Scientists said yesterday that they have unearthed the first conclusive evidence of a glass factory in ancient Egypt, offering new insights into production techniques for a commodity so highly prized that nobles used it interchangeably with gemstones.

Analyzing glass and clay fragments at Qantir-Piramesses in the eastern Nile Delta, researchers described a two-step process in which factories melted crushed quartz to form "semifinished" glass, then re-melted and colored it to make glass "ingots" for shipment to artisans elsewhere. They melted the glass again and shaped it into inlays, ornaments and other objects.

"For years, there was no direct evidence of the production of glass," said archaeologist Thilo Rehren of University College London. "Somebody was making it, but the only thing we had were museums full of glass objects."

Rehren, reporting in this week's edition of the journal Science, said he first visited Qantir 12 years ago to examine artifacts at the ruins of an ancient industrial complex dated 1250 B.C.

"There were lots of bronze castings, but among all the debris we found a number of things that didn't fit," Rehren said in a telephone interview. "It took me a year or two to figure out that the unusual finds were related to glass."

But it was not until additional excavations at Qantir in 2003 that researchers were able to say that Egypt had a thriving glassmaking industry of its own instead of simply importing glass from Mesopotamia. The Qantir artifacts have allowed archaeologists for the first time to show in detail how glass was made in the ancient world.

"We have thought that people were actually making glass in earlier excavations, but it has been very hard to interpret some of the evidence," said Egyptologist Christine Lilyquist of New York's Metropolitan Museum of Art. "Qantir is a very important excavation."

Common glass is made mostly of silicon dioxide -- the same compound as quartz or sand -- that has been melted and cooled rapidly so the molecules remain amorphous instead of forming a crystal lattice. Other compounds give glass its color or other properties.

Lilyquist said the first true glass appeared in Mesopotamia between 1600 B.C. and 1550 B.C. Ancient Egyptian texts describe how a pharaoh brought home skilled glassworkers, along with buckets of glass ingots, after a Mesopotamian expedition in 1500 B.C.

"The mainstream opinion was that the Egyptians were importing it from Mesopotamia," Rehren said. "In Egypt, we had only evidence that they were making it into artistic objects."

Glass was highly prized, not because it was inherently expensive but because "not that many people knew how to do it," Lilyquist said. Artisans used it as a gemstone equivalent in gold jewelry and in making small bottles, beads and sculptures.

It is unclear when glass production began in Egypt. At Amarna, archaeologists have found finished glass and intact furnaces from the time of the pharaoh Akhenaten, about 1350 B.C., but there is debate about whether the furnaces were for glass production or glassworking.

"We have wires of glass that are used for making objects, and it's true that glassworking doesn't necessarily mean glassmaking," said Cardiff University archaeologist Paul T. Nicholson, a glass expert who works at Amarna. "But I think we actually do have the factory."

Qantir, the ancient capital of the pharaoh Ramses II about 60 miles northeast of Cairo, has no intact furnaces or kilns but is littered with about 1,100 fragments of ceramic vessels marked by heat and frequently encrusted with bits of glass.

Rehren outlined a two-phase process that began by loading crushed quartz powder into half-gallon ceramic beer jars and heating it to about 1,650 degrees Fahrenheit. The quartz was mixed with an equal amount of ash from desert plants rich in sodium carbonate, which lowers the melting temperature of quartz.

Rehren said the artisans then crushed the semifinished glass into a powder and leached it with water to remove salt and other impurities. Then they put it into a flowerpot-shaped crucible and heated it to 2,000 degrees Fahrenheit, probably with the aid of a bellows.

"On the second melting, the gas bubbles disappear and you can get the colors," Rehren said. Qantir appears to have specialized in red glass, made by adding copper oxide during the second melting in carefully controlled amounts and conditions. Once cooled, the ceramic was broken and chipped off, and the ingots were sent away.

About 1,100 fragments of ceramic vessels, in which artisans would place crushed semifinished glass and heat it to 2,000 degrees Fahrenheit, have been found in Qantir, about 60 miles northeast of Cairo.