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"Goldstone" - Aventurine Glass

Last Updated: 24th May 2009

Goldstone: Man-made Aventurine Glass

by Daniel E. Russell



Goldstone is a man-made glass which consists of minute octahedral crystals of copper dispersed in glass to created a glittering appearance. It is usually cut en cabochon for use in jewelry, and its popularity as a gemstone has waxed and waned over the past three or four centuries. Also called "Aventurine Glass," goldstone introduced the term of "aventurescence" into the mineralogical lexicon, and varieties of quartz which contain small, reflective spangles of other minerals such as muscovite are still referred to as "aventurine" quartz today.

Goldstone has been produced in reddish-brown, blue, purple and green colors. It is created by the melting of soda-lime glass containing copper salts in a reducing (oxygen depleted) environment, causing the copper salts to decompose to metallic copper. (A green variation of the formulation is the result of chromium salts being decomposed to form a green glass with silvery metallic crystals.)
Historical Notes:

A litany of apocryphal “urban folklore” mythologies regarding how and when goldstone was created have devolved. Foremost among these is that goldstone was accidentally discovered by Italian monks (religious order unknown) when they dropped copper shavings into a batch of molten glass in the 19th century (or 18th century. Or 17th century. Or 13th century); the “accidental copper shavings” hypthosis would not yield evenly sized, evenly distributed copper particles in the glass groundmass. Webster (1949) suggested that it was developed in the 13th century by Christoforo Briani, one of the great early glassmakers of Murano; while Briani did attempt simulations of agate and chalcedony in glass, there is no evidence of his creating goldstone. Another more colorful claim has been advanced that “goldstone” was the result of medieval alchemists’ search to create gold.

Fanciful notions aside, the best available evidence suggests that goldstone was developed by Vincenzo Miotti (sometimes spelled Miozzi), a member of a prominent family of Venetian glass makers, in the 17th century. Keeping with the tradition of the medieval craft-guild, the formulation was maintained a closely guarded secret. It was not until the early 19th century, after the family had stopped making glass, that the formulation was divulged by one of the surviving members of the family to restart the tradition of goldstone manufacture in Venice. In the 1860’s, two French chemists independently developed their own formulations for making goldstone, with the recipe created by Theophilus Jules Pelouze based on chromium salts being decomposed to form silvery chrome oxide crystals in a green glass groundmass. The basic process of making goldstone can be simplified as follows: a simple copper rich glass “metal” is prepared by fusing high grade sand, sodium carbonate, calcium carbonate, and copper oxide. When the glass in extremely liquid, iron or iron oxide dust is stirred in and dissolved to form a red, opaque glass that was once known as “hematine”. Rather than cooling the glass immediately, the “draught of the furnace is then stopped, the ash pan closed, the pot with its lid on covered with ashes, and it is allowed to cool very slowly” (Sauzay 1870). This creates a low-oxygen (reducing) environment which converts the copper salts into metallic copper. The process suggests that the origins of goldstone may lie in a failed experiment at creating a variation of “hematine” in the 17th century. Washington (1894) alternatively hypothesized a more direct process in which iron was added in the form of a flux charge of siderite (iron carbonate) along with the calcium and sodium carbonate fluxes and copper oxide during the initial creation of the glass.

In 1894, Henry S. Washington (who would later serve as President of both the Geological Society of America and Mineralogical Society of America) was permitted to visit the Venezio Murano Glass Co. at Murano, the small island off Venice to which the Venetian glass industry had been relocated in the 13th century. “The manufacture of this glass being a trade secret I could extract no information from the foreman who gave me the specimens as to the process or details of the manufacture; a fact which is greatly to be regretted.” He was, however, provided with samples of goldstone as well as samples of an unsuccessful batch of goldstone, and provided a detailed account of the shape of the copper crystals he observed in the sample, which are illustrated below.


Copper Crystals in Goldstone (after Washington, 1894)


In 1876, French gemologist Louis Dieulafait offered the following comments on goldstone:

For several centuries Venice has had the monopoly of the fabrication of aventurine; and even now, it is a Venetian artist, Bibaglia, who furnishes to commerce the artificial aventurine that is most highly prized. Aventurine is a glass the base of which is soda ash, lime, and magnesia, coloured yellow by oxide of iron, and holding in suspension a large number of small particles of oxide of copper. The distribution of these particles in a regular manner through the whole vitreous mass appears to be the chief difficulty in its manufacture. The dexterity requisite to accomplish this must be very difficult to attain, for the profits realized from the manufacture of aventurine are remarkably large.

According to its quality, the artificial gem sells for $5 to $15 the pound, while the raw materials that enter into the composition of a pound of it are certainly not worth a quarter-dollar. French chemists--M. Hautefeuille in 1860, and M. Pelouze in 1865--have published processes by which productions have been obtained equal to that of Venice, and, in the latter case, perhaps superior. The new aventurine of M. Pelouze has a beautiful lustre, and a hardness exceeding that of glass and ordinary aventurine. It is obtained by melting together 250 parts sand, 100 parts carbonate of soda, 50 parts carbonate of lime, and 40 parts bichromate of potassium. It will be seen that by this formula the spangles with a basis of copper are replaced by spangles with a basis of chrome. – Dieulafait (1876)


The uniform shape and size of the metallic crystals as well as the uniform distribution of the copper crystals through the glass body helps differentiate goldstone from aventurine quartz and sunstone. The copper crystals in goldstone are readily visible under 20x magnification. The crystals may appear more silvery than coppery red in the blue, purple and green variations of goldstone, sometimes appearing tinted as a result of the filtering effects of the colored glass comprising the body of the material. Large masses of rough may sometimes preserve flattened contact surfaces, rounded edges, and surfacial flow lines that were created when the molten glass was poured out onto a hard surface to chill.

Bibliography:
• Dieulafait, Louis (1876) Diamonds and Precious Stones - A Popular Account of Gems Co. 1876 Scribner, Armstrong & Co. New York pp. 245-246
• Hautefeuille, E (1861) Artificial Aventurine (Aventurine Glass) Bull. Soc. Encour. Id. Nut., 60, pp 609-17 (1861)
• Sauzay, A. (1870) Marvels of Glassmaking in All Ages. London, 1870 pp. 173 - 175
• Washington, Henry S. (1894) On copper crystals in Aventurine Glass American Jornal of Science, Third Series, Vol. 48 no. 287 pp. 411 – 418
• Webster, Robert (1949) Aventurine: Glass, Feldspar, Quartz Gems and Gemology, Vol 6 No. 7 (Fall, 1949) pp. 207-211





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