Core-Formed Glass Vessel — Ancient Glass Technique

The core is a temporary form around which the glass vessel is built. Mix clay with animal dung and chopped straw or grass in approximately equal parts by volume. Mold the mixture around a metal rod into the shape of the desired vessel interior, typically 8-15 cm long. Allow the core to dry completely, then pre-fire it gently over charcoal to harden the surface.

Ancient glass used the soda-lime formula: approximately 70% silica sand, 15% soda, and 15% lime. Soda lowers the melting point from over 1700°C to approximately 1050-1100°C. For colored glass, add metallic oxide colorants: cobalt oxide for deep blue, copper oxide for turquoise, manganese dioxide for purple, or antimony trioxide for opaque yellow-white. Mix each batch thoroughly and place in separate clay crucibles.

Place the filled crucibles into a charcoal-fired furnace and raise the temperature to approximately 1050-1100°C. This requires sustained bellows operation for 4-6 hours as the raw batch slowly fuses into molten glass. The glass is ready when a metal rod dipped into the crucible draws out a smooth, continuous thread without lumps or bubbles. Keep the furnace at working temperature throughout the core-forming process.

Heat the clay core in the furnace opening until it glows red-hot. Dip a metal rod into the crucible and gather molten glass. Trail the gathered glass in a tight spiral around the core, starting from the base and working upward. Each trail must fuse to the previous one while both are still hot. Build up 2-3 mm of glass thickness across the entire surface.

While the trailed glass is still hot and pliable, roll the vessel on a flat stone slab to smooth the ridged surface into a uniform wall. Press gently and roll evenly. Reheat the vessel between marveling passes. Two or three marveling passes are needed to achieve a smooth, even surface. Wall thickness should be approximately 2-3 mm.

While the body glass is still hot, gather a contrasting color of molten glass on a thin metal rod and trail it in thin lines around the vessel. The classic pattern uses opaque yellow and white trails on a dark blue body. For the 'feather drag' pattern, trail horizontal lines and then drag a pointed metal tool vertically through them. Work quickly while both body and decoration glass are above 600°C.

Shape the vessel rim by gathering additional glass and trailing it around the top opening. While the rim glass is still hot, use a flat tool to form a smooth, slightly flared edge. For handles on larger vessels, gather a small amount of glass and draw it into a thin trail. Attach securely and reheat to equalize temperature between handle and body.

Place the completed vessel in the furnace and reduce temperature slowly over 4-6 hours. This relieves internal stresses that would cause cracking. Once cool, extract the metal rod and scrape out the clay-dung core using a long hooked wire tool. The pre-fired core crumbles easily. Clean the interior thoroughly — the finished vessel should be hollow and translucent, revealing the layered colors of body and decoration glass.