Well, carbon-iron melts at 1200°C, while glass melts at 1400-1600°C. But to get glass from sand you actually need 1500-1700°C. And if the sand is pure silica -- needed for clear, transparent glass -- up to 2300°C.
The kilns available probably can only get up to 1200-1300°C, so still 300-500°C...