What this step adds; in part B, the heat, the hot glass and the lehr dialParts already in placeA pot furnace 1.6 m across in 22.5 cm firebrick, fired with charcoal on an iron grate, with one clay pot holding about 80 kg of glass (32 L at 2.5 kg/L). Soda-lime glass is about 74% silica, 16% soda and 10% lime; soda ash and limestone give off CO₂ as they melt (106 g of Na₂CO₃ leaves 62 g of soda, 100 g of CaCO₃ leaves 56 g of lime), so the pot takes 59 kg of sand, 22 kg of soda ash and 14 kg of limestone and 15 kg bubbles away. Medieval forest glass used potash from wood ash in place of soda. The batch becomes glass at about 1,320 °C, is worked at about 1,090 °C, blown mostly at 870–1,040 °C and annealed at 371–482 °C. The hours are the model's assumption: 12 h to heat up, melted down by hour 14, held to hour 18 for the seeds to rise, then let down to working heat; the jar soaks 1 h at 482 °C and cools to 371 °C over 3 h. A 20 × 26 cm jar with a 3 mm wall is about 1.7 kg, a gather 11 cm across; one pot makes about 47. Glow starts near 525 °C; its colours are the usual eye estimates. This is a guide to how the parts go together, not engineering drawings. Molten glass at 1,300 °C burns through skin and clothing and the furnace radiates heat you feel metres away: wear wool or leather, never synthetics, and infrared-filtering glasses when you look into the furnace or glory hole. Wear a dust mask when you weigh and mix the batch: fine silica dust scars the lungs.