Ancient Glass Blog of The Allaire Collection

WHAT IS THE CHEMISTRY OF ROMAN GLASS ?

Posted in Uncategorized by Allaire Collection of Glass on May 19, 2014

Chemistry of Roman Glass

Roman Glass, Allaire Collection

Roman Glass, Allaire Collection

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
Fe2O3                                                                                                                    0.5
Sb2O5                                                                                                                     1.5
MnO2                                                                                                                       0.5
PbO                                                                                                                        0.01

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.

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