Home Glass Chemistry Lutz or Aventurine

Lutz or Aventurine

(A) A lutz marble is any marble containing goldstone, a form of glass made with copper or copper salts. Goldstone is also sometimes called aventurine glass, but aventurine is green. Lutz marbles feature one or more unmistakable swirls or bands of shimmering, reddish brown.

Lutz style marbles are particularly beautiful and very collectible, especially antique handmades.

Contemporary glass artists also produce some stunning lutz marbles which, unlike many antique lutz marbles, are always in "wet" mint condition and truly showcase the beauty of lutz.

(B) Goldstone is a type of glittering glass made in a low-oxygen reducing atmosphere. Another common name for the material is aventurine glass, based on the original Italian name avventurina (from avventura, "adventure" or "chance"). It is also sometimes called "stellaria" or "sun sitara" (sitara = "star" in Sanskrit) for its starry internal reflections, or "monk's gold" or "monkstone" from folkloric associations with an unnamed monastic order.

Curiously, "aventurine" glass is one of the few synthetic simulants to provide the eponym for the similar natural stones. The mineral name "aventurine" is used for forms of feldspar or quartz with mica inclusions that give a similar glittering appearance; the technical term for this optical phenomenon, "aventurescence", is also derived from the same source.

The original manufacturing process for goldstone was invented in seventeenth-century Venice by the Miotti family, which was granted an exclusive license by the Doge. Persistent folklore describes goldstone as an accidental discovery by an unnamed Italian monastic order or medieval alchemists, but there is no pre-Miotti documentation to confirm this.

The most common form of goldstone is reddish-brown, containing tiny crystals of metallic copper that require special conditions to form properly. The initial batch is melted together from silica, copper oxide, and other metal oxides to chemically reduce the copper ions to elemental copper. The vat is then sealed off from the air and maintained within a narrow temperature range, keeping the glass hot enough to remain liquid while allowing metallic crystals to precipitate from solution without melting or oxidizing.

After a suitable crystallization period, the entire batch is cooled to a single solid mass, which is then broken out of the vat for selection and shaping. The final appearance of each batch is highly variable and heterogenous. The best material is near the center or "heart" of the mass, ideally with large, bright metal crystals suspended in a semitransparent glass matrix.

(C) Copper colloid size and failure modes:  Copper-based "red goldstone" aventurine glass exists on a structural continuum with transparent red copper ruby glass and opaque "sealing wax" purpurin glass, all of which are striking glasses whose reddish colors are created by colloidal copper. The key variable is controlling the colloid size: goldstone has macroscopic reflective crystals; purpurin glass has microscopic opaque particles; copper ruby glass has submicroscopic transparent nanoparticles.

The outer layers of a goldstone batch tend to have duller colors and a lower degree of glittery aventurescence. This can be caused by poor crystallization, which simultaneously decreases the size of reflective crystals and opacifies the surrounding glass with non reflective particles. It can also be caused by partial oxidation of the copper, causing it to redissolve and form its usual transparent blue-green glass in ionic solution.

When reheated for lamp-working and similar uses, the working conditions should control the temperature and oxidation as required for the original batch melt: keep the temperature below the melting point of copper (1084.62 °C) and use an oxygen-poor reducing flame, or risk decomposition into the failure modes described above.

Non Copper Goldstone

Goldstone also exists in other color variants based on other elements. Cobalt or manganese can be substituted for copper; the resulting crystals have a more silvery appearance and are suspended in a strongly-colored matrix of the corresponding ionic color, resulting in blue goldstone or purple goldstone respectively.

Green goldstone, or chrome aventurine, forms its reflective particles from chromium oxides rather than the elemental metal, but is otherwise fairly similar.

The non-copper goldstones are easier to work with when reheated, due to the less stringent reduction requirements and higher melting points of manganese (1246 °C) and cobalt (1495 °C).

Aventurine is a form of quartz, characterised by its translucency and the presence of platy mineral inclusions that give a shimmering or glistening effect termed aventurescence.

(D) Aventurine:  The most common color of aventurine is green, but it may also be orange, brown, yellow, blue, or gray. Chrome-bearing fuchsite (a variety of muscovite mica) is the classic inclusion, and gives a silvery green or blue sheen. Oranges and browns are attributed to hematite or goethite. Because aventurine is a rock, its physical properties vary: its specific gravity may lie between 2.64-2.69 and its hardness is somewhat lower than single-crystal quartz at around 6.5.

Aventurine feldspar or sunstone can be confused with orange and red aventurine quartzite, although the former is generally of a higher transparency. Aventurine is often banded and an overabundance of fuchsite may render it opaque, in which case it may be mistaken for malachite at first glance.

The name aventurine derives from the Italian "a ventura" meaning "by chance." This is an allusion to the lucky discovery of aventurine glass or goldstone at some point in the 18th century. Although it was known first, goldstone is now a common imitation of aventurine and sunstone. Goldstone is distinguished visually from the latter two minerals by its coarse flecks of copper, dispersed within the glass in an unnaturally uniform manner. It is usually a golden brown, but may also be found in blue or green.

The majority of green and blue-green aventurine originates in India (particularly in the vicinity of Mysore and Madras) where it is employed by prolific artisans. Creamy white, gray and orange material is found in Chile, Spain and Russia.

(E) Aventurine sits at a 6.5 on the Mohs Scale. It is a member of the Quartz family, along with Carnelian, Amethyst, and Citrine. Aventurine’s name comes from Italian, “A Ventura”, meaning "by chance".

Aventurine is found in many places in the world such as Brazil, China, Japan, Tanzania, and the United States. Green Aventurine is most commonly found in India, while Chile, Spain, and Russia account from good quality white, gray, orange, and gray material.

(F) Aventurine glass has many alternate names. It may be called monkstone, Stellaria, or goldstone, and it is visually quite stunning. It is sometimes confused with several minerals like feldspar or quartz, which may have tiny flecks of glittering material that add shine and sparkle. These are called aventurine, though this is not aventurine glass.

The birth and invention of aventurine glass is credited to an Italian family of glassmakers by the name of Miotti. They created this special and iridescent glass in the mid 17th century and it soon become prized and envied. Their process for creating the glass remained a closely guarded secret for many years, and at first they held exclusive rights to produce it.

Fortunately, the secret of aventurine glass was finally “outed.” Essentially, the glass was combined with copper or copper salts. When the glass melted and cooled, these mineral deposits would clump together to created a gold-flecked and shiny appearance on the glass. As is common with most glass, the glass itself had no color, but the additional minerals added could create varying colors like green and blue, though the most common color is a rich, ruddy brown.

You can also find variants of aventurine glass that appear to have more silvery flecks of shine than gold ones. The types of minerals that are added to the glass can achieve this effect. However made in those early days, aventurine glass became highly prized.

Just as the tiny flecks of mineral seem to sparkle in magical fashion as you gaze at aventurine glass, there are a number of myths, folklore, mysteries and secrets surrounding how the Miotti family created this glassmaking process, that almost suggests magic or alchemy. The name itself may mean discovering something by chance or through luck or adventure. But there were rumors that the glassmaking technique was passed to the family by a secret and unknown order of monks. There’s thus something rather special about the glass that glints like gold, even though we now know how it comes by its glitter and shine.

(G) LUTZ: A hand-made glass marble made from cane, which contains a sparkling powdered goldstone. A highly desirable and very valuable marble. In a pamphlet titled “Marbles: Identification and Price Guide” by Mel Morrison and Carl Terison, published without a date (1970’s?) claimed this type of marble was the product of the famous American glass-master Nicholas Lutz. However, this is not supported in the historic record nor in scholarly secondary sources, published biographies of Nicholas Lutz or the companies that he helped make famous.