What is iridescence on ancient glass?
The iridescence on ancient glass was unintentional unlike what is found on modern Tiffany, Loetz, and Steuben glass. Caused by weathering on the surface, the iridescence, and the interplay of lustrous, changing colors, is due to the refraction of light by thin layers of weathered glass. How much a glass object weathers depends mainly on burial conditions and to a lesser extent the chemistry of it. These conditions are humidity, heat and type of soil the glass was buried in. The chemistry of ancient glass though basically the same as our soda glass differed in the purity of raw materials and compositional ratio. There were also differences in flux alkali used such as natron (sodium carbonate) or potash (potassium carbonate). Generally glass made in the Western Provinces with potash has less iridescence than glass from the Eastern Mediterranean areas using natron. At the same time burial conditions also were different. Natural iridescence is sometimes found on modern glass bottles from digs in the back yards of old houses or pulled out of river beds. The word iridescence comes from Iris, the Greek Goddess of rainbows and refers to rainbow-like colors seen on the glass which changes in different lighting. It is simply caused by alkali (soluble salt) being leached from the glass by slightly acidic water and then forming fine layers that eventually separate slightly or flake off causing a prism effect on light bouncing off and passing through the surface which reflects light differently, resulting in an iridescent appearance. Modern iridescence sometimes called iris glass is made by adding metallic compounds to the glass or by spraying the surface with stannous chloride or lead chloride and reheating it in a reducing atmosphere. Examples below are ancient glass from the Allaire Collection with natural iridescence.
Opal glass: A translucent white glass made by adding a finely ground powder tin oxide or bone ash to the glass batch to give it a. First made in Roman times and fully developed in Venice, Germany and Bohemia in the 17th/18th Centuries.
Milk Glass: Opaque white glass, usually opacified by tin oxide or arsenic.
Dichroic Glass: Glass which shows different colors depending on whether the light upon it is being reflected or transmitted. This iridescent effect is due to multiple micro-layers of metals or oxides in the glass. In Roman times colloidal silver-gold alloy was used; today uranium or copper oxide are also used. Example: Lycurgus Cup Roman 4th C
Opalescent glass: Is similar to iridescent glass. It is a surface treatment applied on translucent white glass. Frederick Carder (1863- 1963) introduced alabaster glass at Steuben Glass Works in Corning, New York. Carder’s alabaster glass has an iridescent finish made by spraying the object with stannous chloride and then reheating it.
Iridescence: Optical property in which fine colors are produced on a surface by the interference of light that is reflected from both the front and back of a thin film. Things that appear iridescent show rainbow-like luminous colors, and often change color as their positions change. The iridescence on ancient glass is different. It was cause by natural processes and was unknown by the glass makers at that time. see this link to Iridescence on Ancient Glass
Chemistry of Roman Glass
The Chemistry of Roman Glass is the same as modern soda-lime-silica glass only cruder. Glass is a unique material made of a mixture of fused silicates. Its properties are neither those of a solid nor those of a liquid, but rather a mixture of both. Physically glass is nearly an amorphous solid liquid or a super cooled liquid whose form and viscosity change at high temperatures. An object made of glass must be annealed or gradually cooled to keep it from shattering. The appeal of glass has always been its utility and transparency.
The composition of ancient glass is complex mainly because of impurities in the raw material used by the early glassmakers. To them the glass formula may have been as simple as 2 parts sand and 1 part natron. The lime in ancient glass may have come from calcium carbonate found in Natron and sand (quartz). Natron is sodium carbonate, an alkali used as a flux to reduce the temperature needed to produce glass and to make it more pliable. Natron is found in dry lake beds or made from the ash of sea plants from salt marshes. Potash (potassium carbonate), another flux alkali is made from wood ash. Potash was used in inland areas and in the western Roman provinces in the post Roman era. Natron (sodium carbonate) was the preferred flux aide. Natron, a natural product, also contains a small amount of potash carbonate and calcium carbonate. Sand is a mixture of many oxides and its composition is different from place to place. It is generally 80% silica dioxide (quartz), 9% calcium oxide, aluminum oxide, magnesia oxide and many other metal oxides, one of which is iron oxide. In addition to sand, other types of silica were used along with broken or waste glass which is known as cullet. The Romans recycled glass because the addition of cullet (10-25%) to a new batch of glass lowered the working temperature and improved the quality of glass.
Table from Dr. Brill, 1962 CMOG
|Components: Modern Soda-Lime-Silica % Typical Roman %|
|SiO2 73.6 67.0|
|Na2O 16.0 18.0|
|K2O 0.60 1.0|
|CaO 5.2 8.0|
|MgO 3.6 1.0|
|Al2O3 1.0 2.5|
Colored or colorless glass was obtained by adding metal oxides to the batch or by selecting the raw materials which produced the color and shade they wanted. Different shades of green glass for example come from the levels of iron oxide in the glass. The addition of manganese oxide can produce colorless, yellow or purple glass. Copper could be added to make reds or blues, and tin to make white. They also made vessels out of colorless, opaque white and blue glass. The largest use of glass in Roman times was for windows not vessels.
Iridescence found on much of ancient glass is accidental rather than intentional. Caused by weathering on the surface, the iridescence, and the interplay of lustrous, changing colors, is due to the refraction of light by layers of weathered glass. How much a glass object weathers depends on burial conditions and what type of flux aid was used and other impurities. Generally glass made in the western provinces has less iridescence than glass from the Eastern Mediterranean areas.
American Pattern-Molded Bottles and Flasks
American bottles and flasks with pattern-molded designs have been produced from 1765 on. This same type has been made for centuries in Europe and England. A flask is a bottle, which has been flatten so it fits into a jacket pocket and also called a pocket bottle. The pattern-molded bottles and flasks were blown from a single gather of glass, patterned in either rib molds or pattern piece-molds having a simple (diamond pattern) or more elaborate designs. The Pitkin-type flask is part of this group and made by the half-post method and ornamented by pattern-molded ribbings. Both flasks can look alike; however the Pitkin flasks has a tell-tail ring of thicker glass around the neck (post) from the second dip of the half-post method. The examples below are from the Allaire Collection.