate slabs are soaked for a duration depending on their thickness, e.g. 3 mm thick, 6 – 10 days, 7 – 10 mm from 3 – 4 weeks. Stones are then heated in a closed crucible for 2 – 3 or 8 – 10 days again according to the original thickness. Dreher found the iron nitrate supplied by a chemical house less satisfactory than the somewhat cumbersome nail method. S. Hoffman worked through the same method to obtain red agate.

For blue Dreher used 250 g of yellow potassium ferrocyanide dissolved in 1 litre of lukewarm water. Stones were immersed for 8 – 14 days. They are then washed and subsequently placed in a lukewarm saturated solution of ferrous sulphate.

For black the stones are immersed in a solution composed of 375 g of sugar per litre in which they are soaked for 2 – 3 weeks, with the occasional addition of water to replace losses through evaporation. They are rinsed and dried and then placed in a bath of concentrated sulphuric acid. This is warmed for one hour until it is hot. The stones are soaked for 1 – 2 hours while the acid is brought close to boiling point (340ºC). They are carefully washed on removal. It was not necessary to bring the acid to boiling point to achieve carbonization of the sugar.

Green staining is accomplished by the use of a saturated solution of chromium trioxide in 1 litre of water. Immersion lasts from 8 – 14 days for thin slabs, 2 – 8 weeks for thickness of 3 – 10 mm. After removal and rinsing the agates are placed in ammonium carbamate acid carbonate followed by heating to redness.